Metallic Glasses - No Disdain for Disorder

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

The concept of metallic glass as a frozen-in liquid suggests an atomic arrangement that resembles the atomic structure of a liquid. It is indeed the hallmark of glasses that atoms are not arranged periodically over distances of several interatomic distances or more. A transmission-electron microscopy (TEM) image of a bulk metallic Fe₅₀Cr₁₅Mo₁₄C₁₅B₆ 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 in metallic glasses are arranged has captivated the glass research community for decades^2-6. The recent, mostly computer simulation-based research indicates that a significant fraction of atoms in metallic glasses are arranged in clusters with sizes of a few nanometers. These clusters might reveal some connectivity^3,4. It has furthermore been demonstrated that even small amounts of alloying additions can drastically affect the fraction of atoms that are arranged in clusters³. Advances in computer simulations, first principle calculations, and atomic level experimental characterization of materials promises further advances and insight in the near future into the "structure" of metallic glasses, the dependence of cluster volume fractions on composition, and the thermal history.

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

Synthesis of Metallic Glasses

A common strategy to synthesize metallic glasses is to quickly limit the mobility of atoms and thus to prevent atoms from moving into crystal lattice positions. Atomic mobility can be limited, for example, with rapid quenching from the liquid⁷ or vapor state^8,9 onto substrates with a high thermal conductivity. Deposition techniques, including electro-deposition or vapor phase deposition are additional synthesis techniques that have proven successful for metallic glass synthesis. An entirely different approach is based on the destabilization of crystalline materials. Intense deformation^10-12, irradiation with electrons¹³ or ions¹⁴, or even loading with hydrogen¹⁵ are approaches that 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 can form metallic glasses. The necessity to avoid crystallization during quenching suggests a low liquidus temperature for glass forming compositions, i.e. a low solidification onset temperature during quenching. The liquid transforms into a frozen-in liquid when the viscosity exceeds about 10¹⁵ poise¹⁶ and the temperature range at which the viscosity crosses this critical value represents the glass transition range. For most glass forming compositions, the ratio of liquidus and glass transition temperature exceeds a value of about 0.5-0.6. To synthesize bulk metallic glasses Inoue proposed three empirical rules: the number of components in the alloy should exceed two, the size difference should be more than 12 %, and the heat of mixing between the major alloy components should be negative¹⁷. Exceptions to these rules are known and several additional criteria have been developed to address these exceptions¹⁸.

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

Properties and Applications of Metallic Glasses

The lack of long-range periodicity in metallic glasses precludes the plastic deformation mechanisms that are operative in crystalline materials. The mechanical properties of metallic glasses are characterized by a large elastic limit of about 2 %--compared with about 0.2 % for crystalline metallic materials-and yield strength values that are about 1.5 to twice of those of their crystalline counterparts²⁰. For example, tensile strength levels were reported for Al-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 tensile strength follows from the deformation mechanism of metallic glasses that is based on shear bands. During deformation at room temperature, metallic glasses slide internally along bands with thicknesses of about 10-20 nm that can propagate through the entire sample if they are not impeded, for example, by precipitates. The challenge will remain for the foreseeable future to design metallic glasses with improved ductility but without loss in strength and elastic limit.

Metallic glasses differ greatly in their solute content compared with engineering alloys. The solute content in metallic glasses is typically on the order of tens of percent and thus far exceeds the solute content of conventional engineering 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 applications^24,25. Structural applications include sporting 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 as tooth implants, or casings for electronic devices. Metallic glasses have been used since the late 1960s for magnetic applications, 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 applications continues to increase²⁸. Once thought of as a lab curiosity, metallic glasses have come a long way, but still provide ample opportunities for new discoveries.

References

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