Atomistic origin of stress overshoots and serrations in a CuZr metallic glass

Abstract

In this work we use molecular dynamics simulations to study the stress overshoots of metallic glass Cu50Zr50 in three scenarios (unloading-reloading, slide-stop-slide, and stress serrations) that are associated with shear band relaxation. We found that, after the elastic recovery effect is factored out, atomic volume in the shear band barely changes during compressive relaxation but decreases during tensile relaxation, while local fivefold symmetry increases consistently for both cases. We propose that the atomistic mechanism for the related stress overshoots is due to the relaxation of structural symmetry, instead of free volume, in the shear band. Upon unloading, a propagating shear band continues for some time before arrested, which results in a stress undershoot and could contribute to material fatigue under cyclic elastic loads. We did not directly observe stress serrations via molecular dynamics simulations due to the very high simulated strain rates. While athermal quasistatic simulations produce serrated flow stress, we note that such serrations result from global avalanches of shear events rather than the relaxation of the shear band.

Mechanical Properties of Glassy Nanopillars: A Comparative, Computational Study of Size Effects in Nanoglasses and Homogeneous Bulk Glasses

“…These results are in agreement with recent MD simulation results which showed that a cyclic loading after the formation of a shear band in a Cu 50 Zr 50 HG leads to structural relaxation, and consequently, a stress increase occurs in the stress-strain curve (Tang et al, 2018). Experiments and theoretical models have shown that this phenomenon depends on temperature, strain rate, and structural relaxation (Kawamura et al, 1997;Lu et al, 2003;Maaß et al, 2012;Jiang et al, 2015;Yang et al, 2019).…”

“…MD simulations were carried out using the LAMMPS simulation package (Plimpton, 1995).The interatomic interactions are described by the Finnis-Sinclair-modified-type potential for Cu-Zr (Mendelev et al, 2009) and the embedded atom model potential for Pd-Si (Ding et al, 2012). These potentials have been optimized to predict accurately the structure of liquid and amorphous Cu-Zr and Pd-Si alloys and were successfully applied to Cu-Zr and Pd-Si glasses (Cheng et al, 2013;An et al, 2016;Tang et al, 2018;Adjaoud and Albe, 2019).…”

“…It is known that cyclic loading may significantly affect the deformation behavior of materials. Cyclic loading is usually performed in the elastic regime (Sha et al, 2015;Sha et al, 2017) or in the plastic regime (Zhu et al, 2017;Tang et al, 2018). The cyclic loading of metallic glasses in the plastic regime leads usually to the phenomenon of stress overshoot (Maaß et al, 2012).…”

Temperature rise in shear bands in a simulated metallic glass

“…Indeed, the recently reported ∆T up to 2 K is comparable to our lowest simulated ∆T (∼20 K) when considering the sliding velocity difference. Although flow stress serrations are beyond the capacity of MD simulations [37], some insightful comments can be made here. As mentioned above, ∆T is limited by the elastic energy released during the shearing process.…”