Superplasticity of Mechanically Alloyed Nanocrystalline and Amorphous Materials

Abstract

A wide variety of natural and technical materials exhibit superplastic or superplastic-like flow under external or internal stress in certain temperature regimes. Phenomenological analysis of mass-transfer mechanics producing such flow gives the basis for dividing these materials into two principal groups: (1) polycrystalline superplastic materials which exhibit so called fine-structure superplasticity i.e. superplastic flow where mass-transfer units are crystalline grains of size in micro-, submicro- or nanometer range; and (2) materials exhibiting superplastic-like flow where the basic mass-transfer units are either single atoms (molecules) or groups of them. The latter group includes inorganic non-metallic and metallic glass-forming systems and polymers. Amorphous alloys (metallic glasses) with small volume fraction of nanocrystalline dispersed phase are known to have very high mechanical strength and low ductility at low and high temperatures compared with those for crystalline alloys. On the other hand, some recently developed bulk amorphous alloys with large glass-forming ability have shown striking superplastic-like behavior and very high formability in supercooled liquid state which seems to be promising for future development as a new type of superplastic metallic materials [1–3]. One of the most powerful tools for producing bulk amorphous alloys is severe plastic deformation (SPD) in the form of mechanical alloying. Other forms of SPD can be used to yield traditional fine-structure superplastic alloys.