Development of visco-plastic deformation in metallic glasses

Abstract Inelastic deformation of metallic glasses above 0.6 Tg is envisioned to be the result of uncorrelated shear transformations. In the early stages of deformation when all transformations are spatially isolated, back stresses set up in the transformation sites by the undeformed surroundings can fully reverse the deformation upon removal of the applied stress, and the deformation is purely anelastic. With larger accumulations of inelastic strain, the probability increases for transformed volume elements to acquire additional, transformed near neighbors. Interpreting experimental results with the help of a probabilistic theoretical model that is proposed has suggested that when the number of contacts between transformed near neighbor regions exceeds two, back stresses in the central transformations are lost and they become visco-plastic. Substantial relief of back stresses in two or more adjacent contacting transformed volume elements is expected to be by free volume redistribution that becomes possible when the deformed region is large enough. Based on the model it is predicted that transient inelastic shear strains in excess of 0.02 are required before steady state flow can be achieved. Finally, the model permits also the reaching of some basic conclusions on structural relaxations vis-a-vis unrestrained visco-plastic flow.