The first aim of this project was to disclose the atomic-level mechanism underlying elastic and plastic deformation in MGs. By simulating athermal quasistatic shearing processes, we concluded that shear transformation zones (STZ) start to activate from a very early stage of loading (see Fig. 1 a) and the variation in their magnitude defines the boundary between elastic and plastic events. During loading, elastic heterogeneities undergo a continuous distortion that generates small strain variations and elastic excitations. Eventually, the elastic events exceed the local yield stress and the energy overcomes the barrier for large plastic strain. In general, during STZ activation the complex strain and the corresponding directional rotation fields display quadrupolar distributions (Fig.1 b-c) with long-range signatures that overlap to some extent, constraining the atomic rearrangement during plastic, and even elastic unloading.
As the propagation of a shear band is a stress-driven process [5], we designed and tested the stress-modulated heterogeneity MG structures. The correlation between the deformation behavior and the effect of stress and structure heterogeneities in MGs was investigated. In general, strain hardening together with enhanced tensile ductility in monolithic MGs can be attained by only modulating the internal stress state without changing their local structure. The stress heterogeneity caused by the residual stresses can change the shear band dynamics leading to the formation and interaction of multiple shear bands, which consequently enhances the deformability. Moreover, the residual compressive stress offsets the external tensile stress, which delays shear band formation and, therefore, is responsible for efficient strain hardening and toughness.
Through systematic control of the annealing parameters, we have been able to simulate MGs resembling structures usually obtained by quenching at cooling rates used in the melt spinning process. Density, local order, and local entropy calculations predict MGs with structures prepared at cooling rate orders of magnitude lower than those typically realized in atomistic modeling. Hence, annealing above Tg is an alternative to melt quenching simulations and offers the prospect of modeling well relaxed glassy structures that were not achievable before in molecular dynamics simulations.