metallic glasses no disdain for disorder

Metallic Glasses - No Disdain for Disorder

by Professor Rainer J. Hebert

Professor Rainer J. Hebert, Institute of Materials Science, University of Connecticut

Corresponding author: [email protected]

Metallic glasses were borne out from rapid cooling experiments with binary metallic alloys in the late 1950s at theCalifornia Institute of Technology under the aegis of Pol Duwez. The idea was to quench molten metal mixtures veryrapidly and thereby bypass crystallization of the liquid alloy1. If crystallization, i.e., the formation of a crystallinephase from a parent liquid phase can be avoided, the atomic movement will be sufficiently restricted below a criticaltemperature for the liquid to be "frozen-in".

The concept of metallic glass as a frozen-in liquid suggests an atomic arrangement that resembles the atomicstructure of a liquid. It is indeed the hallmark of glasses that atoms are not arranged periodically over distances ofseveral interatomic distances or more. A transmission-electron microscopy (TEM) image of a bulk metallic Fe50Cr15

Mo14C15B6 glass is shown in Fig. 1. The image is typical for metallic glasses in that it does not reflect periodicity but

instead a pattern that is often referred to as "salt and pepper" structure. The question how exactly the atoms inmetallic glasses are arranged has captivated the glass research community for decades2-6. The recent, mostlycomputer simulation-based research indicates that a significant fraction of atoms in metallic glasses are arranged inclusters with sizes of a few nanometers. These clusters might reveal some connectivity3,4. It has furthermore beendemonstrated that even small amounts of alloying additions can drastically affect the fraction of atoms that arearranged in clusters3. Advances in computer simulations, first principle calculations, and atomic level experimentalcharacterization of materials promises further advances and insight in the near future into the "structure" of metallicglasses, the dependence of cluster volume fractions on composition, and the thermal history.


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Fig.1: TEM image of bulk metallic Fe50Cr15Mo14C15B6 glass.

Synthesis of Metallic GlassesA common strategy to synthesize metallic glasses is to quickly limit the mobility of atoms and thus to prevent atomsfrom moving into crystal lattice positions. Atomic mobility can be limited, for example, with rapid quenching fromthe liquid7 or vapor state8,9 onto substrates with a high thermal conductivity. Deposition techniques, includingelectro-deposition or vapor phase deposition are additional synthesis techniques that have proven successful formetallic glass synthesis. An entirely different approach is based on the destabilization of crystalline materials.Intense deformation10-12, irradiation with electrons13 or ions14, or even loading with hydrogen15 are approachesthat destabilized crystalline pre-cursor materials and induced amorphous phases.

Even at the highest cooling rates not all alloys can be quenched into glassy phases. Only specific compositions canform metallic glasses. The necessity to avoid crystallization during quenching suggests a low liquidus temperaturefor glass forming compositions, i.e. a low solidification onset temperature during quenching. The liquid transformsinto a frozen-in liquid when the viscosity exceeds about 1015 poise16 and the temperature range at which theviscosity crosses this critical value represents the glass transition range. For most glass forming compositions, theratio of liquidus and glass transition temperature exceeds a value of about 0.5-0.6. To synthesize bulk metallicglasses Inoue proposed three empirical rules: the number of components in the alloy should exceed two, the sizedifference should be more than 12 %, and the heat of mixing between the major alloy components should benegative17. Exceptions to these rules are known and several additional criteria have been developed to addressthese exceptions18.

From the requirement for high cooling rates to bypass crystallization, it is clear that metallic glasses can not be castin large sizes. Currently, the "record" size is 72 mm diameter for a Pd40Cu30Ni10P20 bulk metallic glass19.

Properties and Applications of Metallic GlassesThe lack of long-range periodicity in metallic glasses precludes the plastic deformation mechanisms that areoperative in crystalline materials. The mechanical properties of metallic glasses are characterized by a large elasticlimit of about 2 %--compared with about 0.2 % for crystalline metallic materials-and yield strength values that areabout 1.5 to twice of those of their crystalline counterparts20. For example, tensile strength levels were reported forAl-based metallic glasses of up to 1500 MPa21 compared to about 750 MPa for the strongest crystalline Al alloys.Co-based bulk metallic glasses were measured with yield strengths of about 5 GPa22. These strength levels,however, only occur in compression. In tension much lower strength levels are observed. The lack of tensilestrength follows from the deformation mechanism of metallic glasses that is based on shear bands. Duringdeformation at room temperature, metallic glasses slide internally along bands with thicknesses of about 10-20 nm



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that can propagate through the entire sample if they are not impeded, for example, by precipitates. The challengewill remain for the foreseeable future to design metallic glasses with improved ductility but without loss in strengthand elastic limit.

Metallic glasses differ greatly in their solute content compared with engineering alloys. The solute content in metallicglasses is typically on the order of tens of percent and thus far exceeds the solute content of conventionalengineering alloys. At the same time, fully amorphous alloys are homogeneous. The combination of homogeneity,lack of grain boundaries, and concentrated solute content can play out very favorably for corrosion properties23.

The unique properties of metallic glasses appeal to a range of applications24,25. Structural applications includesporting goods such as baseball bats or tennis rackets, where metallic glasses excel with their high elastic limits,micro-meter sized gears and springs that reveal exceptional wear resistance26, biomedical applications such astooth implants, or casings for electronic devices. Metallic glasses have been used since the late 1960s for magneticapplications, for example, as transformer core materials27. With an ever expanding range of glass-forming systems,processing improvements, and a better understanding of fundamental properties the number of applicationscontinues to increase28. Once thought of as a lab curiosity, metallic glasses have come a long way, but still provideample opportunities for new discoveries.

References1. Duwez, P., The Edward DeMille Campbell Memorial Lecture for 1967. Transactions of the ASM, 1967. 60: p.

606-633.2. Bakai, A.S., et al., Field emission microscopy of the cluster and subcluster structure of a Zr-Ti-Cu-Ni-Be bulk

metallic glass. Low Temperature Physics, 2002. 28(4): p. 400-5.3. Cheng, Y.Q., E. Ma, and H.W. Sheng, Atomic level structure in multicomponent bulk metallic glass. Physical

Review Letters, 2009. 102(24): p. 245501 (4 pp.).4. Takeuchi, A., et al., Molecular dynamics simulations of critically percolated, cluster-packed structure in

Zr-Al-Ni bulk metallic glass. J. Mat. Sci., 2010. 45: p. 4898-4905.5. Bernal, J.D., Geometry of the structure of monatomic liquids. Nature, 1960. 185: p. 68-70.6. Polk, D.E., The structure of glassy metallic alloys. Acta met., 1972. 20: p. 485-491.7. Falkenhagen, G. and W. Hofmann, Die Auswirkung extrem hoher Abkuehlungsgeschwindigkeit auf die

Erstarrung und Gefuege binaerer Legierungen. Z. Metallkde., 1952. 43: p. 69.8. Buckel, W., Z. Phys., 1954. 138: p. 136.9. Kramer, J., The amorphous state of metals. Z. Phys., 1936. 106: p. 675-691.

10. Koch, C.C., et al., Preparation of 'amorphous' Ni60Nb40 by mechanical alloying. Appl. Phys. Lett., 1983.43(11): p. 1017-1019.

11. Sagel, A., et al., Synthesis of an amorphous Zr-Al-Cu-Ni alloy with large supercooled liquid region bycold-rolling of elemental foils. Acta mater., 1998. 46(12): p. 4233-4241.

12. Atzmon, M., K.M. Unruh, and W.L. Johnson, Formation and characterization of amorphous erbium-basedalloys preapred by near-isothermal cold-rolling of elemental composites. J. Appl. Phys., 1985. 58(10): p.3865-3870.

13. Mori, H., H. Fujita, and M. Fujita, Electron irradiation induced amorphization at dislocations in NiTi. JapaneseJournal of Applied Physics, Part 2 (Letters), 1983. 22(2): p. 94-6.

14. Hung, L.S., et al., Ion-induced amorphous and crystalline phase formation in Al/Ni, Al/Pd, and Al/Pt thinfilms. Applied Physics Letters, 1983. 42(8): p. 672-4.

15. Aoki, K. and T. Masumoto, Hydrogen-induced amorphization of intermetallics. Journal of Alloys andCompounds, 1995. 231(1-2): p. 20-28.

16. Turnbull, D., Under what conditions can a glass be formed? Contemp. Phys., 1969. 10(5): p. 473-488.17. Inoue, A., Bulk amorphous alloys: preparation and fundamental characteristics. Materials Science

Foundations. Vol. 4. 1998, Uetikon-Zuerich: Trans-Tech Publications.18. Suryanarayana, C. and A. Inoue, Glass-forming ability of alloys, in Bulk metallic glasses. 2011, CRC Press. p.

49-135.19. Inoue, A., N. Nishiyama, and H. Kimura, Preparation and thermal stability of bulk amorphous Pd40Cu30Ni10P

20 alloy cylinder of 72 mm in diameter. Mater. Trans. JIM, 1997. 38: p. 179-183.20. Schuh, C.A., T.C. Hufnagel, and U. Ramamurty, Overview No.144 - Mechanical behavior of amorphous alloys.

Acta Materialia, 2007. 55(12): p. 4067-4109.21. Yeong-Hwan, K., A. Inoue, and T. Masumoto, Ultrahigh mechanical strengths of Al88Y2Ni10-xMx (M=Mn, Fe

or Co) amorphous alloys containing nanoscale FCC-Al particles. Materials Transactions, JIM, 1991. 32(7): p.599-608.

22. Inoue, A., et al., Ultra-high strength above 5000 MPa and soft magnetic properties of Co-Fe-Ta-B bulk glassyalloys. Acta mater., 2004. 52: p. 1631-1637.

23. Scully, J.R., A. Gebert, and J.H. Payer, Corrosion and related mechanical properties of bulk metallic glasses.Journal of Materials Research, 2007. 22(2): p. 302-313.



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24. Telford, M., The case for bulk metallic glass. Materials Today, 2004. 7(3): p. 36-43.25. www.liquidmetal.com.26. Ishida, M., et al., Wear resisitivity of super-precision microgear made of Ni-based metallic glass. Mater. Sci.

Engr. , 2007. A 449-451: p. 149-154.27. McHenry, M.E., M.A. Willard, and D.E. Laughlin, Amorphous and nanocrystalline materials for applications as

soft magnets. Progress in Materials Science, 1999. 44(4): p. 291-433.28. Greer, A.L., Metallic glasses...on the threshold. Materials Today, 2009. 12(1-2): p. 14-22.

Copyright AZoM.com, Professor Rainer J. Hebert (University of Connecticut )Date Added: Dec 13, 2010 | Updated: Nov 4, 2012



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metallic glasses no disdain for disorder

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