FIGURE 27 Isothermal formation of a metastable metallic glass by interdiffusion at temperature Td between two crystalline phases A and B when the stable phases A3B and AB do not nucleate. Tg=glass transition temperature. From Perepezko.47

the carbon content of the low-carbon regions becomes virtually nil, and in view of the coherency between the high- and low-carbon modulations, the elastic strains may reach the incredible level of 17 percent. It has not been realized before that such layered structures and the attendant coherency strains might be playing a substantial role in the classic strengthening of iron-carbon martensites—a matter of vast industrial importance. This long-standing challenge in ferrous metallurgy has defied all theories of steel hardening over the years, but now the existence of fine-scale compositional modulations offers a new outlook for a reinvigorated theoretical attack on the problem.

The use of layered structures for interdiffusion measurements is well known45 and need not be reviewed here. However, there is one related phenomenon that warrants particular attention as a technique for synthesizing bulk metallic glasses.46 In particular, the hypothetical alloy system shown in Figure 27 can form metastable intermediate phases if the more stable intermediate phases are sufficiently slow to nucleate during interdiffusion between layers of the pure metals A and B.47 In such a case, one of the metastable phases may be a liquid, and, if the interdiffusion temperature happens to lie below the glass transition temperature of the supercooled liquid, a metallic glass will then form at the expense of the crystalline A and B components, starting at each A/B interface in the multilayered specimen, as in Figure 28.46 Examples of alloy systems that behave in this way are gold-lanthanum, nickelhafnium, and cobalt-zirconium; in each instance, the first component is a “fast diffuser” whereas the second is a “slow diffuser.” Apparently, this condition tends to inhibit the nucleation of crystalline phases while favoring the rapid formation and thickening of a glassy phase.47

In a real sense under these circumstances, the two crystalline metals A and B dissolve in, or melt into, the growing glassy phase, notwithstanding the relatively low interdiffusion temperature involved. This process can result in the production of homogeneous glassy alloys for the kinds of bulk property measurements and test purposes that have not been feasible before. It is also

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