MECHANISMS OF FORMATION OF SUBMICRON GRAIN STRUCTURES DURING SEVERE DEFORMATION

摘要

Severe deformation processing is an emerging method for refining the grain structures of conventional alloys to submicron levels. Although severely deformed metallic alloys generally still contain a significant proportion of low angle boundaries and can be inhomogeneous, materials can be produced using this approach that contain a high angle grain boundary (HAGB) fraction of greater than 70% and with HAGB spacings in the submicron range. In this paper the mechanisms by which grain refinement takes place over the extreme strain range exploited in severe deformation processing are explored, using an example of a simple Al-0.13%Mg model alloy. Grain refinement occurs through grains subdividing by new HAGBs being formed discontinuously, on finer and finer length scales, until a limit is reached where the grain size merges with the subgrain size, which is the smallest available length scale within which dislocations are absorbed. The formation of new HAGBs is a natural consequence of crystal plasticity that is initiated by grains splitting into deformation bands, and is promoted by heterogeneity and instability in plastic flow.