Role of thermal expansion heterogeneity in the cryogenic rejuvenation of metallic glasses

Abstract

Cryogenic rejuvenation in metallic glasses reported in Ketov et al ʼs experiment (2015 Nature 524 200)has attracted much attention, both in experiments and numerical studies. The atomic mechanism of rejuvenation has been conjectured to be related to the heterogeneity of the glassy state, but the quantitative evidence is still elusive. Here we use molecular dynamics simulations of a model metallic glass to investigate the heterogeneity in the local thermal expansion. We then combine the resulting spatial distribution of thermal expansion with a continuum mechanics calculation to infer the internal stresses caused by a thermal cycle. Comparing the internal stress with the local yield stress, we prove that the heterogeneity in thermo mechanical response has the potential to trigger local shear transformations, and therefore to induce rejuvenation during a cryogenic thermal cycling.

Atomistic modeling of heat treatment processes for tuning the mechanical properties of disordered solids

“…In particular, it was concluded that internal stresses due to heterogeneity in the local thermal expansion can induce local shear transformations, provided that the system size is sufficiently large [35]. The efficiency of thermal rejuvenation generally increases with the number of thermal cycles and becomes relatively large when the system dimensions exceed about 10 nm [35]. In the present study, however, the appearance of rejuvenated states in Figs.…”

“…It should also be commented that the possibility of attaining higher energy states upon cryogenic thermal cycling (well below T g ) was recently discussed by Shang et al [35]. In particular, it was concluded that internal stresses due to heterogeneity in the local thermal expansion can induce local shear transformations, provided that the system size is sufficiently large [35]. The efficiency of thermal rejuvenation generally increases with the number of thermal cycles and becomes relatively large when the system dimensions exceed about 10 nm [35].…”

The effect of cryogenic thermal cycling on aging, rejuvenation, and mechanical properties of metallic glasses

“…It was argued that atomic-scale structural rejuvenation upon thermal cycling might be caused by internal stresses that arise due to spatially heterogeneous thermal expansion of the amorphous material [9]. More recently, the probability distribution of local thermal expansion coefficients was estimated using atomistic simulations, and it was shown that internal stress can exceed the local yield stress and therefore trigger local shear transformations [10]. Similar to thermal agitation, it was originally demonstrated that a shear cycle can either overage or rejuvenate the glass depending on whether the strain amplitude is smaller or larger than the yield strain [11].…”

Atomistic Modelling of Thermal-Cycling Rejuvenation in Metallic Glasses

“…S8), heterogeneity in local coefficient of thermal expansion is important, as suggested in early work [ 9 , 35 ]. However, the correlation length for local thermal expansion, computed by some of us [36] , shows a power-law dependence on coarse-grain size, which means that the activation probability induced by the local thermal expansion mismatch is negligible in our present simulated system.…”