FIGURE 28 Schematic representation of metallic glass interdiffusion layer growing isothermally between crystalline A-rich and B-rich phases, corresponding to metastable equilibrium relationships at temperatures Td in Figure 27. From Schwarz and Johnson.46 Reprinted with permission.

evident that, in such metallic systems, bulk glasses can likewise be synthesized from compacted powders of the component metals, thus greatly facilitating appropriate specimen preparation. One can easily visualize new directions of research activity emerging from these recent studies of interdiffusion on a fine scale.

SOME SPECIAL METALLIC SYSTEMS FOR STRUCTURAL PURPOSES
Ductile Ordered Alloys

Long-range ordered phases and intermetallic compounds have long been a familiar subject in physical metallurgy, particularly from the standpoints of crystal structure, thermodynamic equilibrium, kinetics of formation, and second-phase strengthening dispersions. Both long- and short-range ordering have been studied in detail. But more recently there has been a new wave of interest in such alloy systems because of the ductility-related mechanical behavior of aluminides and similar phases, especially those having cubic Ni3Al-type (L12) structures. It is a nice coincidence that intermetallic compounds have also come into prominence because of unusual ferromagnetic properties; this is discussed later in the section on magnetic alloys.

Excellent reviews of structure-property relationships in ductile ordered alloys have appeared within the past few years.4853 One reason for this focus has been the surprising degree of ductility and fabricability exhibited by these materials—a kind of mechanical behavior not commonly associated with intermetallic phases. It turns out that many L12 compounds are ductile at ambient temperatures both in the monocrystalline and polycrystalline states,



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