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

Baoshuang Shang,Jean-Louis Barrat,Pengfei Guan

doi:10.1088/2515-7639/aad451

Abstract

Cryogenic rejuvenation in metallic glasses reported in Ketov et al 's 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.

Highlights

Glasses are solid materials, disordered at small scale, but homogeneous and isotropic at large scales
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 to induce rejuvenation during a cryogenic thermal cycling
When the metallic glasses are heated, due to the heterogeneity of local thermal expansion coefficient (LTEC), internal stresses are generated by the mismatch in the local thermal deformation

Summary

INTRODUCTION

Glasses are solid materials, disordered at small scale, but homogeneous and isotropic at large scales. It was discovered that an alternative to this mechanical rejuvenation could be obtained, in some metallic glasses (MGs), by thermal cycling towards low temperatures [5] This is quite surprising, as from a purely thermal point of view one would expect that bringing the system to a lower temperature does not populate higher energy states, and should only result in slow aging. One possible interpretation of the effect, suggested by Hufnagel [6], is that the rejuvenation is due to the creation of internal stresses during the temperature cycling The origin of such stresses could be twofold: local heterogeneities in the temperature field due to the very fast heating process which were investigated by some of us in a recent molecular dynamics study [7]. Our main conclusion is that, while the differential thermal expansion is capable of generating plastic activity and thermal rejuvenation upon cryogenic cycling, the effect is unlikely to be observable at the scale of molecular dynamics simulations

THE LOCAL THERMAL EXPANSION COEFFICIENT

INTERNAL STRESS BY THERMAL EXPANSION HETEROGENEITY

PROBABILITY OF TRIGGERING AN ATOMIC REARRANGEMENT

DISCUSSION AND CONCLUSION