Metallic glasses from “alchemy” to pure science: Present and future of design, processing and...

Materials and Design 35 (2012) 518–556

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Materials and Design

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Review

Metallic glasses from ‘‘alchemy’’ to pure science: Present and future of design,processing and applications of glassy metals

Eugen Axinte ⇑The Gheorghe Asachi Technical University of Lasi, Faculty of Machine Manufacturing & Industrial Management, 59 A Prof. Dimitrie Mangeron Blvd., 700050 Iasi, Romania

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 August 2011Accepted 11 September 2011Available online 28 September 2011

Keywords:A. (metallic) glassesF. Atomic structureH. Selection for material properties

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Metallic glasses, first discovered a half century ago, are currently among the most studied metallic mate-rials. Available in sizes up to several centimeters, with many novel, applicable properties, metallic glasseshave also been the focus of research advancing the understanding of liquids and of glasses in general.

Metallic glasses (MGs), called also bulk metallic glasses (BMGs) (or glassy metals, amorphous metals,liquid metals) are considered to be the materials of the future. Due to their high strength, metallic glasseshave a number of interesting applications, for example as coatings. Metallic glasses can also be corrosionresistant. Metallic glasses, and the crystalline materials derived from them, can have very good resistanceto sliding and abrasive wear. Combined with their strength – and now, toughness – this makes them idealcandidates for bio-implants or military applications. Prestigious Journals such as ‘‘Nature Materials’’,‘‘Nature’’ frequently publish new findings on these unusual glass materials. Moreover Chinese and Asianscientists have also been showing an interest in the study of metallic glasses.

This review paper is far from exhaustive, but tries to cover the areas of interest as it follows: a shorthistory, the local structure of BMGs and the glass forming ability (GFA), BMGs’ properties, the manufac-turing and some applications of BMGs and finally, about the future of BMGs as valuable materials.

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1. Introduction

When it was started the researching for this review, it was apleasant surprise to learn that a large number of original papershave either been published or accepted for publication in scientificliterature. However, it was easy to understand that this immenseflow of information may pose a risk, that of ‘‘not seeing the woodfor the trees’’! By querying only the Scopus database, it wasobserved an exponential growth in the number of research studiesin the field of metallic glasses, as presented in Table 1.

The scopes and target audience of ‘‘Materials and Design’’ Jour-nal offers the possibility to create a filtration barrier in the path ofthis ‘‘tsunami of information.’’ This filter has helped in making adecision regarding the chapters and the weight of each chapterin the composition of this study, as illustrated in the chart below(see Fig. 1).

In early middle age up to the Renaissance, the alchemy, physicsand chemistry overlapped to fusion. The alchemists were viewed as

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sorcerers, with magic powers. The alchemists’ primary objectivewas the creation of the mythical ‘‘philosopher’s stone’’ which wassaid to be capable of turning base metals into gold or silver, and alsoact as an elixir of life that would confer youth and immortality uponits user. Remarkable scientists from middle age were also wellknown alchemists. Some examples are: the persian Jabir ibnHayyan (well known as Geber, 8th century, pharmacist, chemist,physician), Roger Bacon (also known as Doctor Mirabillis – ‘‘fatherof teachers’’, 13th century); Saint Thomas Aquinas (Thomas ofAquin, or Aquino, known as Doctor Angelicus and Doctor Universalis,13th century); Bombastus von Hohenheim (well known asParacelsus – Swiss Renaissance physician, botanist, 16th century);sir. Isaac Newton (17th century). The list will continue in 20th cen-tury with Albert Riedl, Jean Dubuis or Terrence McKenna [1].

Today, the modern science and philosophy, view the alchemy asa protoscience, a precursor to modern chemistry, having providedprocedures, equipment, and terminology that are still in use. How-ever, alchemy also included various non-scientific mythological,religious, and spiritual concepts.

In 1960, in the modern sanctuary of science from California(California Institute of Technology), an act of inversed ‘‘alchemy’’was produced: a metal, a gold based alloy was transformed in aglass (. . . a metallic glass). And the philosopher’s stone was theextremely rapid cooling of melted alloy (cca. 106 K/s).

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Table 1Evolution of BMGs information flow from 1960 to date.

Years (since 1960) 1960–1970 1970–1980 1980–1990 1990–2000 2000–2010 2011 Accepted and published online

Papers in Scopus 133 903 4969 4956 13,154 637 – May 2011, 869 – to date

Fig. 1. The virtually filter of informational flow.

E. Axinte / Materials and Design 35 (2012) 518–556 519

2. Short history of BMGs

The first reported metallic glass, scientific obtained, was the al-loy Au75Si25 produced at Caltech by Klement, Willens and Duwezin 1959 [2] In a book chapter, entitled Metallic glasses-historicalbackground, Duwez describes the first experiment, based on ‘‘thegun technique’’ as ‘‘a success combined with a failure’’. The successwas given by sufficient metallic glass obtained to make the Debye–Scherrer patterns. The failure was the destruction of experimentalapparatus. Duwez says: ‘‘the shock pressure was too high andabout half of apparatus disintegrated, sending hot broken piecesinto the laboratory’’ [3].

This and other early glass-forming alloys had to be cooled extre-mely rapidly to avoid crystallization. An important consequence ofthis was that metallic glasses could only be produced in a limitednumber of forms (typically ribbons, foils, or wires) in which onedimension was small so that heat could be extracted quickly en-ough to achieve the necessary cooling rate. As a result, metallicglass specimens (with a few exceptions) were limited to thick-nesses of less than one hundred micrometers. In 1969, H.S. Chenand D. Turnbull formed amorphous spheres of Pd–M–Si (whereM = Ag, Cu, or Au) at critical cooling rates of 100–1000 �C s�1.

In 1976, (based on the concept of the melt spinner developed byPond and Maddin in 1969), Liebermann and Graham developed anew method of manufacturing thin ribbons of amorphous metalon a supercooled fast-spinning wheel. This was an alloy of iron,nickel, phosphorus and boron [4]. The material, known as Metglas,was commercialized in 1980s and used for low-loss power

distribution transformers (amorphous metal transformer). In theearly 1980s, Turnbull’s group produced glassy ingots of Pd40Ni40P20

with a diameter of 5 mm using surface etching followed by heatingand cooling cycles. In 1980s A. Inoue (Tohoku University’s Institutefor Materials Research) and William L. Johnson (Caltech) havediscovered strongly glass forming multicomponent La-, Mg-, Zr-,Pd-, Fe-, Cu-, and Ti-based alloys with large undercooling and lowcritical cooling rates of 1–100 �C s�1, comparable to oxide glasses [5].

In 1988, A. Inoue discovered that alloys of lanthanum, alumi-num, and copper ore are highly glass-forming.

In the 1990s, however, new alloys were developed that formglasses at cooling rates as low as 1 K/s. These cooling rates canbe achieved by simple casting into metallic molds. These ‘‘bulk’’amorphous alloys can be cast into parts of up to several centime-ters in thickness (the maximum thickness depending on the alloy)while retaining an amorphous structure. The best glass-forming al-loys are based on zirconium and palladium, but alloys based oniron, titanium, copper, magnesium, and other metals are alsoknown. Many amorphous alloys are formed by exploiting a phe-nomenon called the ‘‘confusion’’ effect. Such alloys contain somany different elements (often a dozen or more) that upon coolingat sufficiently fast rates, the constituent atoms simply cannot coor-dinate themselves into the equilibrium crystalline state beforetheir mobility is stopped. In this way, the random disordered stateof the atoms is ‘‘locked in’’.

In 1992, the first commercial amorphous alloy, Vitreloy 1 – Vit1(41.2% Zr, 13.8% Ti, 12.5% Cu, 10% Ni, and 22.5% Be), was developedat Caltech (by W.L. Johnson and A. Peker), as a part of Department

Table 2A synthesis of historic evolution of BMGs.

Year BMGs composition BMGs commercial name Authors BMGs place of birth

1959 Au75Si25 – W. Klement Jr., Pol Duwez, and R.H.Willens

Caltech

1969 Pd–M–Si (whereM = Ag, Cu, or Au)

H.S. Chen, and D. Turnbull

1976 Fe40Ni40B20 MetGlass (since 1980) H. Liebermann, and C. Graham Univ. of Pennsylvania, Philadelphia, PA1992 Zr41Ti14Cu12Ni10Be22 Vitreloy1 (produced by Liquidmetal

Technologies http://www.liquidmetal.com)W.L. Johnson, and A. Peker Caltech

2001 Zr–Ti–Cu–Ni–Alalloys

Similarity with Vit105(Zr52.5Ti5Cu17.9Ni14.6Al10)

A.A. Kündig ETH Zürich – The Swiss Federal Institute ofTechnology Zurich

2004 Bulk amorphoussteel

– Liu and Zhao Ping Lu; Wallace Porter;James R. Thompson

Oak Ridge National Laboratory (ORNL) http://www.ornl.gov, University of Virginia

2010 Pd79Ag3.5P6Si9.5Ge2 – Demetriou, Launey, Garrett, Schramm,Hofmann, Johnson, and Ritchie

California Institute of Technology and TheUniversity of California, Berkeley

520 E. Axinte / Materials and Design 35 (2012) 518–556

of Energy and NASA research of new aerospace materials. In 2001,Kündig at ETH Zürich has been investigating Zr–Ti–Cu–Ni–Al alloysfocusing on those similar to Vit105 (Zr52.5Ti5Cu17.9Ni14.6Al10) – oneof the best glass-forming alloys [5].

In 2004, two groups succeeded in producing bulk amorphoussteel, one at Oak Ridge National Laboratory (‘‘glassy steel’’), theother at University of Virginia. The product is non-magnetic atroom temperature and significantly stronger than conventionalsteel, though a long research and development process remainssecret.

In 2010, the concept that metallic glasses not being very toughwas made history by Marios Demetriou and colleagues from theCalifornia Institute of Technology and The University of California,Berkeley [6]. Demetriou, Launey, Garrett, Schramm, Hofmann,Johnson and Ritchie developed a palladium based metallic glass(with formula Pd79Ag3.5P6Si9.5Ge2 [7]) that is not only strong, butalso tough as steel. The damage tolerance of this metallic glass,its combination of strength and toughness, is higher than anyknown and studied material. In [7] Demetriou announce anotherpotential application: ‘‘Many noble-metal alloys, including palla-dium, are currently used in dentistry due to their chemical inert-ness and resistance to oxidation, tarnish and corrosion. Owing toits superior damage-tolerance capacity, the present palladium al-loy can be thought of as a superior alternative to conventional pal-ladium dental alloys. [...] The absence of any elements consideredtoxic or allergenic from the composition of the present glass willlikely promote good biological compatibility.’’

In Table 2 are synthesized these historical dates.

3. Local structure of metallic glasses

3.1. Generally discussion

Metals and glasses are well known by humanity by thousandyears. The first (meteoric) iron manmade objects are from Sumerand Ancient Egypt, 4000 B.C. It is historical accepted that the firstmanufactured glass was in the form of a glaze on ceramic vessels,about 3000 B.C. The first glass vessels were produced about 1500B.C. in Egypt and Mesopotamia. Mineral soda–alumina (m-Na–Al)glass has been found across a vast area stretching from Africa toEast Asia. m-Na–Al glass appears around the 500s B.C. [8].

These two materials possess distinctly different properties, andwere explored and developed independently. Metals are made ofmetallic elements via metallic bonding. Atoms in metals are knownto reside on a crystalline lattice with long-range translational or-der. Glasses are solids (frozen liquids) with a randomized threedimensional structure, and involve covalent and ionic bonds, orVan der Waals interactions,

Cheng and Ma, in their monumental review [9] says: ‘‘The dif-ferent atomic and electronic structures underlie the contrasting

properties of metals and glasses. It is not until the 1960s thatmetallic glasses (MGs) were successfully synthesized, overlappingthese two categories of materials. MGs consist of predominantlymetallic elements and metallic bonds, but at the same time havean amorphous internal structure. Such a combination of ‘‘metal’’and ‘‘glass’’ leads to unique properties and unprecedented oppor-tunities. Since the discovery of the first MG (Au75Si25), there hasbeen increasing interest in developing and understanding thisnew family of materials. Among the many unresolved puzzles,the atomic-level structure and structure–property relationshipare one of the central topics.’’

Hirata et al. in [10] make direct observations of the local atomicorder of BMGs and revealed that: ‘‘Amorphous materials do nothave any translational and rotational symmetry down to the sub-nanoscale because of their disordered atomic arrangement. Thus,it is quite difficult to experimentally characterize their atomicstructure by conventional diffraction, spectroscopic and imagingtechniques. Various structural models, such as Bernal’s ‘dense ran-dom packing’, Gaskel’s ‘short-range order’ and the recent ‘solute-centred quasi-equivalent cluster models’, have been proposed inthe past 50 years’’ (excerpt with permission from [10] � 2011 Nat-ure Publishing Group).

According to [9–11] there are two major challenges in the studyof BMGs structures: how to construct a realistic three-dimensional(3-D) amorphous structure, using experimental and/or computa-tional tools and how to effectively characterize a given amorphousstructure and extract the key structural features relevant to thefundamentals of glass formation and properties, using appropriatestructural parameters. In Table 3 are listed the structure parame-ters of BMGs.

In Fig. 2 is exemplified a Voronoi tessellation (named after Rus-sian–Ukrainian scientist G.F. Voronoi 1868–1908) The 3-D config-uration of an amorphous structure can be represented by spacetiling of Voronoi cells, while the motif of each cell (of the centeratom) is determined by the spatial arrangement of the nearestneighbors. The example Voronoi cell in (b) has three quadrangularfaces and five pentagonal faces, and thus has a Voronoi index of h0,3, 6, 0i. The A–B pair forms a 5-fold bond, and the example in (c)has five common neighbors (1–5) forming a loop, with a CNA indexof 555.

For BMGs, the direct reconstruction of the locally 3-D structureis very difficult. Cheng and Ma, in review paper [9] says: ‘‘Someexperimental techniques can be used to extract statistical informa-tion about the average glass structure, but the data usually cannotprovide a complete picture. Nevertheless, the experimental resultsstill serve as a yardstick against which any hypothetical structuralmodel should be verified.’’ Structural studies have been trans-formed in recent years by acceleration in the acquisition of X-rayand neutron scattering data, and by improved computationalmethods, including the reverse Monte-Carlo method to fit

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Fig. 2. Voronoi tessellation (a); Voronoi cell (b); the nearest neighbors of A (c) reproduced with permission from [9] � Elsevier.

Table 4List of experimental and computational methods used in study of local structure of BMGs.

Experimental techniques Computational methods

X ray based methodsX-ray/neutron diffraction (XRD) Reverse Monte Carlo modeling (RMC)X-ray absorption fine structure (XAFS) Molecular dynamics simulation (MD)

Fluctuation electron microscopy (FEM) Quantum molecular dynamics – also called ab initio MD or first-principles MDTransmission electron microscopy (TEM) Classical molecular dynamics (classical MD)Nuclear magnetic resonance (NMR)

Table 3Structure parameters of BMGs (dates excerpted and adapted from [9]).

Structure parameter Abbreviations Definition – characterization (from [9])

Pair distribution function PDF Pair correlation representing the probability of finding atoms as a function of distance r from an average center atomStructure factor S(q) The partial PDFs are directly related to partial structure factors in reciprocal space via Fourier transformationCoordination number CN The number of atoms that are in the nearest-neighbor shell of a given center atomChemical short-range order SRO For a structure with more than one species, it is useful to analyze the chemical make-up of the nearest-neighbor atoms,

and compare with the composition of the alloy. This reflects how far the local chemistry deviates from the expectationof a random solution, i.e., the degree of chemical SRO

Bond angle distribution BAD (h) Bond angle distribution is a three-body correlation describing the spatial relations of three neighboring atoms. One firstdetermines the nearest-neighbor atoms that are ‘‘bonded’’ with the center atom. Then the angles between each andevery two bonds are calculated. The same is repeated for all atoms in the system to obtain the distribution

Bond orientational order BOO The BOO is a many-body correlation and is a quantitative description of the bond orientational symmetry around thecenter atom, which concerns multiple bond angles and their spatial relationship. The normalized parameter Wi(evaluates the bond orientational order, and differentiates the various local environments

Common neighbor analysis CNA CNA is a multi-body correlation between neighboring atoms In CNA, each pair of nearest-neighbor atoms is given in athree-number index, jkl. j is the number of nearest neighbors common to both atoms in the pair. k is the number ofbonds between the j atoms themselves. l is the number of bonds in the longest continuous chain formed by the k bonds.CNA can detect various local atomic arrangements. Example: CNA index of 555 corresponds to a pentagonal bipyramid,which is the building block of an icosahedron

Voronoi tessellation VT VT is a scheme to divide the 3-D space into cells centered by each atom. A plane is drawn to bisect each line connectingthe center atom and one of the neighboring atoms, and the cell enclosed by all the inner planes is called a Voronoi cell,or Dirichlet cell, or Wigner–Seitz primitive cell


measured data, and molecular-dynamic simulations [11]. A syn-thesis of experimental and computational methods used in studyof local structure of BMGs is presented in Table 4.

An example how fluctuation electron microscopy (FEM) worksis given in Fig. 3a, reproduced with permission from [9], with ori-ginal explanation of authors.

The typical constituent elements of the BMGs can be groupedas: alkali and alkaline earth metals (AM), semi- or simple metals(SM) in IIIA and IVA groups neighboring the semiconductors, tran-sition metals (TM), including early transition metals (ETM) and latetransition metals (LTM), rare earth metals (RE), and nonmetals(NM).

The simplified classification of common constituent elements ofBMGs and also the simplified classification of typical MGs based onthe binary prototypes are listed in Table 5.

3.2. Structure of monoatomic liquids

In 1960 Bernal considered the structure of monatomic metallicliquids to be dense random packing. Bernal, Scott and Finney arepioneers in solving the structure of metallic liquids work overthe intervening decades has emphasized that densest packing isa key factor governing the structure of metallic liquids and glasses.

The atoms in metals are approximated as hard spheres. Theproblem was how to pack the 3-D space with identical hardspheres as densely as possible, without introducing crystallineorder.

In [9], Chen and Ma write ‘‘This is Bernal’s original idea of denserandom packing (DRP) of hard spheres (DRPHS). Obviously, in aDRPHS, there should not exist a hole (empty space) that is suffi-ciently large to accommodate one more identical sphere without

Fig. 3. Fluctuation electron microscopy (FEM). (a) A sketch showing how FEM works in detecting medium-range structural correlation (reproduced with permission � 2005by Institute of Physics). (b) Imaging modes for the FEM (reproduced from [13]). (c) Cartoon picture showing how V(k, Q) is sensitive to medium range order (reproduced withpermission from [12] � 2010 Elsevier).

Table 5Simplified classification of common constituent elements of BMGs and of typical MGs based on the binary prototypes (adapted and processed after [9]).

Abbreviation Description Examples

AM Alkaline earth metals group IA and II A metals Mg, Ca, BeSM Semi- or simple metals, metals in groups IIIA and IVA Al, GaTM, ETM Transition metals (TM), including early

transition metals (ETM) – metals in IVB to VIIB groupsTi, Zr, Hf, Nb, Ta, Cr, Mo, Mn

LTM Late transition metals, metals in groups VIIIB, IB, IIB Fe, Co, Ni, Cu, Pd, Pt, Ag, Au, ZnRE Sc, Y, lanthanides (Ln) Sc, Y, La, Ce, Nd, GdNM Nonmetals and metalloids B, C, P, Si, Ge

Prototype Base metal Examples

Metallic glasses on binary prototypesLTM + NM LTM Ni–P, Pd–Si, Au–Si–Ge, Pd–Ni–Cu–P, Fe–Cr–Mo–P–C–BETM + LTM ETM/LTM Zr–Cu, Zr–Ni, Ti–Ni, Zr–Cu–Ni–Al, Zr–Ti–Cu–Ni–BeSM + RE SM/RE Al–La, Ce–Al, Al–La–Ni–Co, La–(Al/Ga)–Cu–NiAM + LTM AM Mg–Cu, Ca–Mg–Zn, Ca–Mg–Cu

Fig. 4. Bernal canonical holes (reproduced with permission from [9] � Elsevier).


Fig. 5. The structure model for Al-rich BMGs (reproduced with permission from [9] � Elsevier).

Fig. 6. Typicall 3-D configurations of the (a) Cu46Zr54 and (b) Cu46Zr47Al7 BMGs (reproduced with permission from [9] � 2011 Elsevier).


adjusting its neighbors. Given this, Bernal proposed that five typesof holes with edges of equal length (i.e., equilateral triangle faces)are likely the basic structural units of monatomic liquids’’ (quotewith permission � 2010 Elsevier).

In Fig. 4 are presented the Bernal’ canonical holes. In each panel,the left figure shows the hard sphere packing surrounding the hole,and the right figure shows the hole in the center (the radius of thepink sphere corresponds to the size of the hole).

3.3. Structure of Al-rich metallic glasses

The structure of Al-rich BMGs has been studied using bothexperiments and computer simulations. Al90FexCe10�x was studiedby using XRD techniques. Also, ab initio MD was to simulatethe structure of Al-rich MGs. In Fig. 5 is presented the structuremodel for Al-rich BMGs (reproduced with permission from [9] �Elsevier).

3.4. Structure of RE-rich metallic glasses

The structure of RE-rich MGs has not been systematically stud-ied. Nevertheless, an interesting observation is the polyamorphismfound in some Ce-based MGs. Sheng et al. by using in situ XRD, firstreported an amorphous-to-amorphous phase transition in Ce55Al45

MG under hydrostatic pressure. The feature of this transition is thesignificant increase of density with compression, and a hysteresisloop in the loading–unloading volume–pressure plot [9,14].

3.5. Structure of ETM–LTM metallic glasses

ETM–LTM are considered the most usual metallic glasses, manyETM–LTM-based compositions can be made into BMGs of rela-tively large sizes, with properties that are currently being inten-sively studied. The most popular metallic glasses from thiscategory are: Cu–Zr binary metallic glasses, Cu–Zr–Al metallicglasses, Cu–Zr–Ag metallic glasses, (Ti,Zr,Nb,Hf,Ta)–(Ni,Cu,Pd,Ag)–(Al)–(Be) metallic glasses.

Typically 3-D configurations of the Cu46Zr54 and Cu46Zr47Al7

BMGs samples obtained in MD simulation are presented in Fig. 6(reproduced with permission from [9] � 2011 Elsevier).

In Fig. 7 is observed that the population and degree of connec-tivity are obviously higher in (b) Cu46Zr47Al7 than in (a) Cu46Zr54.

3.6. Structure of AM–LTM metallic glasses

Metallic glasses from this category (Mg–Cu, Ca–Mg–Zn, Ca–Mg–Cu) exhibits some unique properties. For example, Mg–Ca–Zn isbiodegradable and was suggested for bio-medical applications.Unfortunately, the structure of these BMGs has not been systemat-ically studied.

3.7. Structure of LTM + NM metallic glasses

This glasses are from category of binary prototype (Ni–P, Pd–Si,Au–Si–Ge, Pd–Ni–Cu–P, Fe–Cr–Mo–P–C–B).

Fig. 7. The population and degree of connectivity in (a) Cu46Zr54 and (b) Cu46Zr47Al7

(reproduced with permission from [9] � 2011 Elsevier).

Fig. 8. Structure of Ni81B19 metallic glass obtained from an ab initio MD simulation(adapted with permission from [12]� 2009 Elsevier).

Fig. 9. The spatial distribution of nickel atoms and interconnected niobiumisosurface (reproduced and adapted with permission from [14] � 2007 NaturePublishing Group).

Fig. 10. The spatial distribution of yttrium atoms (reproduced and adapted withpermission from [14] � 2007 Nature Publishing Group).


Palladium based metallic glasses, owing to their superior dam-age-tolerance capacity can be thought of as a superior alternativeto conventional palladium dental alloys. A typical simulated struc-ture is presented in Fig. 8. The chemical ordering is such that soluteboron atoms are fully coordinated by solvent nickel atoms and donot make contact with each other [12].

3.8. Direct observations of structural aspects of metallic glasses

Shariq and Mattern, in [15] make a microstructural character-ization of Ni66Nb17Y17 metallic glass ribbon by using atom probetomography. Nickel, niobium and yttrium with purities of 99.9%were (multiple times) arc-melted in a Ti-gettered argon atmo-sphere and pre-alloyed with a nominal composition of Ni66N-b17Y17. For ensure the homogeneity, the samples were remeltedseveral times. The glassy state of the ribbon was proven by X-raydiffraction (XRD) and transmission electron microscopy (TEM).Fabricated samples were analyzed for different experimental con-ditions both in pulsed voltage and pulsed laser mode. In Fig. 9 thespatial distribution of nickel atoms and interconnected niobiumisosurface (17 at.%) is presented in orange for Ni66Nb17Y17 as spunribbon.

In Fig. 10 appears the spatial distribution of yttrium atomsshowing phase separated yttrium- enriched and yttrium-depletedregions for Ni66Nb17Y17 as spun ribbon.

The authors concluded that voltage pulsed atom probe analysisprovides misleading composition of the sample due to preferentialevaporation of yttrium, which has relatively low evaporation field.However, proper laser pulsed evaporation conditions need to beselected to avoid preferential evaporation of Y for reasonably cor-rect quantitative analyses for this system. In letter [10] to NatureMaterials, Hirata and colleagues, reports local atomic configura-tions of a metallic glass investigated by nanobeam electron diffrac-tion combined with ab initio MD simulation. This study providescompelling evidence of the local atomic order in the disorderedmaterial and has important implications in understanding theatomic mechanisms of metallic-glass formation and properties.Nanobeam electron diffraction (NBED) with a coherent electronbeam smaller than 1 nm in diameter enables authors of this studyto acquire two-dimensional diffraction patterns from a nanoscaleregion to detect local atomic structure. The schematic method usedin [10] is given in Fig. 11.

The structure of glassy Zr66.7Ni33.3 using ab initio MD simulation(using the Vienna ab initio simulation package) was first

Fig. 11. The schematic method used in NBED (reproduced with permission from [10] Copyright � 2010, Nature Publishing Group).

Fig. 12. Simulated atomic configuration of glassy Zr66.7Ni33.3 (extracted andreproduced with permission from [10] Copyright � 2010, Nature Publishing Group).


investigated. Fig. 12 shows the simulated atomic configurationwith 198 atoms (132 Zr and 66 Ni).

Fig. 13 is an example showing the characterization of two inter-connected polyhedra (a). By analyzing the diffraction vectors, twosets of diffraction patterns from polyhedra can be identified fromthe NBED image (b), which are fully consistent with the simulatedNBED patterns of each polyhedron (d and e). On the basis of the geo-metric correlation of the two polyhedra revealed by the diffraction,the most probable configuration of the two clusters can be deter-mined in the form of a face-sharing assembly, as shown in (a0).

(Beam diameter: 0.72 nm). (a) Two face-sharing (polyhedrawith a common on-axis orientation for Bragg diffraction. (a) The

super-cluster viewed from a direction showing the face-sharingconfiguration. (b) Experimental NBED pattern including two setsof possible rectangle diffraction patterns. (c) Simulated NBED pat-tern obtained from the super-cluster shown in a with the on-axiselectron incidence. (d) Simulated NBED pattern obtained only fromcluster A in the super-cluster. (e) Simulated NBED pattern onlyfrom cluster B (explanations reproduced with permission from[10] Copyright � 2010, Nature Publishing Group).

Finally, in their study [10], Hirata et al. concluded: ‘‘By employ-ing a state-of-the-art electron nanoprobe combined with ab initiomolecular dynamics simulation, we have found that subnanoscaleordered regions can give distinctly symmetric NBED patterns thatoriginate from individual and interconnected atomic polyhedra.Our observation offers compelling evidence of the local atomic or-der in the disordered metallic glass, which is consistent with therecent cluster models and previous predictions that the metallicglasses possess short range order (SRO) and MRO as opposed tothe long-range periodicity of a crystalline solid. This study providesan important approach to determine the local atomic structure ofdisordered materials, which may pave a new way to exploringthe atomic mechanisms for the formation and properties of metal-lic glasses.’’ (text reproduced with permission from [10] � 2010,Nature Publishing Group).

In [16] the structure of the glass system, Ca60Mg40�xCux, isexamined by using direct fitting of total correlation functions,T(r), measured using both neutron and X-ray diffraction. The reli-ability of the coordination numbers extracted is assured by com-parison the obtained data with partial correlation functions, tu0(r),calculated using the Percus–Yevick (PY) approximation for a binaryhard sphere system – Fig. 14 (reproduced with permission from[16] � 2011, Elsevier).

The results concerning metallic and covalent radii show that itwould be advantageous to have a theoretical solution of the P–Yequation for a binary or ternary hard sphere system in which the

Fig. 13. Characterization of medium range order (MRO) by NBED (reproduced with permission from [10] Copyright � 2010, Nature Publishing Group).

Fig. 14. A comparison the experimental T(r) for Ca60Mg15Cu25 with the Cu–Cu, Cu–Ca and Ca–Ca contributions simulated according to the Ca2Cu crystal structure(reproduced with permission from [16] � 2011, Elsevier).


interatomic distances for the hard sphere cutoffs of the potentialsare not additive [16].

Fig. 15. Quasicrystals (adapted from http:

3.9. Metallic glasses and quasicrystals

Quasicrystals are ordered structures, predictable but not peri-odic crystals, according to the classical crystallographic restrictiontheorem, can possess only 2, 3, 4, and 6-fold rotational symmetries.The Bragg diffraction pattern of quasicrystals shows sharp peakswith other symmetry orders, for instance 5-fold. The quasicrystalshistory begins in 1984, when D. Shechtman et al. in paper ‘‘MetallicPhase with Long-Range Orientational Order and No TranslationalSymmetry’’ demonstrated a clear diffraction pattern with a 5-foldsymmetry. Since Shechtman’s discovery (1984) hundreds of quasi-cristals were reported in aluminum alloys (Al–Li–Cu, Al–Mn–Si,Al–Ni–Co, Al–Pd–Mn, Al–Cu–Fe, Al–Cu–V) and in other composi-tions (Cd–Yb, Ti–Zr–Ni, Zn–Mg–Ho, Zn–Mg–Sc, In–Ag–Yb, Pd–U–Si.

There are two types of quasicrystals: (a) polygonal (dihedral)quasicrystals, have an axis of 8, 10, or 12-fold local symmetry(octagonal, decagonal, or dodecagonal quasicrystals, respectively).They are periodic along this axis and quasiperiodic in planes nor-mal to it. The second type, icosahedral quasicrystals, are aperiodicin all directions. Regarding thermal stability, there are three typesof quasicrystals: (1) stable quasicrystals grown by slow cooling orcasting with subsequent annealing, (2) metastable quasicrystals

//en.wikipedia.org/wiki/Quasicrystal).

http://www.en.wikipedia.org/wiki/Quasicrystal

Table 6Brief summary of structural models for MGs (processed and adapted from [9]).

Structural model Symbol Descriptors

Densest packing with identical hardspheres

DRPHS Starting from a single sphere, Bernal’s holes including the tetrahedron, essentially a frustrated polytetrahedralpacking

Polytetrahedral packing model PPM Triangulated coordination shells surrounding the center atom with nearly equal edges. The polytetrahedral modeldescribes the SRO only

Stereochemical model (proposed byGaskell)

SM The nearest-neighbor interaction is the strongest of all, the SRO in MGs is the same as that in the correspondingcrystals, while the differences lie in the MRO (Gaskell). Available for a particularly a group of LTM–NM MGs

Efficient cluster packing (recentlyproposed by Miracle)

ECP To achieve stable solute-centered clusters by selecting the size ratio R between center solute and surroundingsolvent and how these clusters are connected and arranged in medium range

Fig. 16. Engineering aspects of GFA. Fig. 17. Schematized diagram TTT (picture adapted after [17] � 2011 WILEY-VCHVerlag, GmbH and Co.).


prepared by melt-spinning, and (3) metastable quasicrystalsformed by the crystallization of the amorphous phase.

In Fig. 15 are presented a atomic model of an Ag–Al quasicrystaland a A Ho–Mg–Zn icosahedral quasicrystal formed as a dodecahe-dron, the dual of the icosahedron.

The 5-fold environment is quite common in liquids and BMGsand some structural similarities between quasicrystals an metallicglasses certainly exists. The icosahedral quasicrystals werefounded in many metallic glass alloys as Cu-rich alloys and Cu–Zr. A summary of structural models for MGs is presented in Table 6.

4. Properties of metallic glasses

The central theme in materials researches is the relationship be-tween the materials structure and materials properties. The struc-ture of materials influences their properties in multiple and verydifferent ways. At amorphous materials, particularly at MGs, thestructure–property correlation is very difficult to study. The MGsstructures are far from being completely understood and eluci-dated and are very difficult to describe and quantify (see Section2). As a result, the predictions about how the atomic structureinfluence the macroscopic properties of MGs is difficult to do.

4.1. Kinetics and the glass forming ability (GFA)

Glass forming ability (GFA) is a influential factor in studying theformation of BMGs. There is no standard definition for this param-eter up to now, and many indicators have been developed and pro-posed. From the engineering aspect, the lower the critical coolingrate and the larger the critical thickness are, the higher the glassforming ability of a metallic glass will be as in scheme from Fig. 16.

The difficulty is to measure with accuracy the critical coolingrate. The critical thickness depends on processing parameters.But, characterizing the glass forming ability with measurable andreproducible parameters is of major importance in designing, fab-ricating and processing metallic glasses. A parameter used in char-acterizing the GFA is the reduced glass transition temperature Trg,

defined as a report between glass transition temperature Tg and li-quid state temperature Tl(Trg = Tg/Tl). The metallic glasses with thehigh GFA are considered to have a Trg in the range of 0.66–0.69.

Packing in BMGs is very dense, with a low content of free vol-ume resulting in viscosities that are several orders of magnitudehigher than in pure metal melts. The dense packing accomplishedby structural and chemical atomic ordering also brings the BMG-forming liquid energetically and entropically closer to its corre-sponding crystalline state. These factors lead to slow crystallizationkinetics and consequentially to high GFA.

A schematic time–temperature-transformation (TTT) diagram ispresented in Fig. 17. To avoid crystallization and for the glassystructure forming, is necessary that the cooling to do after curve1 (a great decreasing of temperature in a short time).

All metallic-glass-forming liquids show an excess specific heatat the liquidus temperature that increases on cooling until theglass-transition temperature is reached. This reflects ordering inthe liquid limits the increase in the thermodynamic driving forcefor crystallization and can yield a glass with an entropy barelyhigher than that of the crystal, as in Fig. 18.

The fraction of Cu-centered clusters within which the coordina-tion is icosahedral for three compositions of Cu–Zr glass simulatedusing molecular dynamics. The fraction is appreciable at theliquidus temperature T1 and rises on cooling towards the glass-transition temperature Tg.

In [18] the theoretical analysis shows that crystallizationresistance is in proportion to the viscosity of ‘‘nose’’ temperature(Tn) while crystallization driving force is inversely proportional tothe viscosity of crystallization onset temperature (Tx) on reheatingin time–temperature-transformation (TTT) curve, and therefore aGFA parameter x0, defined as (Tg � T0)/(Tx � T0) � (Tg � T0)/(Tn � T0), was proposed (wherein Tg and T0 are glass transition tem-perature and Vogel temperature respectively). The parameter x0

shows an excellent correlation with the critical cooling rate forglass formation of BMGs.

In [19], four types of criteria for the glass-forming ability arecategorized and reviewed: Indicators with characteristic

Fig. 18. The fraction of Cu-centered clusters (reproduced with permission from [12]� 2009 Elsevier).

Table 7The GFA influential temperatures of some representative BMGs (adapted andextracted from [22]).

BMGs Tg (K) Tx (K) Tm (K) Trg

Mg80Ni10Nd10 454.2 477.7 725.8 0.63Zr41.2Ti13.8Cu12.5Ni10Be22.5 623.0 705.0 932.0 0.67Zr53Ti5Cu16Ni10Al16 697 793 1118 0.62Zr66Al8Ni26 672.0 707.6 1188.5 0.57Pd40Ni40P20 590.0 671.0 877.3 0.67Cu60Zr30Ti10 713.0 763.0 1110.0 0.64La66Al14Cu20 395.0 449.0 681.9 0.58Nd60Al10Cu10Fe20 485.0 610.0 773.0 0.63Ti50Ni24Cu20B1Si2Sn3 726.0 800.0 1230.0 0.59Au77.8Si8.4Ge13.8 293.0 293.0 606.0 0.48

Tg – glass transition temperature.Tx – onset temperature of crystallization.Tm – onset melting temperature (point).Trg – reduced glass transition temperature-representing the glass-forming abilityGFA.


temperatures; Indicators involving structural factors; indicatorsbased on Miedema’s model; indictors based on phase diagram.The conclusion is that ‘‘is a still long way to develop proper indica-tors for GFA, which are theoretically strict and composed of verysimple and fundamental parameters’’.

In [20], Qin et al. investigated the effects of alloy additions onthe GFA and mechanical properties of a typical glass-forming com-position, Mg65Cu25Tb10, which approaches the ideal brittle behav-ior associated with silicate glasses. Authors selected Ag, Zn and Be(including their combinations) as alloying elements. The atomic ra-dius of these elements is quite different from that of Mg, Cu or Tb.The conclusions are that the small amount of Ag, Zn and Be addi-tions not only improve the GFA, but also enhance the strengthand plasticity of Mg–Cu–Tb based BMGs. Authors prepared amor-phous Mg65Cu20Ag5Tb10 rods with a diameter of 10 mm by coppermold casting. The increased GFA is a result (by authors opinion) ofimproved atomic packing efficiency and decreasing Gibbs free en-ergy difference between the liquid and crystal phases when appro-priate amounts of Ag, Zn and Be are added into the material.

In the research paper [21] authors added a small amount of Mnto a ternary Gd55Ni25Al20 good glass forming alloy, as a replace-ment for Ni, and a Gd55Ni22Mn3Al20 bulk metallic glass was ob-tained by suction casting. Its glass forming ability wascharacterized by X-ray diffraction and differential scanning calo-rimetry. A new quaternary BMG (Gd55Ni22Mn3Al20) with excellentGFA and magnetocaloric effect was synthesized.

In Table 7 (adapted and extracted from [22]) are presentedsome representative BMG systems with their glass transition

temperature, Tg, temperature of crystallization, Tx, melting point,Tm, and glass-forming ability represented by reduced glasstransition temperature, Trg.

In [23], Suo and colleagues proposed a new parameter to eval-uate the glass-forming ability of BMGs. This new criteria would af-fect metallic glass alloy development and modeling. It is proposedbased on the consideration of the liquid phase stability, the resis-tance to crystallization and the glass transition enthalpy. Thethermodynamics and energy are adequately integrated whenthe glass-forming alloys solidified. It is shown schematically inFig. 19 (adapted with permission from [23] � 2010 Elsevier).

Melting enthalpy (DH) is an important parameter for influenc-ing the Gibbs free energy difference (DG) between undercooled li-quid and crystal state. DG has played an important role inpredicting the glass forming ability of metallic alloys. The less isthe DG, the smaller is the force of crystallization and the easieris the formation of BMGs. The relation between DG and DH was ex-pressed by Turnbull as DG = DH (Tm � T/Tm) where Tm is the start-ing melting temperature of alloys. Elg – glass transition enthalpydefined by DE/DH is close to 1. From the energetic point of view,the parameter Elg is fit for describing GFA of different alloy systems.The new dimensionless criterion for evaluating the GFA of an alloyis defined by Q = [(Tg + Tx)/Tl] � DE/DH. The reliability and benefitsof the new criterion with respect to other parameters (Tx, Trg and c)ave been demonstrated in different glass-forming alloy systems[23].

4.2. Mechanical properties and performances of BMGs

The mechanical properties of bulk metallic glasses (their supe-rior strength and hardness, corrosion and wear resistance, com-bined with their general inability to undergo homogeneousplastic deformation have been a subject worthy of investigationsfor scientists and engineers in past 50 years. All studies summa-rizes that BMGs have much higher tensile strengths and much low-er Young’s moduli. The difference in these values between the BMGand crystalline alloys is as large as 60%. The significant difference inthe mechanical properties is thought to be a reflection of the differ-ence in the deformation and fracture mechanisms between BMGsand crystalline alloys. Plastic deformation in metallic glasses isgenerally associated with inhomogeneous flow in highly localizedshear bands. In their large review paper about mechanical proper-ties of BMGs [24], Trexler and Thadhani says (quote): ‘‘The scien-tific interest stems from the unconventional deformation andfailure initiation mechanisms in this class of materials in whichthe typical carriers of plastic flow (dislocations) are absent.


Metallic glasses undergo highly localized, heterogeneous deforma-tion by formation of shear bands, a particular mode of deformationof interest for certain applications, but which also causes them tofail catastrophically due to uninhibited shear band propagation.Varying degrees of brittle and plastic failure creating intricatefracture patterns are observed in metallic glasses, quite differentfrom those observed in crystalline solids. The tension–compressionanisotropy, strain-rate sensitivity, thermal stability, stress-inducedcrystallization and polyamorphism transformations, are some ofthe attributes that have sparked engineering studies on bulkmetallic glasses. Understanding of the glass-forming ability andthe deformation and failure mechanisms of bulk metallic glasses,has given insight into alloy compositions and intrinsically-formingor extrinsically-added reinforcement phases for creating compositestructures, to attain the combination of high strength, tensileductility, and fracture toughness needed for use in advancedstructural applications. The relative ease of fabricating metallicglasses into bulk forms, combined with their unique mechanical

Fig. 19. A new approach for understanding GFA of amorphous ma

Fig. 20. Elastic limit (yield stress) ry plotted against the Young modulus E for 1507permission from [5] � 2010 Elsevier).

properties, has made these materials attractive options for possibleapplications in aerospace, naval, sports equipment, luxury goods,armor and anti-armor systems, electronic packaging, and bio-medicaldevices’’ (excerpt with permission from [24] � 2010 Elsevier).

4.2.1. Elasticity of BMGsMetallic glasses clearly cannot have the crystallographic de-

fined slip-systems of polycrystalline metals. In the absence of dis-location-mediated slip, they show high yield stresses, much closerto the theoretical limit than their crystalline counterparts – seeFig. 20 (reproduced with permission from [5] � 2010 Elsevier).

4.2.2. ToughnessFracture toughness is a measure of the load-bearing capacity of

a material before fracture, and is a critical property determiningthe overall mechanical performance of a mechanic piece. Amor-phous metals therefore exhibit toughness values that vary fromas low as the values characteristic of brittle ceramics to as high

terials (adapted with permission from [23] � 2010 Elsevier).

metals, alloys, metal–matrix composites, and metallic glasses (reproduced with

Fig. 21. Fracture toughness vs Young modulus for different materials (reproducedwith permission from [25] � 2008, Nature Publishing Group).


as the values characteristic of engineering metals. An Ashby plotfor materials selection exhibit a large range of common engineer-ing materials along with selected metallic glass ribbons and BMGs,as it can see in Fig. 21 (reproduced with permission from [25] �2008, Nature Publishing Group).

A typical fracture of a BMG specimen (commercial Vitreloy 1) isshown in Fig. 22 (adapted with permission from [25] � 2008, Nat-ure Publishing Group).

In Fig. 23 are illustrated the fracture characteristics of this BMGupon compression, testing at a strain rate of 10�4 s�1. The macro-scopic view of the fractured cylinder (a) shows two distinct sur-faces (regions A and B) oriented at 44� and 50�, respectively. Thestress–strain curve (c) illustrates elastic deformation followed by�0.5% plastic strain. The top view of the fractured sample withthe dashed line marking the boundary between regions A (flat sur-face revealing typical shear fracture) and B (randomly distributedflat and mirror regions) is shown in (b). The combination of thetwo types of fracture surfaces are illustrative of the complex stressstate experienced by the BMG, and resulting bifurcation in fractureprocess preceding the final catastrophic failure. The authors spec-ulate that the BMG rod fails under compression initially by a slowshear (mode II) crack, which then transforms to fast tensile (mode I)

Fig. 22. Brittle fracture in a monolithic BMG (a) and stress vs strain curves for a BMG andPublishing Group).

at the final stage of failure. This was supported by observance oftypical vein patterns, shown in (d) typical of shear fracture, as wellas dimples at the 100-nm scale, shown in (e) [24].

Another comparative and suggestive chart of fracture toughnessas a function of yield strength is given in Fig. 24.

4.2.3. HardnessIs the mechanical property that mostly influences the wear

resistance capabilities of a material. Since hardness is understoodto be a measure of flow stress, it correlates linearly with the mate-rial yield strength. The Vickers hardness plotted from differentmaterials (Fig. 25, adapted from [27]) shows that the amorphousmetals demonstrate an advantage over crystalline metals in termsof hardness.

4.2.4. Fatigue endurance of BMGsFatigue endurance is the very important property that dictates

the overall service life of machine pieces. Demetriou et al. in[26], revealed that the fatigue endurance is ‘‘the single most impor-tant property that dictates the overall service life of a load-bearingimplant’’. It is estimated that the average non-active person im-poses several millions of cycles of stress on the hip joint per year.Nineteen over a period of 20–30 years, this rate corresponds toapproximately 108 loading cycles. Therefore, the resistance of amaterial to cyclic loading is critical in ensuring longevity of thedevice.

The fatigue performances of BMGs has not been extensivelyinvestigated. Some investigations were performed on Zr-basedglasses. The fatigue endurance limit for this class of amorphousmetals against the corresponding yield strengths is plotted inFig. 26.

In conclusion, BMGs are a subject of interest due to their supe-rior specific strength, large ductility in bending, low coefficient offriction, high hardness, high resistance to corrosion, oxidationand wear. These properties are accompanied by their inability toundergo homogeneous plastic deformation due to the absence ofdislocation-mediated crystallographic slip. Metallic glasses deformby shear banding, a particular mode of deformation of interest forcertain applications, but which also causes them to be quite brittleand fail catastrophically due to uninhibited propagation of thebands. A lot of studies investigated the shear band formation, shearband directions, shear band toughness of BMGs, etc.

three glass matrix composites (adapted with permission from [25] � 2008, Nature

Fig. 23. Fracture characteristics observed in a Vitreloy 1 BMG following compression testing at 10�4 s�1 (adapted with permission from [24] � 2010 Elsevier).

Fig. 24. Comparative chart of fracture toughness vs yield strength for BMGs andother metallic materials (adapted from [26] � 2010 JOM).

Fig. 25. Vickers hardness for different materials (adapted from [27] � 2011Elsevier).


4.2.5. Discussion about the shear bands in BMGsMotto: ‘‘TEM shows that the shear is sharply localized; thick-

ness of shear band is 10–20 nm. The origins of localization remaincontroversial: structural change, or temperature rise?’’ (A.L. Greer,Materials on the Horizon, Cambridge, 9 December 2008).

In the research paper [27], Gao and colleagues shows that it isinappropriate to relate the angle between the loading axis andthe shear-band (or fracture) plane in metallic glasses under uniaxial

loading conditions to the coefficient of internal friction in theMohr–Coulomb model. Shear bands in metallic glasses are a resultof material instability (which can be predicted from constitutiveparameters and loading conditions), which does not correspondto the material yield condition. Specifically, the shear-band direc-tions depend on the Poisson’s ratio, the ratios of three deviatoryprincipal stresses to the von Misses stress, the coefficient of inter-nal friction, and the dilatancy factor. The last parameter describes

Fig. 26. Fatigue endurance limit (stress-range based) vs yield strength data forBMGs and other metallic alloys (adapted from [26] � 2010 JOM).

Fig. 28. Shear bands produced by bending (adapted with permission from [28] �2011 Elsevier).


whether the plastic flow is associative or non-associative. By usingthe elastic contact solutions and the Rudnicki–Rice model, theauthors identify three (two) regimes under the two-dimensionalcylindrical (three-dimensional spherical) contact where differentshear-band directions may occur. When using a bonded-interfacetechnique to visualize shear bands under three-dimensional con-tacts, it should be noted that the stress component normal to thebonded interface is released, resulting in the commonly observedsemicircular shear bands whose directions are predicted to followthe larger in-plane principal stress. In Fig. 27 the bonded-interfacetechnique was used to reveal shear-band patterns underneath thespherical indentation [27].

In [28], nanoindentation experiments were conducted on Zr-based metallic glass samples, which were elastically and plasticallybent in order to investigate the effect of residual stresses on hard-ness. It was found that tensile residual stress reduced the hardnesssignificantly, while compressive residual stress produced only asmall effect on the hardness. In this paper is presented a detailedstress analysis based on yield asymmetry under tension andcompression to describe the distribution of residual stresses inbent metallic glass specimens. The calculations agree well withthe hardness variations measured experimentally. In Fig. 28 [28],shear bands produced by bending a Zr52.5Al10Ti5Cu17.9Ni14.6 plate(0.6 mm/3 mm/15.3 mm) exhibit asymmetric directions on thetension and compression sides.

Fig. 27. Radial and semicircular shear bands in Zr52.5Al10Ti5Cu17.9

Jiang and Dai, in [29] performed dynamic ‘‘forced’’ shear andquasi-static tensile tests on a typical Vitreloy 1 bulk metallic glass(BMG). A theoretical model that takes into account momentum,energy and free-volume balance is developed to quantitatively de-scribe the dissipation system of shear-band propagation in the lo-cal plastic region.

The authors obtained analytical expressions for shear-localizationtime, shear-band thickness and critical energy dissipated withinthe band. Calculations demonstrate that the shear-band propaga-tion process is dominated by the free-volume softening, ratherthan the thermal softening. The latter increases the band thicknessslightly but decreases the corresponding critical dissipation energysomewhat. The concept of shear-band toughness is further intro-duced to measure the inherent resistance capability of materialsto the propagation of shear bands in BMGs. These results assistin more comprehensively understanding the evolution mechanismof the shear bands, and in guiding alloy design to enhance thetoughness of BMGs [29].

The schematic of forced shear test is presented in Fig. 29 (from[29] � 2011 Elsevier).

Based on the critical energy dissipated within the shear band,the shear-band toughness is proposed by [29] to quantitativelymeasure the susceptibility of the shear band to catastrophic frac-ture in BMGs. The schematic diagram of shear band initiating in lo-cal plastic regions in a BMG subjected to external loading isproposed in Fig. 30.

Ni14.6 (adapted with permission from [27] � 2011 Elsevier).

Fig. 29. The schematic of forced shear test of hat-shaped specimen ([29] � 2011 Elsevier).

Fig. 30. The schematic diagram of shear band initiating and fracture features of Vitreloy 1 BMG under quasi-static tension (reproduced with permission from [29] � 2011Elsevier). (a) Schematic diagram of shear band initiating in local plastic regions in a BMG subjected to external loading. (b) Side view of the fractured specimen. (c) SEM imageshowing a top view of the main fracture surface. (d) Details corresponding to the area circled in (c) showing a nanoscale shear band followed by a crack.


In [30] six MG alloys: Zr55Cu23Ni5Al10Nb2, Zr55Cu30Ni5Al10,Zr50Cu37Al10Pd3, Cu46Zr44.25Al7.5Er2, Mg58Cu31Nd5Y6 and (Fe44.3Cr5-

Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5 – in at.% – were compressed toextensive plastic deformation without sudden fracture in themicrocompression tests. Based on the theoretical framework a cor-relation between the plastic energy dissipation and the sample sizeeffect is established, which can be then utilized to explore theinfluence of the alloy’s chemical compositions on the behavior ofshear-induced material softening. In Fig. 31 (reproduced from[30]) are presented the micrographs showing the typical morphol-ogies of the shear banding mediated plasticity in the BMGsmicropillars.

Takeuchi and Edagawa provides a large study [31] about sheardeformation in MGs. In this article, author review atomistic simu-lation studies of deformation processes in metallic glasses, i.e., localshear transformation (LST), structural characterization of the localshear transformation zones (STZs), deformation-induced softening,shear band formation and its development, by use of elementaland metal–metal alloy models. Authors also review representativemicroscopic models so far proposed for the deformation mecha-nism: early dislocation model, Spaepen’s free-volume model,Argons’s STZ model and recent two-state STZ models by Langeret al. The authors revealed that: ‘‘Two pioneering deformationmodels were proposed in the late 1970s, which have survived till


today; they provide basic and important concepts in deformationmechanism of metallic glasses. One of the models is the free-volume model by Spaepen, and the other is the shear transformationzone (STZ) model by Argon. These two models are based on theassumption that the fundamental unit process of deformation isa local rearrangement of atoms which accommodates the localshear strain’’ (excerpt from [31]). These processes are schemati-cally represented in Fig. 32. The group of atoms change its config-uration under a shear stress from one relatively low energyconfiguration to a next such configuration by thermal activation.

Because the STZ disappears after larger strain. Langer et al. havedeveloped a ‘‘two-state STZ model’’, which is based on the previoustwo models. ‘‘The basic assumption in this model is similar to theSpaepen’s and Argon’s models: it is assumed that the fundamentalunit process of deformation is a local rearrangement of atoms,which is essentially the same as an STZ operation. The major revi-sion in this model for the earlier models lies in the assumption thatthe STZs are intrinsically two-state systems. In response to an ap-plied stress, they may transform from one orientation to another,but they cannot transform further in the same direction. Here, theymay transform back to the original orientation when a stress is ap-plied in the reverse direction’’ (excerpt with permission from [31]� 2011 Elsevier). Also in [31] Takeuchi and Edagawa illustrated thedislocation model. They have studied the stability of edge and screw

Fig. 31. Typical morphologies of the shear banding mediated plasticity in the BMGsMg58Cu31Nd5Y6, (e) (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5, and (f) Cu46Zr44.25Al7.5Er2

dislocations in a model amorphous structure of a Ni33Y67 alloy. Fig33 shows (a) the atomic displacement field after the initial intro-duction of an edge dislocation and (b) shows the displacement fieldafter relaxation. The displacement of each atom with respect to theatom position before the introduction of the dislocation isindicated by an arrow, where the length of the arrow is three timeslarger than the actual atomic displacement. The magnitude of thex-component of the displacement is shown in the color scale.

On Greer’s opinion [32] there are two approaches for improvingthe mechanical properties of BMGs (in Table 8).

5. Design and fabrication of BMGs

5.1. Design and material selection principles

In Section 1, about the history of BMGs, is briefly described thebasic principle of BMGs obtaining: extremely rapidly cooling ofmelted glass-forming alloy. The cooling rate necessary to avoidcrystallization is extremely great (106 K/s). An important conse-quence of this was that the first metallic glasses could only be pro-duced in a limited number of forms (typically ribbons, foils, orwires) in which one dimension was small so that heat could beextracted quickly enough to achieve the necessary cooling rate.Along the past 50 years, the great interest for BMGs of scientific

micropillars: (a) Zr50Cu37Al10Pd3, (b) Zr55Cu30Ni5Al10, (c) Zr55Cu23Ni5Al10Nb2, (d)(reproduced with permission from [30] � 2011 Elsevier).

Fig. 32. The models of Spaepen’s free-volume and the (STZ) model by Argon (adapted with permission from [31] � 2011 Elsevier).

Fig. 33. The displacement field after the initial introduction of an edge dislocation (a) and the displacement field after relaxation (b) (reproduced with permission from [31] �2011 Elsevier).

Table 8Approaches for improving BMGs’ properties (adapted after [32]).

Approach Aims, descriptors and possibilities

A.1. To make a glass metallic composite ) Encouraging results by introducing a ductile crystalline phase into the glassy matrix) Increases ductility) Lowers strength (but better compromise of properties)

A.2. To control the operation of shear bands within the glassy phase ) Aim to deflect shear bands) Prevent failure by one dominant band) Proliferation of bands makes deformation more diffuse and increases energy absorption) Also reduces shear offset at each shear band

Reduces cracking) Possibility of work-hardening?


Table 9The methods of component design of BMGs (adapted with excerpts from [33]).

Design method Description

1. Trial and error method It is primarily a main method. It is applied to design the component of the BMG on the basis of existing phasediagrams, thermodynamic parameters, physical and chemical parameters and the component of the developedBMGs. And then, the components are experimented one by one

2. Nearly-free-electron method The method, proposed by Nagel and Tauc [Nearly-free-electron approach to the theory of metallic alloys. Phys RevLett 1975;35(6):380–3.], treats the alloy as a nearly-free-electron gas and employs many of the concepts used inZiman’s theory of liquid metals to describe the system

3. Valence electron concentration method The method is to calculate chemical bond length, overlapping population and biding energy by using quantumchemistry on the basis of the cluster model, predicting the GFA and designing the component of metallic glasses

4. Discrete variational method The method is a molecular orbital method titled by numerical self-consistent field method based on the densityfunction theory. It is suitable for the macromolecules, macro-clusters and solid system, especially the system withheavy-atoms

5. Thermodynamic and dynamic methods The method is to establish the thermodynamic or dynamic model for the alloys and calculate the thermodynamicand dynamic parameters, predicting the GFA or designing the component of the alloy

6. Similitude principle and artificial neuralnetwork method (SPANNM)

This method is to model based on similitude principle and artificial neural network because of the similaritybetween all kinds of the metallic glasses. In addition, the artificial neural network can extract usable informationfrom large quantity of discrete data with noise and is usable for resolving the highly non-linear and uncertainproblems

Table 10Strategies for application selection and their implementation (adapted from [34]).

Strategy Descriptors Implementation

Search through function(STF)

Finding the most promising properties or performance indices forthe new material

Requires the computation of the performance indices of the newmaterial, and comparison with the performance indices derivedfrom available material-property data

Parasitical substitution (PS) Consists in finding the ‘‘classical materials’’ which are most ‘‘alike’’the new one, The strategy requires the definition of a distance inthe space of materials’ performance indices defined by theproperties of materials

Requires the definition of a ‘‘distance’’ between materials. Thepurpose is 2-fold: either to find the closest possible material to the‘‘new’’ one, or finding the materials which are definitely worsethan the ‘‘new’’ one and, thereby, potential victims for substitution

The metric in ‘‘materialsspace’’

Is derived from the (STF + PS) strategies. It consists in developing adatabase of generic applications, to identify from the currentlyused materials for these applications the relative importance of theproperties and performance indices, and then to define a distance,taking into account this weighting, and allowing assessment of thesuitability of the new material for the particular application

The information on ‘‘known solutions’’ for a given application arenecessary The implementation of the third strategy requires thedevelopment of an ‘‘Application Database’’ – currently in progress

Fig. 34. Selection map showing good applicability for BMGs in springs of smallvolume (reproduced with permission from [34] � 2005 Elsevier).


community make that new glass forming alloys and new techniquesfor fabrication to be remarkable developed. Generally, BMGs aremulti-component metallic alloy systems. As a result, the hugedifficulty for the component design of new multi-componentglass-forming metallic alloy system. In [33], Cai et al. summarizesthe development and application of the component designmethods for bulk metallic glasses. Is also proposed a new methodtitled as similitude principle and artificial neural network model(SPANNM) for the component design and glass forming ability ofbulk metallic glasses. The methods of component design of BMGsis briefly summarized in Table 9.

In [34], Salimon et al. revealed a systematic methodology forsearching for possible applications for BMGs (treated as new mate-rials). By starting to fact as any engineering component has one ormore functions (to support a load, to contain a pressure, to transmitheat, and so forth), the authors make an overview on methods formaterials and process selection. ‘‘Functions, objectives and con-straints define the boundary conditions for selecting a materialand – in the case of load-bearing components – a shape for itscross-section. The performance of the component, measured byperformance Pj, depends on control variables xi. The control vari-ables include the dimensions of the component, the mechanical,thermal and electrical loads it must carry, and the properties ofthe material from which it is made. Performance is described interms of the control variables by one or more objective functions.Each combination of function, objective and constraints leads toa performance metric containing a group of material propertiescalled a material index; the index is characteristic of the combina-tion. The point of interest here is that materials with high values ofcertain indices are well suited to meet the functional requirements

from which they were derived’’ (excerpt with permission from [34]� 2005 Elsevier). The strategies applicable for find possible appli-cations for a new material are presented in Table 10 (adapted from[34]).

The application of these three strategies for BMGs is largelydetailed by authors and example for applying the first strategy(STF) is given in Fig. 34, based on The Cambridge EngineeringSelector – CES software.

Fig. 35. The experimental scheme to obtain a metallic glass that is ordered at a largescale (adapted from [37] � Carnegie Inst.).


5.2. Fabrication and processing techniques of BMGs

The most recent method to obtain a metallic glass that is orderedat a large scale was reported in a study conducted by GeophysicalLaboratory at the Carnegie Institution for Science Zhejiang Univer-sity from China, Stanford University and SLAC. Ho-Kwang Maofrom Carnegie and other scientists are conducting research on

Fig. 36. (a) Optical microphotograph of micro-forged surface of Pt-based BMG; reflectansurface of Pt-based BMG with periodic intervals of (b–d) 800 nm and (e) 400 nm. (adap

Fig. 37. Imprinted BMG nano-pillars obtained by pressing the glassy alloy int

metallic glass produced from aluminum and cerium. The team, cre-ated a single crystal by applying 25 GPa of pressure (equivalent of1800 tons per square inch!) to the cerium–aluminum glass and thenew order formed is preserved even when the glass is restored toambient pressure [35–37]. In Fig. 35 is schematized the experi-ment in where a metallic glass ordered at large scale (LRO) wasobtained.

In Acta Materialia Gold Medal Lecture [38], Inoue and Takeuchireviews past developments and present understanding of theglass-forming ability, structure and physical, chemical, mechanicaland magnetic properties of bulk glassy alloys (BGA) with theemphasis on recent results obtained since 1990, together withapplications of BGA, achieved mainly in Tohoku University. Thepresent authors also reported that various nano-imprinted pat-terns can be produced using Newtonian flow. For example,Fig. 36 shows imprinted patterns with intervals of 800 nm and400 nm for Pt–Pd–Cu–P glassy alloys produced by die-forge press-ing in the supercooled liquid region against focus ion beam (FIB)-machined Zr–Al–Ni–Cu glassy alloy dies.

Imprinted glassy alloy nano-pillars with a diameter of 200 nmand length �5 lm, obtained by pressing the glassy alloy into por-ous alumina in the supercooled liquid region are presented inFig. 37. The nano-pillars were tested for commercialization as

ce at k = 533 nm. (b–d) SEM micrographs of die-forged periodically nanostructuredted with permission from [38] � 2011 Elsevier).

o porous alumina (adapted with permission from [38] � 2011 Elsevier).

Fig. 38. X⁄-element-based large centimeter-sized BMG (adapted with permission from [38] � 2010 Elsevier).

Fig. 39. The continuous casting setup (adapted with permission from [39] � 2011Elsevier).

Fig. 40. Glassy alloy rod (Zr48Cu36Ag8Al8) obtained by novel continuous castingmethod (adapted with permission from [39] � 2011 Elsevier).


anti-reflection material cell culture medium for bio-chips, andelectrode material [35].

Is revealed also how the dramatic increase in the thermal stabil-ity of supercooled liquid has enabled the production of large-scaleBGA with various outer shapes: massy ingot shape for a Pd–Cu–Ni–P system, cylindrical rods 25 mm in diameter for a Pd–Cu–Ni–Psystem, hollow pipes 10 mm in outer diameter and 1 mm thickfor Pd–Cu–Ni–P, Zr–Al–Ni–Cu and La–Al–Ni–Cu systems (seeFig. 38).

Zhang et al. in research paper [39], developed a new continuouscasting method for the massive production of bulk metallic glassingot with centimeter-scale diameter without length limitation.An intermittent withdrawal procedure was practiced for continu-ous casting of bulk glassy alloy. The new developed continuouscasting method can provide a cooling speed as high as that pro-vided by the stationary mold casting method. A Zr-based glassy al-loy rod with a diameter of about 10 mm and length of tens ofcentimeters was prepared by the continuous casting method forthe first time. The schematic illustration of the continuous castingsetup is shown in Fig. 39.

A thermal open-ended mould surrounded by a heater is con-nected to a water-cooled open-ended mould coaxially. A thermo-couple is placed at the exit end of the thermal mould. Thethermocouple and the heater are adopted to adjust the tempera-ture of the thermal mould. The glassy alloy melt is kept in a cruci-ble for continuous casting process. The crucible is connected to thethermal mould. A starting block is inserted into the thermal mouldfor initiation of casting. The starting block is moved by a roller de-vice which is not shown in Fig. 39. All the units are put into a stain-less steel vacuum chamber. At the beginning of the experiment, themelt is guided by the starting block passing through the thermalmould into the water-cooled mould and then cooled to form glassyalloy (or supercooled liquid) in the water-cooled mould. At last thealloy is gradually pulled out of the water-cooled mould by the roll-er device. An intermittent withdrawal procedure was practiced forthe continuous casting of BMG. A Zr48Cu36Ag8Al8 glassy alloy rod

with a diameter of 10 mm (see Fig. 40) was produced by the con-tinuous casting method [39].

In [40], Schroers and colleagues developed Pt57.5Cu14.7Ni5.3P22.5

bulk metallic glass (Pt–BMG) nanowires. The Pt–BMG nanowireshave high surface areas, thereby exposing more of the catalyst,and also maintain their activity longer than traditional fuel cell cat-alyst systems. After 1000 cycles, these nanowires maintained 96%of their performance – 2.4 times as much as conventional Pt/C cat-alysts. (see Fig. 41).

In [42] master alloys of TaNiCo were prepared by arc melting ofpure Ta, Ni and Co (>99.5%) under a purified argon atmosphere.These alloys were remelted at least four times to ensure the chem-ical homogeneity. Then, the alloy rods with different diameters

Fig. 41. The Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass (Pt–BMG) nanowires (reproduced from [40] available at [41]).

Fig. 42. Comparison of mechanical properties of Ta42Ni40Co18 alloy and other BMGs (adapted with permission from [42] � 2011 Elsevier).


were prepared by copper-mold suction casting and the amorphousribbons were prepared by the melt-spinning. The optimized Ta42-

Ni40Co18 BMG has the best glass forming ability, which can be castinto 2-mm glassy rods. The BMG exhibits very high thermal stabil-ity, high glass transition temperature and high crystallization tem-perature. The glass also has excellent mechanical properties suchas high micro-hardness, high Young’s modulus and high fracturestrength, and high density (see Fig. 42).

In review paper [43], Schroers et al. discusses about the thermo-plastic based processing of bulk metallic glass. ‘‘The main challengeassociated with net shape processing of BMGs is to avoid crystalli-zation. The stability against crystallization exhibited by BMG form-ers allows for two principally different processing methods. One isdirect casting, where the liquid BMG former must fill a mold andsimultaneously be cooled sufficiently fast to avoid crystallization.Only a careful balance of processing parameters can satisfy thesecontradictory requirements and, as a consequence, this only allowsthe casting of some limited geometries . . . With blow molding,

high strength bulk metallic glasses can be formed in a manner sim-ilar to plastics when the specifics of these alloys are considered.This allows one to net shape complex geometries in an economicaland precise manner, including shapes, which cannot be producedwith any other metal processing method. Furthermore, blow mold-ing of BMGs enables combination of three traditional processingsteps (shaping, joining, finishing) into one processing step. Thesuperior properties of BMGs relative to plastics and typical struc-tural metals, combined with the ease, economy, and precision . . .’’(excerpt with permission from [43] � 2011 Elsevier).

A comparison between processing parameters of BMGs, plas-tics, and superplastically formable (SPF) metallic alloys is givenin Table 11.

By reducing the heat losses during deformation, shapes thatcannot be produced with any other metal processing method canbe precisely net shaped within <1 min through thermo-plasticallyformed (TPF) based blow molding. Surface patterning and func-tionalization can be integrated into the blow molding processing

Table 11Processing parameters of BMGs, plastics and SPF alloys (adapted with permission from [43] � 2011 Elsevier).

Processing parameter Materials

Plastics (polypropylene) BMGs Zr44Ti11Cu10Ni10Be25,Pt57.5Cu14.7Ni5.3P22.5,Au49Ag5.5Pd2.3Cu26.9Si16.3

SPF alloys Ti (6Al–4V)Al (2004 SPF)

Temperature (�C) 160–260 160 (Au-based); 280 (Pt-based);350 (Pd-based) 430 (Zr-based)

900 (Ti6Al4), 465 (Al 2004)

Pressure (Pa) 1–10 � 105 Pa 1–4 � 105 Pa 1–4 � 105 PaMaximum strain (%) 1 �10,000 <400Typical strain rate (%) 10�1–1 10�1 10�3

m, Strain rate sensitivityexponent, m = dr/de whichreflects the material’sthinning resistance,

�1 1 0.4–0.7

ms = d(logg/G)/dTg/T thesteepness index-temperaturedependence of the viscosity(G: shear modulus, Tg: glasstransition temperature)

137 52 (Pt-based); 70 (Zr-based) Not applicable

K (W/mK) thermal conductivity 0.3 10 170Schematics of the temperature

distribution during blowmolding of SPF metal,polymer, and BMG

Fig. 43. Zr-based BMG pieces obtained by TPF based blow molding (adapted withpermission from [43] � 2011 Elsevier).

Fig. 44. BMGs pieces obtained at Liquidmetal Technologies) by die casting method(adapted from [44] � 2010 WILEY-VCH Verlag, GmbH and Co.).


step. Joints can be created by blow molding around fastener sites,resulting in a mechanical bond. Surface patterning or finishing canalso be integrated into one processing step with the blow moldingas in examples given in Fig. 43 [43].

In [44] are extensively presented other methods of fabricationof BMGs: the direct casting of BMGs (suction and die casting)The suction casting method yields high-quality parts with particu-larly low porosity compare to samples prepared by die casting. Abenefit of die casting is, however, the ability to scale up to high-volume production of small- to medium-size articles. Currently,Liquidmetal Technologies are using die casting, which is adoptedfrom conventional aluminum die casting for high-volume manu-facturing of Zr-based BMGs. Commercially produced (by Liquid-metal Technologies) BMG articles using a die casting method arepresented in Fig. 44.

The strength and weaknesses of die casting method for BMGsare presented schematic in Fig. 45.

BMGs combined with copper cores possess many advantages inprecision casting and coring of alloys. In [45] inner holes with dif-ferent dimensions and complex sectional shapes were fabricatedthrough direct casting and coring in MGs. The MG castings showgood surface finish and the cored inner holes exhibit high aspectratios larger than 80 while holding a tight dimension tolerance.Commercial copper wires with different diameters ranging from�130 lm to �2.5 mm were used as high-aspect-ratio cylindricalcores, and cores with complex sectional shapes were preparedfrom bulk copper sheets by mechanical machining or electrical-discharge machining methods. The surfaces of the cores were pol-ished using high-grade abrasive papers before fixed into the coppermolds. Alloy ingots of Zr55Al10Ni5Cu30 (at.%) were prepared byarc-melting mixtures of pure metals (purity P 99.5 at.%) underTi-gettered high-purity argon atmosphere. The ingots were remelted

Fig. 45. Schematic of advantages/disadvantages of die casting method for BMGs(extracted and adapted from [44]).


in a quartz tube by induction heating followed by injection intocopper molds assembled with copper cores. After mechanicallyseparating the molds, copper cores were extracted from thecastings manually [45].

The image of the transverse section of a casting without core-removal and the dimensions of casting and core after contractionto room temperature simulated by FEM method are presented inFig. 46. This study has important implications in the near net shapefabrication of accurate inner holes in metallic devices and in theassembly of BMG components.

Fig. 46. SEM image of the transverse section of a casting (a) and FEM simulation of dimen2011 Elsevier).

In [46], Wang et al. fabricated an intrinsically plastic Zr53.8Cu31.6-Ag7.0Al7.6 bulk metallic glass. A new BMG Z1 (Zr53.8Cu31.6Ag7.0Al7.6)with good bending and compressive plasticity was obtained aftermeticulous scanning of the Zr–(Cu, Ag)–Al composition map.Microstructural observations were systematically performed. Plas-ticity of BMG can change dramatically with slight composition var-iation and The Zr53.8Cu31.6Ag7.0Al7.6 as-cast sample exhibits uniformdistribution of all components, and neither crystallization norphase separation can be detected. In Fig. 47 are examples of bentsamples with representative compositions (a) and (b) pieces of Z1sample successfully bent after 10 mechanical bending tests.

In research paper [47] sand-blasting has successfully been usedfor inducing plasticity in as cast Zr58.3Cu18.8Al14.6Ni8.3 bulk metallicglass (BMG). Button of alloy was prepared by arc melting of highpurity Zr (4 N), Cu (4 N), Ni (4 N) and Al (4 N) under Ti-getter puri-fied argon atmosphere. The sample was remelted four times toimprove homogeneity. This button was finally remelted andsuction-cast into rod-shape sample of dimensions 3 mm � 50 mmin copper die. The rod was machined to produce specimens havingdimension 3 mm � 6 mm. In Fig. 48, are presented variation ofyield strength, fracture strength, plastic strain and Vickers hard-ness with different blasting time and SEM image showing few par-allel shear bands.

In [48] the effects of sample geometry on the mechanical prop-erties of a Zr-based bulk metallic glass was investigated. Authorsrevealed that the specimen geometry strongly influences the defor-mation behavior of as-cast Zr-based BMG. As the geometry of thesample was changed from cylindrical shape to conical shape,BMG sample with negligibly small plasticity tends to deform in aductile manner (as is shown in Fig. 49).

Authors anticipates that the findings of this work would be veryuseful for future engineering applications of BMGs. A finite ele-ment analyze (FEA) was performed using ANSYS 10.0. Von Misesstress distribution in cylindrical sample with very small stress gra-dient indicating the end effect resulting from the interface betweenthe crosshead and the specimen. Conical samples with semi-vertexangle a = 2� (sample 2) and a = 5� (sample 3) show higher stressgradients as evident by stress contours. High stress gradients inthe conical samples lead to the evolution and interaction of multi-ple shear bands.

In [49] study revealed time–temperature windows in whichmetallic glasses can be shaped by thermal annealing while remain-ing fully ductile. These include binary (Fe83B17, Co80B20 andPd82Si18), ternary (Fe81.5B14.5Si4 and Cu60Zr30Al10) and morecomplex quaternary Zr70Ni16Cu6Al8 glassy alloys of both metal–metalloid and metal–metal types. All showed full bending ductilityafter shaping. The thermomechanical processing leading to full

sions of casting and core (b) (reproduced and adapted with permission from [45] �

Fig. 47. Bent samples pieces of Zr53.8Cu31.6Ag7.0Al7.6 sample successfully bent after 10 mechanical bending tests (reproduced with permission from [46] � 2011 Elsevier).

Fig. 48. Variation of plasticity factors with different blasting time and SEM image of fractured sample (adapted with permission from [47] � 2011 Elsevier).

Fig. 49. Variation of load displacement for different sample forms (adapted with permission from [48] � 2010 Elsevier).


shaping without thermal embrittlement was successfully modeledusing the approach of Taub and Spaepen (Acta Metallurgica, 1980)based on the free-volume model of structural relaxation. Theresults are of direct interest for applications of metallic glasses assprings (see Fig. 50).

In [50], laser processing of Fe–Cr–Mo–W–C–Mn–Si–B glassforming alloy components without losing feedstock’s partialamorphous structure is successfully demonstrated using LaserEngineered Net Shaping (LENS) – a solid freeform fabricationtechnique. Current experimental results demonstrate that bulk

amorphous components, for structural applications, can poten-tially be fabricated via process optimization of laser based directmanufacturing techniques. Initially X-ray diffraction was used toanalyze the structure in processed alloy parts of Fe-basedNSSHS7574 (gas atomized NanoSteel NSSHS7574 glass formingalloy powder – The NanoSteel Company Inc., Idaho Falls, ID)Cross-sectional SEM micrographs of the LENS™ processed Fe-based BAA part shows excellent interfacial bonding and micro-structural features at the top region of the bulk sample, as inFig. 51.

Fig. 50. (a) Shaped Pd82Si20 glassy tape showing double bending ductility; (b) example of more complex shaping (reproduced with permission from [49] � 2011 Elsevier).

Fig. 51. Cross-sectional SEM micrographs of the LENS™ processed Fe-based BAA: (a) interfacial bonding and (b) microstructural features at the top region (reproduced withpermission from [50] � 2011 Elsevier).

Fig. 52. Schematic of the results of resistance spot welding of a Zr-based metal glassy alloy (adapted with permission from [52] � 2008 Elsevier).


In study [51] two ways of production of amorphous alloys wereexplored. Amorphous ribbons and rods of Zr75�xAlxNi10Cu10Ti5

(x = 15–20 at.%) alloys were successfully manufactured. Authorsused a special Zr crystal rod with an oxygen concentration of lessthan 0.02 at.% to maintain a low oxygen concentration in the alloy.All investigated alloys were manufactured under argon atmo-sphere. The ribbons thickness of about 120 lm and width of

2 mm were melt spun on a copper wheel. Rod shape samples withdiameter of 2 mm were produced by casting method into a coppermould.

Welding possibilities and the weldability of BMGs was recentlystudied in [52,53]. In [52], a Zr-based metal glassy alloy was suc-cessfully welded by small-scale resistance spot welding (RSW).TEM and XRD analyses indicated that no crystallization occurred

Fig. 53. Schematic of the results of dissimilar friction welding between Vit-1 BMG/Al-alloy A5083 (adapted with permission from [53] � 2010 Elsevier).


in both the weld nugget and the heat-affected zone. The amor-phous structure in the weld nugget as well as the heat-affectedzone was maintained during welding and the joints had sufficientstrength. The supercooled liquid was drained out, resulting in theexpulsion of the metal, and many small liquid droplets wereformed outside the expelled metal. The cooling rate of small scaleSRW was estimated to be at least 102 K/s and it probably reached104 K/s, at which crystallization never occurs. In Fig. 52 are pre-sented cross-sections of joints made with different welding cur-rent, the weld metal expulsion and fracture surface of the jointmade with welding condition of 400 A.

In [53], the similar and dissimilar friction welding of tubular Zr-based BMGs to BMGs and crystalline metals have been tried andcompared with the cases of BMG rods. A successful joining of thebulk metallic glass (BMG) to crystalline metals could be obtainedfor certain pairs of the material combination through the precisecontrol of friction conditions. The residual strength after friction

Table 12Synthesis of design and engineering methods for BMGs.

Component Method name and/or description

Design of BMGalloy

Similitude principle and artificial neural network model (SPANNdesign and glass forming ability of bulk metallic glassesMethods for materials and process selection-based on CambridgSelector – CES software

Engineering ofBMGs

Gun technique for cooling melted alloys

Ultra-high pressure method (metallic glass ordered at large scalSingle/multiple arc melting of components in argon purified atmby a rapid cooling (water quenching, fluxing or copper mold)Melt spinning methodsLaser melting–Laser Engineered Net Shaping (LENS)

Engineering ofBMG parts

Direct casting (suction and die casting)

Thermoplastic forming TPF also known as: hot forming–hot presviscous flow forming–viscous flow working (TPF-based compresblow molding; miniature fabrication; nanoforming; extrusion; cmetallic foams)Surface patterningSurface finishingCoating and coringFriction weldingSmall-scale resistance spot welding

welding was evaluated by the 4-point bending test and comparedwith the cases of BMG rods. Appearances after dissimilar frictionwelding of tubular BMGs to Al-alloy, the cross-sectional viewaround the interface after dissimilar friction welding of tubularVit-1 BMG to A5083 and SEM fractographic images at BMG sideafter 4-point bending test for dissimilar friction weld Vit-1 BMG/Al-alloy A5083 are presented in Fig. 53 (after [53]).

The conclusions and highlights of this chapter are synthesizedin Table 12.

6. Applications of BMGs

In 2004, Salimon et al. launched the question ‘‘Bulk metallicglasses; what are they good for?’’, as a title of their paper [34].And in a very well structured formula they give some answers tothis question, answers given in Table 13.

Reference

M) for the component [33]

e Engineering [34]

Rapid cooling to avoidcrystallization (100–106 K/s)

[3,54]

e) [35–37]osphere followed [47,12,21]

[5][50]

[43,38,17]

sing–super plastic forming–sion and injection molding;old/hot rolling; amorphous

[17][17][45][53][52]

Table 13Application and possible applications of BMGs available in 2004 (adapted from [34]).

Application and possible applications Technical reasons Economic reasons

Golf clubs (commercialized) High strength A, B, C, D, EKnives for ophthalmology (commercialized) High hardness, wear and corrosion resistance A, B, EEdges for sport goods (skis, skates) High hardness, wear and corrosion resistance, ability to be sharpened A, B, C, D, EPrecision mechanical elements High strength, high hardness, wear and corrosion resistance,

good abilities for surface finishingA, B, C, E

Materials for digital master disks Wear resistance, no grain structure C, EDental materials High strength, hardness, wear and corrosion resistance, castability,

abilities for surface finishingB, C, E

Wires for musical instruments High strength, high workability C, EJewellery (Pd and Pt-based alloys) High hardness, wear and corrosion resistance, lustre, precious metal content C, D, EMachinery structural material (high performance springs) High fatigue strength, high workabilityMarket indicators

A Cost of material constitutes small part of the priceB Performance is highly valuedC Cost of failure is modest;D Fashion and ‘‘novelty’’ are welcomedE Market does not require large volumes of material

Fig. 54. Grand challenges for engineering for 21st century (adapted from [8]).

Fig. 55. Applications of BMGs in 21st century.


Fig. 56. Arc melter for producing BMGs for bone surgery (reproduced and adapted from [56] Credit: ETH Zurich/LMPT and (b) laser melting device and (adapted from [57] �Springer/ESAFORM 2008).

Fig. 57. Fibroblast cell attachment on Zr–Ti–Co–Be metallic glass and high-densitypolyethylene (HDPE) disks (reproduced from [26]).


Now, in 2011, the expectations from [34] were far exceeded. In[8] are given the Grand Challenges for Engineering for 21stCentury, identified by National Academy of Engineering (NAE –Washington, DC). By analyzing this challenges is easy to detectthe role and the importance of BMGs for the future developmentof engineering and for humanity (see Fig. 54).

In this moment, the BMGs are practically used in the mannerpresented schematic in Fig. 55. Is easy observable the accordancewith the grand challenges presented above.

6.1. Bio-medical applications of MGs

Actually science demonstrated that is possible to repair brokenbones using metallic glasses. Materials researchers at ETH Zurichhave developed an alloy that could herald a new generation of bio-degradable bone implants. The new metallic glass (MgZnCa) syn-thesized by Dr. Zberg and his colleagues, under the leadership ofProf. Jorg Löffler at the EHT Zurich, shows a fundamentally differ-ent behavior from previous materials synthesized in the past,and appears to eliminate the problem of hydrogen-forming gas[55]. In Fig. 56, a plasma of up to 3000 �C is produced between atungsten tip (center) and a water-cooled copper plate.

In vitro cellular adhesion test where amorphous Zr–Ti–Co–Bedisks were immersed into a fibroblast-enriched environment, thematerial demonstrated a favorable biological performance as celladherence and proliferation were observed to be adequate andcompatible [26]. Micrograph of the amorphous metal surface after7 days (arrows point to cell-layer buildup at the interface) and cellproliferation on the amorphous metal and plastic surfaces areshown in Fig. 57.

Porosity importance in prosthesis in guiding cells and aiding tis-sue repair is increasingly growing. Interconnected-pore architec-tures with pore sizes in the range of 75–250 lm are thought tobe optimal for tissue ingrowth. In addition to eliciting cell attach-ment and tissue ingrowth, porosity functions to reduce the struc-tural properties of monolithic bulk materials to values closer tothose of natural bone. Owing to their high strength combined withrelatively low moduli, amorphous metals can be thought as attrac-tive base materials for developing strong highly-elastic porous sol-ids capable of matching the mechanical properties of cancellousbone. A porous amorphous structure (88% porosity) is shown inFig. 58.

Another approach is to employ the porous particles into theamorphous matrix to form an amorphous matrix composite. Suc-cessful results in Mg based glasses added with 5–25 vol.% of porousMo particles (w30–50 mm) have achieved compressive ductilityover 10%, as shown in Fig. 59.

Zr based BMG can be extended to the medical tool such as thesurgical razor or micro-surgery scissors. Because the razor made

by the Zr BMG presents much smoother edge than the razor madeby martensitic stainless steel. One example is shown in Fig. 60.

Zeng and Gu, in [59] revealed that rapid solidification process-ing was adopted to refine both the matrix grains and intermetallicparticles as well as improve chemical homogeneity of Mg–3Ca al-loy. With extreme fine grain size (200–500 nm), RS alloy ribbonshowed largely decreased corrosion rate (0.36–1.4 mm/yr) anduniform corrosion morphology over the cast one (21 mm/yr). Amagnesium-based bulk metallic glass was also proposed due toits single-phase and chemically homogeneity, eliminating the gal-vanic corrosion caused by second phase. The result indicated thatMg66Zn30Ca4 exhibited decreased corrosion rate, uniform corrosionmorphology and improved cyto-compatibility, with MG63 cell welladhered and growth on its surface. Is always shown a designed (byTan et al., Biomed. Mater., 4 (2009), 015–016.) three-dimensionalhoneycomb-structured magnesium scaffolds with interconnectedpores of accurately controlled pore size and porosity by laser per-foration technique LPT (Fig. 61).


Micro-parts (tools) for medical use are actually in use (commer-cialized) and some examples are presented in Fig. 62.

Fig. 59. SEM micrograph of the homogeneous distribution of porous Mo particles in the M2009 Elsevier).

Fig. 60. Medical tools made from Zr based BMG (adapted and

Fig. 58. �Amorphous porous Pd–Ni–Cu–P foam and micrographs of the cellularstructure at high magnifications (reproduced from [26]).

6.2. Engineering applications

The using of MGs in micro-electro-mechanical systems (MEMS)have great potential but unfortunately is under exploited. Anexample of using of BMGs in MEMS is the spring actuator fromFig. 63.

Another types of actuators based on Pt57.5Cu14.7Ni5.3P22.5 metal-lic glass are proposed by Kumar et al. in [17] and are presented inFig. 64. The comb teeth are 5 lm wide and 20 lm long. The combsmove when an alternating current (AC) voltage is applied.

Metallic glasses are used also for fabrication of micro-gears, mi-cro-springs, micro-motors and some other engineering parts atsmall scale (see Fig. 65).

In research paper [60], Yokoyama et al. succeed in the develop-ment of cap casting and enveloped casting technique to accomplishthe fabrication of centimeter sized BMGs. The former has an advan-tage to increase cooling rate and the later has an advantage to jointanother materials instead of welding. This paper presents the pro-duction of a glassy Zr55Cu30Ni5Al10 alloy rod with a diameter of32 mm and joined glassy Zr55Cu30Ni5Al10 alloy parts with anothermaterials for industrial applications (Fig. 66).

Fe based glassy alloy powders (with Ni, Cr, Mo, B and Si)produced by water atomization are commercialized under the

g based bulk metallic glasses (adapted and reproduced with permission from [58] �

reproduced with permission from [58] � 2009 Elsevier).

Fig. 61. (a) Honeycomb structured magnesium scaffolds fabricated by LPT and lotus-type porous pure magnesium prepared by metal/gas eutectic unidirectional solidificationmethod (adapted from [59]).

Fig. 62. BMG micro-parts for medical use (adapted from [17] � 2011 WILEY-VCH Verlag, GmbH and Co.).


commercial name ‘‘AMO-beads’’. The powder diameter extends in awide range from �0.05 to 0.6 mm because of the high glass-formingability for the developed Fe-based alloy [61]. The ‘‘AMO-beads’’have the advantage of much longer endurance times comparedwith those for cast steel shot and high-speed steel shot. By use ofgood mechanical characteristics (900 HV, high fracture strengthof �3000 MPa and large elastic strain of �0.02) in conjunction withhigh corrosion resistance and a smooth outer surface, the peeningshot treatment using ‘‘AMO beads’’ can generate a higher level ofresidual compressive stress on the surface of high class alloy steelvehicle gears with high Vickers hardness of 750 achieved by carbu-rization treatment. As a result, the alloy steel gears can increase fa-tigue strength by 50–80% compared with the steel gear subjectedto peening shot using high-speed steel balls. This causes a signifi-cant reduction in the weight of alloy steel vehicle gear by �45%[38,61] (Fig. 67).

In [62], Sarac et al. from Schroreslab Yale fabricated 3-D micro-shells by using blow molding method based on thermoplastic

forming of bulk metallic glasses (BMGs) The 3-D microshells are at-tached to the Si wafer through mechanical locking, which isachieved in the same processing step. Versatile sizes and shapesof the 3-D shells can be precisely controlled. High strength(>1 GPa), elasticity �2% and controlled surface roughness �2 nm)are the great advantages of the new method nad suggest their po-tential use in devices, including resonators, microlenses, microflu-idic, and packaging.

In [63] are shown scanning electron microscopy (SEM) imagesof BMGs patterned on different length scales using surface pattern-ing technique described in Fig. 68a. Fig. 68b and c shows Pt–BMGembossed on porous alumina under pressure of 20 MPa at 250 �Cand 280 �C, respectively. The alumina was dissolved in KOH solu-tion resulting in patterned Pt–BMG surfaces. The diameter of thepatterned features is about 100 nm and by changing embossingconditions the height of features can be varied. Fig. 68c demon-strates that surface patterns can be extended into long hair-like fi-bers mimicking structures of sticky feet found in many insects and

Fig. 63. Spring actuator (adapted from [12] � 2009 Elsevier).


animals such as geckos, skinks, and tree frogs. Fig. 68d and e showsthe SEM images of Zr–BMG embossed at 430 �C under 20 MPa onpatterned silicon and nickel, respectively. Both Pt–BMG and Zr–BMG precisely replicate the mold patterns on length scales rangingfrom 20 nm to 100 lm. In 68 f is presented a freestanding BMGcomponent fabricated using combined embossing and hot separa-tion processes [63].

Some engineering and art parts fabricated using blow moldingprocesses are presented in Fig. 69.

6.3. Defense and aerospace application of BMGs

The Department of Defense (DoD) of USA has extensively re-searched Liquidmetal for Kinetic Energy Penetrator (KEP) (seeFig. 70) rod. The KEP, the key component of the highly effective ar-mor piercing ammunition system, currently utilizes depleted ura-nium (DU) because of its density and self-sharpening behavior.Ballistic tests conducted by the US Army have proven that the Liq-uidmetal composites exhibit self-sharpening similar to the DU KEP,but environmentally benign. The high strength and lightweightattributes of Liquidmetal alloys enable the Department of Defense

Fig. 64. BMG comb-drive actuators (reproduced with permissi

to support its transformation toward lighter smaller and more costeffective systems. Liquidmetal Technologies is supporting researchand development for a wide range of military applications [64].

Other defense and aerospace applications are presented inTable 14.

NASA and a team composed by William Johnson, Chris Veazey,Marios Demetriou (Caltech) and William Kaukler (University ofAlabama) investigate the fabrication of BMGs foam in space condi-tions. Investigations tests and produces hardened foam from bulkmetallic glass. The absence of gravity facilitates the creation of amore uniform metallic glass foam, a material with an extremelyhigh strength to weight ratio. Developing lighter and strongermaterials can lead to a more durable spacecraft that will requireless propellant to travel long distances. Three planned runs forthe Foam experiment were successfully completed on station dur-ing Expedition 9. Samples, were returned to Earth in August 2005,have been analyzed and reported. The experiment was designed totest the hypothesis that amorphous metals exhibit foam-makingqualities on the ground that mimic metallic foam textures madein microgravity conditions – Fig. 71 [65].

In 2000, NASA’s Genesis spacecraft has received its final piece ofscience equipment: a solar wind collector made of a new formulaof bulk metallic glass, composed of the same class of material ashigh-tech golf clubs. It and other solar wind collector tiles on thespacecraft collected the first-ever samples of the solar wind asthe spacecraft floats in the oncoming solar stream. The new BMGforming alloy was designed by Dr. Charles C. Hays in the materialsscience laboratories of Dr. William Johnson of Caltech. It is a com-plex mixture of zirconium, niobium, copper, nickel, and aluminum.The new BMG was prepared in a collaborative effort by Dr. Haysand George Wolter (Howmet Corporation, Greenwich, Conn.),using the same process the company uses for the high-tech Vitre-loy-based golf clubs The surfaces of metallic glasses dissolveevenly, allowing the captured ions to be released in equal layersby sophisticated acid etching techniques developed by the Univer-sity of Zurich, Switzerland. Higher-energy ions blast further intothe metal’s surface. After the collection period, the spacecraftclosed-up and returned the samples to Earth in a Stardust-likesample-return capsule (SRC). On 8 September 2004 the SRC en-tered Earth’s atmosphere as planned, but its gravity switches wereoriented incorrectly as the result of a design error and the para-chute system failed to deploy. The high-speed wreck compromisedthe SRC and shattered many of the Genesis collectors. However, theGenesis Preliminary Examination Team was able to show that, be-cause the solar-wind ions were buried beneath the surface of thecollectors, it is possible to detect and quantify elements in the so-lar-wind. When samples are back on Earth, special techniqueswere used to etch the metal layer by layer, releasing the particlesof gas for laboratory study – Fig. 72 [66,67].

on from [17] � 2011 WILEY-VCH Verlag, GmbH and Co.).

Fig. 65. Engineering parts at micro-scale fabricated from BMGs (adapted with permission from [38] � 2011 Elsevier).

Fig. 66. Some pieces fabricated from Zr based BMG (adapted from [60]).


Fig. 67. The mechanic treatment-blasting of a gear teeth with BMG powder – DASP process (adapted with permission from [38] � 2011 Elsevier).

Fig. 68. Miniature forming BMG pieces (adapted from [63] � JOM).


6.4. Environmental capabilities of BMGs

Unfortunately, the environmental aspects of fabrication andusing of BMGs is poor researched. Some aspects can be done:the biocompatibility of most BMGs, the obtaining of BMGsfrom recyclable materials, using of BMGs in environmental

applications (solar cells, hydrogen production, system for reten-tion and purification of dangerous pollutants, nuclear industry,etc.).

A first step was made by DoD of USA and Liquidmetal Technol-ogies Co. by gradual using of glassy metals rods instead of depleteduranium for KEPs fabrication.

Fig. 69. Blow molded BMG parts (adapted from [63] � JOM).

Fig. 70. Kinetic Energy Penetrators ‘‘KEPs’’ (reproduced from [64]).

Table 14Defense and aerospace applications of BMGs [64].

BMGs applications for defense andaerospace industries (already producedand commercialized by LiquidmetalTechnolgies) http://www.liquidmetal.com/applications/defense-applications/

Thin walled casings andcomponents for electronics

Casings for night sights andoptical devicesMissile components fins,nosecones, gimbals, and bodiesFuzes and sub-munitioncomponentsComposite armor light weightcasingsLighter weight fragmentationdevicesLight weight casings forordnanceAircraft fastenersSpacecraft, aircraft, UAV andship components


In [21], Xia et al. studied the using of Gd-based (Gd55Ni22M-n3Al20) BMGs as refrigerant materials. The refrigerant capacity(RC) of the developed BMG can reach a high value of 825 J kg�1

In [68], Suo et al. fabricated metallic glasses (quaternaryW30Fe38B32�xCx (x = 5, 7, 10, 13, 15 at.%) alloys) from low purityindustrial raw materials by melt-spinning method. The low purityindustrial raw materials and a little purity elements were used:ferrotungsten ferroboron, and cast iron. W30Fe38B22C10 full metallicglass was successfully produced. Vickers hardness and density val-ues at room temperature are 11.9–12.9 GPa and 14.5 g/cm3 at

least, respectively. The application of industrial raw materials notonly reduces the cost, bur also improves the manufacturability ofW-based metallic glasses with high crystallization temperature,high modulus and high hardness.

In [69] Luo and Wang revealed the excellent thermoplasticbehavior of rare earth based BMGs at room temperature. This makefrom these materials environmentally friendly because the smallenergy consumption for machining and forming.

http://www.liquidmetal.com/applications/defense-applications/


Fig. 71. NASA images with fabrication of BMGs foam in space conditions (image credit NASA [65]).

Fig. 72. Images with spacecraft (artist rendering during collection phase) and BMG components for Genesis Mission (image credit NASA/JPL – Caltech/USC [66,67]).


7. Conclusions

The first reported metallic glass, practically obtained in 1959and scientific reported in 1960, was the alloy Au75Si25 producedby extremely rapid cooling (cca. 106 K/s) of melted alloy, at Caltechby Klement, Willens and Duwez.

Generally, amorphous alloys are formed by exploiting the ‘‘con-fusion’’ effect. Such alloys contain so many different elements thatupon cooling at sufficiently fast rates, the constituent atoms simplycannot coordinate themselves into the equilibrium crystalline statebefore their mobility is stopped. In this way, the random disor-dered state of the atoms is ‘‘locked in’’.

In 2004, two groups (Oak Ridge National Laboratory and Uni-versity of Virginia.) succeeded in producing bulk amorphous steel(‘‘glassy steel’’). The product is non-magnetic at room temperatureand significantly stronger than conventional steel.

In 2010, the concept that metallic glasses not being very toughwas made history by Marios Demetriou and colleagues from theCalifornia Institute of Technology and The University of California,Berkeley. They developed a Palladium based metallic glass (withformula Pd79Ag3.5P6Si9.5Ge2) that is not only strong, but also toughas steel.

BMGs consist of predominantly metallic elements and metallicbonds, but at the same time have an amorphous internal structure.Such a combination of ‘‘metal’’ and ‘‘glass’’ leads to unique proper-ties and unprecedented opportunities. Since the discovery of thefirst BMG, there has been increasing interest in developing andunderstanding this new family of materials. Among the many

unresolved puzzles, the atomic-level structure and structure–property relationship are one of the central topics.

It is difficult to experimentally characterize the BMGs atomicstructure by conventional diffraction, spectroscopic and imagingtechniques. Various structural models, such as Bernal’s dense ran-dom packing, Gaskel’s ‘short-range order’ and the recent ‘‘solute-centred quasi-equivalent cluster’’ models, have been proposed inthe past 50 years.

According to scientific literature, there are two major challengesin the study of BMGs structures: how to construct a realistic three-dimensional (3-D) amorphous structure, using experimental and/or computational tools and how to effectively characterize a givenamorphous structure and extract the key structural features rele-vant to the fundamentals of glass formation and properties, usingappropriate structural parameters.

For BMGs, the direct reconstruction of the locally 3-D structureis very difficult. Some experimental techniques can be used to ex-tract statistical information about the average glass structure, butthe data usually cannot provide a complete picture. Structuralstudies have been transformed in recent years by acceleration inthe acquisition of X-ray and neutron scattering data, and by im-proved computational methods, including the reverse Monte-Carlomethod to fit measured data, and molecular-dynamic simulations.

The typical constituent elements of the BMGs can be groupedin: alkali and alkaline earth metals (AM), semi/simple metals(SM), transition metals (TM), including early transition metals(ETM), late transition metals (LTM), rare earth metals (RE), andnonmetals (NM).


The structure of materials influences their properties in multi-ple and very different ways. At BMGs, the structure–property cor-relation is very difficult to study. The MGs structures are far frombeing completely understood and elucidated and are very difficultto describe and quantify and as a result, the predictions about howthe atomic structure influence the macroscopic properties of MGsis difficult to do.

Glass forming ability (GFA) is a influential factor in studying theformation of BMGs. but there is no standard definition for thisparameter up to now, and many indicators have been developedand proposed. From the engineering aspect, the lower the criticalcooling rate and the larger the critical thickness are, the higherthe glass forming ability of a metallic glass will be.

The mechanical properties of bulk metallic glasses (their supe-rior strength and hardness, corrosion and wear resistance, com-bined with their general inability to undergo homogeneousplastic deformation have been a subject worthy of investigations.

All studies summarizes that BMGs have much higher tensilestrengths and much lower Young’s moduli. The difference in thesevalues between the BMG and crystalline alloys is as large as 60%.Plastic deformation in metallic glasses is generally associated withinhomogeneous flow in highly localized shear bands.

The fatigue performances of BMGs has not been extensivelyinvestigated. Some investigations were performed on Zr-basedglasses. BMGs are a subject of interest due to their superior specificstrength, large ductility in bending, low coefficient of friction, highhardness, high resistance to corrosion, oxidation and wear. Theseproperties are accompanied by their inability to undergo homoge-neous plastic deformation due to the absence of dislocation-mediated crystallographic slip. Metallic glasses deform by shearbanding, a particular mode of deformation of interest for certainapplications, but which also causes them to be quite brittle and failcatastrophically due to uninhibited propagation of the bands.

Generally, BMGs are multi-component metallic alloy systemsand as a result, the huge difficulty for the component design ofnew multi-component glass-forming metallic alloy system. Is alsopresented a new method entitled ‘‘similitude principle and artifi-cial neural network model (SPANNM)’’ for the component designand glass forming ability of bulk metallic glasses. The methods ofcomponent design of BMGs are briefly summarized in this reviewpaper.

The most recent method to obtain a metallic glass that is orderedat a large scale was reported in a study conducted by GeophysicalLaboratory at the Carnegie Institution for Science Zhejiang Univer-sity from China, Stanford University and SLAC National AcceleratorLaboratory.

Blow molding of BMGs enables a combination of three tradi-tional processing steps (shaping, joining, finishing) into one pro-cessing step. Welding possibilities and the weldability of BMGswas recently studied and a Zr-based metal glassy alloy was suc-cessfully welded by small-scale resistance spot welding (RSW).TEM and XRD analyses indicated that no crystallization occurredin both the weld nugget and the heat-affected zone.

It is possible that broken bones to be repaired by using metallicglasses. The new metallic glass (MgZnCa) synthesized by Dr. Zbergand his colleagues, under the leadership of Prof. Jorg Löffler at theEHT Zurich, shows a fundamentally different behavior from previ-ous materials synthesized in the past, and appears to eliminate theproblem of hydrogen-forming gas.

Owing to their high strength combined with relatively lowmoduli, amorphous metals can be thought as attractive base mate-rials for developing strong highly-elastic porous solids capable ofmatching the mechanical properties of cancellous bone.

Zirconium based BMG can be extended to the medical tool suchas the surgical razor or micro-surgery scissors, because the razormade by the Zr BMG presents much smoother edge than the razor

made by martensitic stainless steel. Micro-tools for medical use areactually in use and are successfully commercialized.

The using of MGs in micro-electro-mechanical systems (MEMS)have great potential but unfortunately is under exploited.

Fe based glassy alloy powders (with Ni, Cr, Mo, B and Si)produced by water atomization are commercialized under thecommercial name ‘‘AMO-beads’’. The ‘‘AMO-beads’’ have theadvantage of much longer endurance times compared with thosefor cast steel shot and high-speed steel shot.

Kinetic Energy Penetrator (KEP) is the key component of thehighly effective armor piercing ammunition system and currentlyutilizes depleted uranium (DU) because of its density and self-sharpening behavior. The DoD of US make steps for using theglassy metals rods (produced by Liquidmetals) instead of depleteduranium for KEPs fabrication.

In 2000, NASA’s Genesis Spacecraft has received it’s final pieceof science equipment: a solar wind collector made of a new for-mula of bulk metallic glass, composed of the same class of materialas high-tech golf clubs.

Another NASA experiment was designed to test the hypothesisthat amorphous metals exhibit foam-making qualities on theground that mimic metallic foam textures made in microgravityconditions.

8. The future developments of BMGs

About the future: the future is promising! Bulk metallic glasses(BMGs) are considered to be the materials of the future; due totheir high strength, metallic glasses have a number of interestingapplications, as coatings. Metallic glasses and the crystalline mate-rials derived from them, can have very good resistance to slidingand abrasive wear. Combined with their strength and toughness– this makes them ideal candidates for bio-implants or militaryapplications.

In [38] from 2011, Inoue and Takeuchi revealed their futureexpectations in BMGs field: ‘‘It is expected that fields of applicationwill be significantly extended in the near future on the basis of theuseful and unique engineering properties of BGA resulting from thesimultaneous achievements of novel atomic configurations, uniquemulti-component alloy compositions, various bulk forms, slow solidifi-cation process, Newtonian flow deformability and net-shape castingformability’’.

Professor Jan Schroers from Schroerslab – Yale University de-clare for this review paper: ‘‘In my opinion the future of metallicglasses stem from the fact that when thermoplastic formed they com-bine previously mutually exclusive attributes, the properties of a veryhigh strength metals and the processibility of plastics. The materialsscience community must develop processing strategies based on ther-moplastic forming, processing protocols, and equipment. In terms ofmaterial, up to date there are only a few BMG compositions that canwidely used for TPF. These are either based on precious metals or onalloys that contain beryllium. A non-toxic and inexpensive BMG form-ing alloy with high TPF processibility, would certainly have tremen-dous ramifications for the wide spread commercial use of BMGs.’’

Professor Marios Demetriou from Caltech declare for this re-view paper: ‘‘Owing to their attractive mechanical properties and un-ique processing capabilities, metallic glasses have the potential todominate metal-hardware engineering in the 21st century.’’

Dr. Physicist Joerg Heber, Senior Editor at Nature Materials andscience writer, comment for readers of Materials and Design and ofthis review paper: ‘‘Metallic glasses have come a long way since theirdiscovery 50 years ago. It is now possible to fabricate a large variety ofcompounds in bulk quantities. What’s more, the properties of metallicglasses have been enhanced considerably. We will see more of this infuture. New alloys designed with certain properties in mind. Whether


as structural materials, or for novel applications. For centuriesresearchers have exploited the properties of crystals, because theirproperties seemed easier to control. But this doesn’t necessarily holdtrue. In future, we will see much more emphasis on amorphousmaterials.’’

Hardened bulk metallic glass foam may be very useful as amaterial for building future spacecraft for long-term space flight.The foams can also be used to build permanent structures on theMoon or Mars. Buildings and spacecraft fuselages made from bulkmetallic glass foams can be extremely tough and light at the sametime, thereby reducing costs while increasing the protection theyprovide to explorers.

The adventure of BMGs as medical, bio- and environmentalfriendly materials is just at the beginning. A long way of findingsis perspective in this area of crucial importance for human health.

Acknowledgments

To Dr. Physicist Joerg Heber, Senior Editor at Nature Materialsand science writer, for his friendly advices and encouragementsand for permission provided to use some information from his sci-entific and editorial work.

To Professor Jan Schroers from Schrorerslab – Yale University forhis statements provided for this review paper and for readers ofMaterials and Design.

To Professor Marios Demetriou from Caltech, for permissionprovided to reuse some information from his published work andfor the statement made for this review and for readers of Materialsand Design.

To researchers and scientists (cited or not cited in this paper)whose work is the true engine of human development.

To Elsevier Ltd., Nature Publishing Group and WILEY-VCH Ver-lag GmbH and Co for copyright licenses and permissions providedby Copyright Clearance Center (CCC) – www.rightslink.com.

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http://www.nasa.gov/mission_pages/station/research/experiments/Foam.html

http://www.nasa.gov/mission_pages/station/research/experiments/Foam.html

http://www.jpl.nasa.gov

http://www.jpl.nasa.gov/releases/2000/genesiscollector.html

http://www.jpl.nasa.gov/releases/2000/genesiscollector.html

http://genesismission.jpl.nasa.gov/

Metallic glasses from “alchemy” to pure science: Present and future of design, processing and...

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