01 introd

Scienza e Tecnologia dei Materiali 2011 - M.Ferraris

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The aim of this course:

• The aim of this course is to provide Mechanical and Automotive Engineering students basic knowledge on different classes of materials

• Relationship between structure of materials and their properties (Materials Science)

• Structure of materials = chemical-physical structure, (chemical bonds, composition, …)

• To design/choose materials (Materials Engineering)• Choice of materials, production process,

structure-properties, performances, cost, life cycle


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MATERIALS OF INTEREST IN ENGINEERING

• Metals and alloys

• polymers

• composites

• glasses

• ceramics

(fuels, lubricants, paints, …....)

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http://helios.augustana.edu/physics/301/periodic-table-fix.jpg


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OVERVIEW ON METALS AND ALLOYS

• ELEMENTS (Fe, Ti, Al, Mg, Cu,…)• Heath and electricity• Prepared by melting• Can be machined• All solids at room temperature• Can form alloys• Corrosion and oxidation issues• Density ranging between 2,7 and 8 g/cm3


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OVERVIEW ON CERAMICS

• Compounds (oxydes, carbides, nitrides, ..)(Al2O3, SiC, Si3N4,…)(density ranging between 3 and 5 g/cm3)

• hard and brittle, difficult to be machined

• Insulators (heat and electricity)

• High melting point

• Prepared by sintering or other processes, never melting

• examples: bricks, concrete, tiles, porcellains, ….


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OVERVIEW ON POLYMERS• Organic macro-molecules,

natural or synthetic • Prepared by organic synthesis• Low density, less than 1g/cm3

• Easy to machine• Insulators (sound, heat and electricity)• Low thermal and mechanical properties• examples: tyres, adhesives, paints, bitumen,..


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OVERVIEW ON GLASSES

• Prepared by melting of oxides

(SiO2, Na2O, CaO, Al2O3, K2O,...)

• Hard, brittle, can be deformed only at suitable temperature, cannot be machined

• insulators (heat and electricity)• Density of about 2,5 g/cm3

• examples: window and car glasses, optical fibers


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OVERVIEW ON COMPOSITES• TWO PHASES:

MATRIX and SECOND PHASE• Classified according to the matrix:• Metal matrix composites• Polymer matrix composites• Ceramic matrix composites• Glass and glass-ceramic matrix composites• examples: wood, reinforced concrete, bones,

bamboo,….


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OVERVIEW ON COMPOSITES

• Can be classified according to the second phase morphology:

• Fiber reinforced composites (long fibers, short fibers)

• Particle reinforced composites

• The second phase can be: metal, polymer, ceramic, glass


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STRUCTURE OF MATERIALS

• CRYSTALLINE ORDERED– metals, ceramics

• AMORPHOUS DISORDERED

• glass, polymers


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EXCEPTIONS….

• Glass-ceramics, polymers, composites::

• Amorphous in some zones, crystalline in others


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CORRELATION STRUCTURE-PROPERTIES….

• …the aim of Materials Science

• CRYSTALLINE STRUCTURE– Melting Temperature

• AMORPHOUS STRUCTURE– They don’t have melting temperature,

but progressive softening


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CRYSTALLINE STRUCTURE: MELTING TEMPERATURE

• Melting temperature = Solid to liquid (Tm)

• directly proportional to the material bond strength

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MELTING TEMPERATURE

Tm (°C)

Tm (°C)

Tm (°C)

W 3410 Mg 651 Al2O3 2045

Ti 1675 Pb 270 SiO2 1730

Fe 1536 Sn 232 MgO 2800

Ni 1453 Al 660 CaCO3 1339

Cu 1083

http://www.metallurgia-italiana.net/csimages/metalli_leggeri.jpg

http://images.google.it/imgres?imgurl=http://www.mcasco.com/images/bonds.gif&imgrefurl=http://www.mcasco.com/eande.html&h=284&w=375&sz=29&hl=it&start=1&um=1&tbnid=yD0NOcoTvn6yaM:&tbnh=92&tbnw=122&prev=/images%3Fq%3Dcrystalline%2Bstructure%26svnum%3D10%26um%3D1%26hl%3Dit


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CRYSTALLINE STRUCTURE

• Chemical bond and crystalline structure: atoms, ions or molecules in crystalline structures are in ordered positions in crystalline cells

• atoms, ions or molecules are at the equilibrium distance between attraction and repulsion forces : bond length or bond distance

ibchem.com


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• “CRYSTALLINE materials: ordered positioning of atoms (ex: metals), or ions (ex: ceramics) or molecules (ex: polymers)”

• CRYSTALLINE

CELL

• a,b,c

0,1-0,26 nm

Video


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CRYSTALLINE CELLS

a,b,c cell parameters cell angles

• identify the structure of crystalline materials by cell parameters

• a,b,c about 0,1-0,26 nm at room T and without external applied forces


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CRYSTALLINE CELLS

• a,b,c changes with T and applied forces

• At zero K they are at the equilibrium distance

• When T increases, vibration around the equilibrium distance cause thermal expansion, then melting.


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SEVERAL CRYSTALLINE CELLS

a = b = c = = = 90° CUBICa = b c = = = 90° TETRAGONALa b c = = = 90°

ORTOROMBIC

a = b = c = = 90° ROMBOEDRICa = b c = = 90° , = 120° HESAGONALa b c = = 90° MONOCLINE

a b c 90° TRICLINE

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Crystalline cells

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Es: Cu, Al, Ag, Au

Es: Fe, W, CrEs: Mg, Ti, Zn

Hexagonal compact


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• Lattice plane: 3 atoms (or ions or molecules) in a cell, define one lattice plane

• COORDINATION NUMBER:

Materiali per l’IngegneriaM.Ferraris

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http://amscampus.cib.unibo.it/archive/00001824/01/5-Silicio_web.doc

http://www.diee.unica.it/~vanzi/Origami.PDF

http://www.lnf.infn.it/esperimenti/rap/docs/silicio.pdf


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Gold (Au) nanocluster, embedded in amorphous silica matrix(SiO2)

Au

silica, SiO2, amorphous

J. Morgiel, Cracovia

http://images.google.it/imgres?imgurl=http://www.phys.uu.nl/~barkema/figures/silica.gif&imgrefurl=http://www.phys.uu.nl/~barkema/&h=768&w=1024&sz=872&hl=it&start=8&um=1&tbnid=5APcAmf-Q7IfTM:&tbnh=113&tbnw=150&prev=/images%3Fq%3Dsilica%26svnum%3D10%26um%3D1%26hl%3Dit


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http://ceramics.org/video/individual-carbon-atoms-in-motion/

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X-ray diffraction (XRD)

XRD of a crystalline material (Au)

XRD of amorphous silica and silica crystalline (SiO2)

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Electromagnetic radiation spectrum

E=h

energy = frequencywavelength

Energy, frequency

wavelength

High Energy, high frequency, short wavelength

Low Energy, low frequency, long wavelength


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X ray diffraction

n = 2 d sin (Bragg law)

n= number, 1, 2, 3…

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X-ray diffraction (XRD): to detect an amorphous or crystalline material

XRD of a crystalline material (Au)

XRD of amorphous silica and silica crystalline (SiO2)


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Examples of XRD


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CRYSTALLINE STRUCTURE: POLYMORPHISM

• A crystalline solid melts when heated

• Polymorphism: change of crystalline structure during heating (solid state reaction)

• examples: Ti, Fe, SiO2

• Polymorfism and variation of materials properties : (V, , k, E, d,….)


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POLYMORFISM


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CRYSTALLINE STRUCTURE: DEFECTS

• Crystalline structure is not completely ordered….LATTICE DEFECTS:

• MONODIMENSIONAL DEFECTS

• BIDIMENSIONAL DEFECTS

• TRIDIMENSIONAL DEFECTS

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MONODIMENSIONAL DEFECT: INTERSTITIAL

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MONODIMENSIONAL DEFECTS:SUBSTITUTIONAL

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MONODIMENSIONAL DEFECTS:SUBSTITUTIONAL


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MONODIMENSIONAL DEFECTS(or POINT DEFECTS)

• Vacancies (lack of one atom or ion or molecule in the lattice)

• interstials• substitutionals

vacancy

substitutionalinterstitial


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ARE POINT DEFECTS «DEFECTS»…?

• ALLOYS and in general solid solutions are possible because of interstitials and substitutionals defects

• presence of substitutional impurities are necessary for in semiconductor science

….


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Solid solutions: substitutional alloys• Metal alloys are solid solutions :

– up to 15% atoms as substitutional defects different from those of the main lattice

– Es: Cu/Ni– RNi = 1.25Å, RCu = 1.28Å– both CFC– Same electronegativity– valence Ni +2, Cu +1, +2

– Atoms with similar radius, crystalline cell, electronegativity and valence


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Solid solutions: interstitial alloys

– Atoms as interstitial defects different from those of the main lattice

– Only small atoms can enter a metallic cell as interstitial defects (max 10%)

– Ex.: carbon is an interstitial defects in the iron lattice in steels, up to about 2%

– RFe = 1.24Å, RC = 0.71Å


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BIDIMENSIONAL DEFECTS: DISLOCATONS

• Edge and screw dislocations: responsible of plastic deformation of materials

• (video)

• Commercial metal or alloy (ex. Cu) about 108 dislocations per cm3


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Dislocations in nature….!• Dislocation: one lattice plane more….

Solfuro di rame (CuS)

cactus!


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Edge dislocations

http://www.uet.edu.pk/dmems/EdgeDislocation.gif

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350Å

Nitruro di gallio(GaN)

Screw dislocations



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Dislocation motion

Vedi moto dislocazioni.ppt

1

2

3

4

5

6


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DISLOCATION AND PLASTIC DEFORMATION

• Dislocation motion means fracture of a bond and formation of another one in a different nearby lattice position

• dislocations are present in ALL crystalline materials (metals, ceramics,…)

• In ceramics, it is NOT possible to form a new bond in a different nearby lattice position due to the ionic or covalent nature of bonds. No plastic deformation (at room T)

• In metals, it IS possible to form a new bond in a different nearby lattice position due to the metal nature of bonds. Plastic deformation.

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Interaction between dislocations and point defects

Mechanical hardening, cold working

Video

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Dislocations interact with themselves: mechanical hardening of alloys


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INTERACTION DISLOCATION AND POINT DEFECTS

• DISLOCATIONS INTERACT with themselves (cold working, hardening).

• Dislocations interact with point defects: dislocation motion is slower (VIDEO) in alloys than in pure metals

• Better mechanical properties of alloys vs pure metals• Without dislocations, the plastic deformation of metals cannot

be explained • Dislocations can be seen with Scanning or Transmission

electron Microscopy (SEM, TEM).

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Dislocation motion


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Interaction dislocation substitutional defect and influence on mechanical properties of alloys vs pure metals

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Interaction dislocation/substitutional defect and influence on mechanical properties of alloys vs pure metals

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Preferential sliding planes for dislocations

Low density plane High density plane


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TRIDIMENTIONAL DEFECTS• surface is a defect : reactivity of atoms or

ions or molecules at the surface. Lower coordination number, higher reactivity

• GRAIN BOUNDARIES (gb)


56http://www.numis.northwestern.edu/Research/Projects/3x1.shtml


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• GRAIN BOUNDARIES:

distorted bonds at the boundary between two grains in a polycrystalline material

TRIDIMENTIONAL DEFECTS



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SOLIDIFICATION OF A LIQUID AND ITS STRUCTURE

1425 °C

or SiO2

a 1730 °C

SLOW SPEEDSOLIDIFICATIONMONOCRYSTALLINE STRUCTURE

MEDIUM SPEED SOLIDIFICATIONPOLYCRYSTALLINE STRUCTURE

HIGH SPEED SOLIDIFICATIONAMORPHOUS STRUCTURE

LIQUIDGRAIN and GB



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Nucleation and growth, formation of GB in polycrystalline materials


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• MONOCRYSTALS: crystalline cells all oriented in the same (ex. Silicon for microelectronics)

• POLYCRYSTALS: crystalline cells oriented in the same way ONLY inside grains: solidification too quick to allow same orientation in entire lattice (ex .metals, alloys, ceramics)


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Monocrystal (a) …. polycrystal(b)

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Monocrystals• Difficult to obtain compared to

polycrystalline materials

• Silicon for electronics– Extra slow molten silicon solidification

(1-10 m per second)– About 1.5m length silicon cylinders

300 mm diameter, sliced as wafers– About 10 defects/ cm 2

http://www.csc.fi/elmer/examples/czmeltflow/growth.gifhttp://www.ami.bolton.ac.uk/courseware/mdesign/ch2/SingleCrystalSiliconIngot.jpg

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http://www.ingegneriadelsole.it/celle_silicio_cristallino.htm

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Polycristalline materials• Natural solidification from a liquid metal gives a

polycrystalline metal• Several grains, same cell orientation inside the

grain, grains separated by grain boundaries(gb)

http://mimp.mems.cmu.edu/~ordofmag/alumina.jpghttp://www.mse.nthu.edu.tw/jimages/Beuty/Steel1.jpg

Grains in Al2O3

Grains in steel


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GRAIN BOUNDARIES

• Irregular bonds, impurities, dislocations, atoms with lower coordination number,…

• High reactivity • Observable by optical

microscopy and chemical etching

• (file micr_acciai_ghise; video crystalline cells, video mech

properties)


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Optical and electron Microscopy

• Optical microscopy (2000x)• Scanning Electron Microscopy (SEM)

(104x)• Transmission Electron Microscopy

(TEM) (106x)


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20 µm

TiTi2(Co,Ni)

Ti(Co,Ni)

TiCo

SEM and compositional analysis (EDS)

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Atomic Force Microscopy (AFM) (109x)



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SOLIDIFICATION OF A LIQUID AND ITS STRUCTURE

1425 °C

or SiO2

a 1730 °C

SLOW SPEEDSOLIDIFICATIONMONOCRYSTALLINE STRUCTURE

MEDIUM SPEED SOLIDIFICATIONPOLYCRYSTALLINE STRUCTURE

HIGH SPEED SOLIDIFICATIONAMORPHOUS STRUCTURE

LIQUIDGRAIN and GB


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AMORPHOUS MATERIALS

• Disordered structure, no melting point

• glasses, polymers

• Short range order (few nm) (ex: Si4+ and O--ions are situated in tetrahedra in silicate glasses

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• Local, short range order; bond angle and length are variable

• No order after few nanometers

• “Undercooled liquids”• Quick solidification of a liquid• “No time to obtain a

crystalline structure” !

http://www.research.ibm.com/amorphous/figure1.gif

Amorphous silica

AMORPHOUS MATERIALS


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Amorphous materials: silica glasses

Si4+

O--

tetrahedron amorphous structure of silica: tetrahedra chains


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Crystalline and amorphous silica (SiO2)


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AMORPHOUS MATERIALS• Amorphous materials are not

thermodynamically stable: • They become crystalline if suitable

heated• All liquids can be solidified quickly

enough to obtain amorphous solids• All liquids can be solidified slowly enough

to obtain poly- or mono-crystalline solids


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• Ex: silica glass: bond angles, force and length differ in different tetraedra

• No melting temperature

• Viscosity vs temperature

• The opposite than with crystalline materials !

AMORPHOUS MATERIALS


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VISCOSITY

• Viscosity is a measure of a fluid's resistance to flow.

• It describes the internal friction of a moving fluid (i.e. tetrahedra chains in a silica glass).


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VISCOSITY

• Viscosity Unit : Poise

10 Poise = 1 Pa∙s = 1 (N/m2)∙s

• Example of viscosity: H2O (room T) = 1 x 10-3 Pa s

SiO2(silica) (1720°) = 1 x 106 Pa s


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VISCOSITY : AMORPHOUS and CRYSTALLINE materials

Viscosity decreases continuosly for amorphous materials.

This is not the case for crystalline materials

Al2O3

Glasses, polymers


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VISCOSITY

annealing

softening

Working point

Melting point



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Viscosity curve of a glass

Viscosity for glass and glass-ceramics

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glassGlass-ceramic

01 introd

Technology

Transcript of 01 introd