Atomistic simulation study of the shear-band deformation mechanism in Mg-Cu metallic glasses

We have simulated plastic deformation of a model Mg-Cu metallic glass in order to study shear banding. In uniaxial tension, we find a necking instability occurs rather than shear banding. We can force the latter to occur by deforming in plane strain, forbidding the change of length in one of the transverse directions. Furthermore, in most of the simulations a notch is used to initiate shear bands, which lie at a 45° angle to the tensile loading direction. The shear bands are characterized by the Falk and Langer local measure of plastic deformation $D_{\min }^2$, averaged here over volumes containing many atoms. The $D_{\min }^2$ profile has a peak whose width is around $10\mathrm{nm}$; this width is largely independent of the strain rate. Most of the simulations were, at least nominally, at $100\mathrm{K}$, about $T_g∕3$ for this system. The development of the shear bands takes a few tens of ps, once plastic flow has started, more or less independent of strain rate. The shear bands can also be characterized using a correlation function defined in terms of $D_{\min }^2$, which, moreover, can detect incipient shear bands in cases where they do not fully form. By averaging the kinetic energy over small regions, the local temperature can be calculated, and this is seen to be higher in the shear bands by about $50–100\mathrm{K}$. Increases in temperature appear to initiate from interactions of the shear bands with the free surfaces and with each other, and are delayed somewhat with respect to the localization of plastic flow itself. We observe a slight decrease in density, up to 1%, within the shear band, which is consistent with notions of increased free volume or disorder within a plastically deforming amorphous material.