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A Novel Low-Density, High-Hardness, High-entropyAlloy with Close-packed Single-phase NanocrystallineStructuresKhaled M. Youssefa, Alexander J. Zaddachb, Changning Niub, Douglas L. Irvingb & Carl C.Kochba Department of Materials Science and Technology, Qatar University, PO Box 2713, Doha,Qatarb Department of Materials Science and Engineering, North Carolina State University,Raleigh, NC 27695-7907, USAPublished online: 09 Dec 2014.

To cite this article: Khaled M. Youssef, Alexander J. Zaddach, Changning Niu, Douglas L. Irving & Carl C. Koch (2014): ANovel Low-Density, High-Hardness, High-entropy Alloy with Close-packed Single-phase Nanocrystalline Structures, MaterialsResearch Letters, DOI: 10.1080/21663831.2014.985855

To link to this article: http://dx.doi.org/10.1080/21663831.2014.985855

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Mater. Res. Lett., 2014http://dx.doi.org/10.1080/21663831.2014.985855

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A Novel Low-Density, High-Hardness, High-entropy Alloy with Close-packedSingle-phase Nanocrystalline Structures

Khaled M. Youssefa, Alexander J. Zaddachb , Changning Niub, Douglas L. Irvingb and Carl C.Kochb

aDepartment of Materials Science and Technology, Qatar University, PO Box 2713, Doha, Qatar; bDepartment ofMaterials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA

(Received 14 May 2014; final form 3 November 2014)

A low-density, nanocrystalline high-entropy alloy, Al20Li20Mg10Sc20Ti30 was produced by mechanical alloying. It formed asingle-phase fcc structure during ball milling and transformed to single-phase hcp upon annealing. The alloy has an estimatedstrength-to-weight ratio that is signicantly higher than other nanocrystalline alloys and is comparable to ceramics. High hardnessis retained after annealing.

Keywords: Mechanical Alloying, X-ray Diraction, Hardness, Nanocrystalline, High-entropy Alloys

High-entropy alloys are a new class of multi-component alloy systems in which the design of thealloys is based not on adding solutes to a single baseelement, but rather on choosing elements that will formsolid solutions when mixed at near-equiatomic con-centrations. This behavior has been attributed to thelarge congurational entropy when ve of more ele-ments at near-equiatomic ratios are mixed together.[1,2]These high-entropy alloys (HEAs) have demonstratedthe potential for superior properties in many cases. Afundamental understanding of the mechanisms of phasestability of HEAs is still a topic of active research. Guoand Liu demonstrated that compositions likely to formsolid solution HEAs have enthalpies of mixing between22 and 7 kJ/mol and very small dierences in atomicradius. The small enthalpy of mixing allows the cong-urational entropy to dominate the free energy and it hasbeen proposed that the system behaves, in this case, morelike an ideal solution.[3] Otto et al. recently published asystematic investigation on the eect of entropy on phasestability of HEAs.[4] The authors demonstrated that highcongurational entropy is not the only sucient criterionto predict whether an equiatomic multi-component alloywill form as a single-phase solid solution. In their study,they found only NiFeCrCoMn to be a single-phase fccsolid solution and HEA. These ndings are consistent

*Corresponding author. Email: cckoch@ncsu.edu

with the work of Guo and Liu [3] as all the enthalpiesof mixing in the respective binary alloys are relativelysmall and the atoms are similar in size. The authors thenreplaced individual elements in an equiatomic NiFe-CrCoMn alloy one at a time with elements that havethe same room temperature crystal structure, similaratomic size, and similar electronegativity as comparedwith the elements being replaced. All other substitu-tions to the NiFeCrCoMn alloy led to a multi-phasealloy. Although the replacement elements were simi-lar according to Hume-Rothery rules, the authors foundthat the substituted elements had a stronger propen-sity to form secondary phases or intermetallics in theirrespective binary alloys. This led the system to act lesslike an ideal solution and resulted in the formation ofa multi-phase alloy with a reduced overall entropy.[4]There has also been interest in nding descriptors to pre-dict the likely crystallographic lattice (fcc or bcc) thatthe solid solution will form upon. One descriptor thathas been reasonably successful in predicting the latticestructure is the average valence concentration (or aver-age periodic table group number) (VEC).[5] Fcc phaseswere determined to be more stable at higher values ofVEC ( 8) while bcc phases were stable at lower VEC(< 6.87). On the boundary between fcc and bcc(VEC = 8), the systems were found to be predominantly

2014 The Author(s). Published by Taylor & Francis.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The moralrights of the named author(s) have been asserted.

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fcc but in some situations the bcc phase has been seen inminute amounts.

A quantitative prediction of whether multi-component systems would form into solid solutions(HEAs), intermetallic containing alloys, or metallicglasses has been presented by Yang and Zhang.[6] Theyproposed parameters and , where is dened asa parameter of the entropy of mixing times the aver-age melting temperature of the elements divided bythe enthalpy of mixing. The parameter is the meansquare deviation of the atomic size of the elements. Theirmodel shows that the high-entropy stabilized solid solu-tion alloys occur for values of 1.1 and 6.6%.These quantitative predictions are mostly consistent withthe empirical data in the literature albeit with a fewexceptions.[6]

While there is continuing high interest in devel-opment of alloys with low densities along with highstrength for energy-saving applications such as in trans-portation and energy, to date there have only been a fewreports of studies of low-density HEAs. For purposes ofour discussion we dene low density as less than 3 gcm3. Juan et al. [7] reported an Al20Be20Fe10Si15Ti35alloy which had an as-cast microstructure composedof one major and two minor phases. It had a densityof 3.91 g cm3. Li et al. [8,9] studied Mg-containingHEAs. These authors prepared Mgx(MnAlZnCu)100xalloys by induction melting and casting.[8] The alloyshad multi-phase structures, composed mainly of hcpphases and AlMn icosahedral quasicrystalline phases.Depending on the composition, the alloys had densi-ties of 2.204.29 g cm3. They exhibited good com-pressive strengths (400500 MPa) at room temperature.However, ductilities were low, 35% in compression.Similar structures and mechanical properties were sub-sequently observed in the equiatomic system cast andcooled in air, water, or brine solutions.[9] Chen et al.used mechanical alloying to prepare BeCoMgTi andBeCoMgTiZn, but the alloys formed only an amor-phous phase.[10] To our knowledge, no single-phaselow-density high-entropy alloy (LDHEA) has beenreported.

In this paper, we report our results on the process-ing, structure, and mechanical hardness of a low-densityHEA.

Phase formation for HEAs can be predicted by cal-culating the parameters and , proposed by Yang etal.[6] represents the competition between enthalpyand entropy, given by

= TmSmix|Hmix| , (1)

where Tm =

ci(Tm)i is the melting temperature ofthe alloy, Smix = R

ci ln ci is the entropy of mix-

ing, and Hmix =

i

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with Cu K radiation. Analysis of the crystal structuresof the contaminated alloys was done using the PM2K Whole Powder Pattern Modeling software.[13] Sam-ples were compacted into 6.25 mm diameter disks 3 mmthick under 2 GPa pressure. Microhardness of the milledpowder, compacted to full density at room temperature,was measured using a Buehler Micromet microhardnesstester with a Vickers indenter at 50 g load and a load-ing time of 15 s. Under these conditions, the indentationdepth is approximately 5.9 m.

A transmission electron microscopy (TEM) speci-men of the as-milled powder was prepared by compact-ing the powder into a 3 mm diameter disk. Thinningwas accomplished using a twin-jet electropolisher witha solution of perchloric acid and methanol. TEM anal-ysis was conducted using a JEOL 2000FX TEM at anaccelerating voltage of 200 kV.

The density functional theory (DFT) calculationswere performed by the package of the exact mun-tin orbital method combined with coherent potentialapproximation or EMTO-CPA.[14] This computationalmethod has been widely used for solid solution systems,for example, stainless steels [15] and NiFeCrCoMn.[16]

The results of our mechanical alloying of theselected alloy, Al20Li20Mg10Sc20Ti30, must be dividedinto two materialsone which contained mainly theve components intentionally added, and another, whichbesides these components had signicant impurity con-tent of nitrogen and oxygen.

The material prepared by mechanical alloyingwhich did not have the high N, O impurity levels wasobserved to have a single-phase fcc crystal structure inthe as-milled condition, as illustrated in Figure 1. It had ananocrystalline grain size estimated by the Scherrer for-mula to be about 12 nm. The lattice parameter of thissample determined by the method of Cohen was foundto be 0.4323 nm. The mechanical hardness of this sam-ple was very high 5.8 GPa. After annealing this sampleat 500C for 1 h, the crystal structure changed as shown

Figure 1. XRD pattern of as-milled alloy.

Figure 2. XRD pattern of uncontaminated alloy after anneal-ing at 500C.

in Figure 2. The new structure was indexed as hcp withc/a ratio of 1.588. The grain size, again estimated by theScherrer method, was 26 nm and the mechanical hard-ness dropped to 4.9 GPa. TEM measurements of theas-milled material, shown in Figure 3, generally agreewith the XRD results, giving an average grain size of20 nm and showing no second phases in the diractionpattern.

The material with the higher N and O concentra-tions (0.4 at% N, 1.39 at.% O as determined by chemicalanalysis) apparently obtained this contamination from abatch of Sc powder which was contaminated during theprocess of converting the as-received pellets into powderby cryomilling. Subsequent batches did not contain theseimpurities and were used in the sample described above.The contaminated sample exhibited an as-milled struc-ture essentially identical to that of the un-contaminatedsample, that is, single-phase fcc with a grain size ofabout 12 nm. However, it had a slightly higher mechan-ical hardness of 6.1 GPa. After annealing at 500C and800C, the structure remained similar to fcc, but somesplitting of the diraction peaks was observed in theXRD results, suggesting either a slight distortion from aperfect cubic lattice or separation into two similar phaseswith a lattice parameter dierence

Mater. Res. Lett., 2014

(a) (b) (c)

Figure 3. (a) Bright eld TEM image and diraction pattern, (b) dark eld image, and (c) grain size distribution of as-milleduncontaminated material.

Table 2. Properties of alloys prepared.

Alloy Annealing temp. (C) Crystal structure X-ray grain size (nm) Hardness (GPa)

Al20Li20Mg10Sc20Ti30 as milled fcc 12 5.8500 hcp 26 4.9

Al20Li20Mg10Sc20Ti30 w/ N, O as milled fcc 12 6.1500 fcca nca 5.9800 fcca nca 5.75

aThe N, O-containing alloys exhibited slight phase separation or distortion upon annealing resulting in a structure similarto fcc. Because the diraction peaks are split, measurement of the grain size by line prole analysis is inaccurate, but thebroadness of the peaks indicates a nanocrystalline structure.

to exhibit a single-phase fcc solid solution in spite ofthe fact that many of the binary systems in this groupexhibit immiscibility (high positive heats of mixing) orform many intermediate phases (high negative heats ofmixing). Comparisons of the enthalpy of formation ofthe solid solution fcc and hcp crystal structures as calcu-lated by DFT show that the hcp phase is the energetically

preferred phase, with an enthalpy of formation 0.977 kJmol1 smaller than the fcc phase.

The sample containing oxygen and nitrogen likelydid not transform to hcp upon annealing as the hcp cellcontains smaller octahedral interstitial sites. The amountof space for an interstitial atom can be approximated asthe distance from the center of the interstitial site to the

Figure 4. Ashby plot of strength vs. density for engineering materials. (Yield strength for metals and polymers, tear strength forelastomers, compressive strength for ceramics,[19] and tensile strength for composites.) Reproduced with permission from Elsevier2010.[20]

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nearest atomic position minus the average atomic radiusof the alloy. Using the lattice parameters measured byXRD, for the fcc octahedral site this distance is 64.4pm, while for the hcp octahedral site it is 54.7 pm, 15%smaller.

The high mechanical hardness of these alloys canpartly be explained by their nanocrystalline grain sizes.However, their hardness values exceed nanocrystallineAl base alloys, for example, by factors of 23 times.[17]Because the equilibrium phase is hcp, this suggests thatthe stacking fault energy of the fcc phase is very low,which may also contribute to the high strength.[18] Acomparison of the strength (estimated as hardness/3) anddensity of these alloys with other structural materials isgiven in Figure 4. It is clear that these materials are com-parable in strength/density values to ceramic materialssuch as SiC. However, our metallic alloys should exhibitmuch higher toughness and ductility, which is indicatedfrom the tendency of the powder to weld during milling,the near full density of the compacted powder at roomtemperature, and the absence of micro cracks around theindents during microhardness indentation.

A low-density (2.67 g cm3) high-entropy alloywith composition Al20Li20Mg10Sc20Ti30 has been pre-pared by mechanical alloying of elemental powders. Theas-milled structure is single-phase fcc, with a nanocrys-talline grain size of 12 nm, and a mechanical hardness of5.9 GPa. The sample without O, N contamination trans-forms to the hcp structure on annealing at 500C. Thesample containing O, N does not transform, but ratherexhibits either a slight phase separation or unit cell dis-tortion upon annealing at 800C. Calculations of theenergies of the competing structures are consistent withthe experimental observations. This material exhibits acombination of hardness (strength) and low density thatis not equaled by any other metallic material.

These results need to be expanded to a study of thisalloy system with various compositions, preparation byprocesses closer to equilibrium than mechanical alloy-ing, and a detailed study of the mechanical behavior ofthese unique metallic materials.

Acknowledgements The authors wish to acknowledge thepartial support of this research by the National Science Foun-dation from the Metals and Metallic Nanostructures programunder grant DMR-1104930.

Disclosure statement No potential conict of interest wasreported by the author(s).

ORCIDAlexander J. Zaddach http://orcid.org/0000-0003-0530-0427

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AcknowledgementsDisclosure statementORCIDReferences