Mechanical Relaxation-to-Rejuvenation Transition in a Zr-based Bulk Metallic Glass

M Zhang,L Liu,F X Li,M Z Li,Y M Wang,S Q Jiang

doi:10.1038/s41598-017-00768-7

Abstract

The relaxation of amorphous materials, i.e., aging, would largely endanger their performances in service. Here we report a mechanical relaxation-to-rejuvenation transition of a Zr35Ti30Be27.5Cu7.5 bulk metallic glass (BMG) in elastostatic compression at ambient temperature, thus provide an accessible way to tailor the mechanical properties of amorphous materials. To unravel the structural evolution underlying the observed transition, atomistic simulations parallel with the experimental tests on a typical model glass system Zr60Cu40 were performed, which successfully reproduced and thus upheld the experimentally observed mechanical relaxation-to-rejuvenation transition. The variations of coordination number and atomic volume during the transition are evaluated to indicate a de-mixing tendency of the constituent atoms in the rejuvenation stage. This de-mixing tendency largely explains the difference between mechanical rejuvenation and thermal rejuvenation and reveals a competitive relationship between activation enthalpy and activation entropy in the stress-driven temperature-assisted atomic dynamics of BMG, such as diffusion and plastic deformation etc.

Highlights

The relaxation of amorphous materials, i.e., aging, would largely endanger their performances in service
Zr35Ti30Be27.5Cu7.5 bulk metallic glass (BMG) in the elastostatic compression at different stress levels, during which the specimen was first loaded with a strain rate of 5 × 10−4 s−1 to a stress level of 50% and 90% of the yield stress σy, respectively and held there for a period of 48 h before unloading
The surface of all the tested specimens shows no traces of shear bands, indicating that the residual strain comes from homogenous deformation of the BMG specimen

Summary

Introduction

The relaxation of amorphous materials, i.e., aging, would largely endanger their performances in service. Several Zr(Cu)-based BMGs shown elastostatic compressive stress-induced softening (i.e., rejuvenation)[15, 16], elastostatic compressive stress-induced hardening (i.e., relaxation) of similar Zr(Cu)-based BMGs has been observed[17, 18] These results remind one of the thermal effect of temperature on the structure of BMGs, where sub-Tg annealing (Tg is the glass transition temperature) would lead to the annihilation of free volume[19], i.e., relaxation, while sup-Tg annealing (i.e., annealing at temperature above Tg) would lead to the creation of free volume, i.e., rejuvenation. Currently, whether such a mechanical relaxation-to-rejuvenation transition exists and what cause the fundamental differences between thermal rejuvenation and mechanical rejuvenation are two important issues in the study of rejuvenation of metallic glasses

Methods

Results

Conclusion