The Mohr-Coulomb criterion from unit shear processes in metallic glass

The experimental and simulation-based evidence for asymmetric plastic yielding of metallic glasses is reviewed, and discussed within the framework of the Mohr-Coulomb criterion. The weight of evidence is supportive of normal stress-dependent yielding, although it is difficult to ascribe the origin of this effect to a particular mechanism given the myriad of complex physical processes that occur during plastic flow of metallic glasses. Using simple unit models of four- or nine-atom 'shear transformation zones', we show that Mohr-Coulomb yielding arises even at this very fine scale. Furthermore, by considering the topological difference between the four- and nine-atom models, we illustrate why the degree of the yield asymmetry in metallic glasses can vary over an appreciable range. We also discuss a potential breakdown of the Mohr-Coulomb criterion for states of high tensile stress in light of these unit models. (C) 2004 Elsevier Ltd. All rights reserved.