Thermal rejuvenation of an aged Au-based metallic glass by fast scanning calorimetry

C.M Meylan,K Georgarakis,A.L Greer

doi:10.1016/j.nocx.2021.100062

C.M Meylan, K Georgarakis + Show 1 more

Open Access

https://doi.org/10.1016/j.nocx.2021.100062

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Abstract

A metallic glass (MG) annealed above its glass-transition temperature T g , and cooled, may show an enthalpy increase Δ H , and other property changes. The extent of this thermal rejuvenation depends on the state of the MG (represented by effective cooling rate Φ i ) and the post-anneal cooling rate Φ c . Previous studies examined effects of (Φ c /Φ i ) up to 10 2 . With a Au-based MG aged for up to 10 years at room temperature, and using fast calorimetry to anneal and then cool at up to 5000 K s −1 , we extend (Φ c /Φ i ) to 10 7 . The rejuvenation is limited by anneal temperature or by Φ c , when, for all MGs, Δ H / T g shows a universal approximate scaling with log(Φ c /Φ i ). We detect decoupling of vitrification from α relaxation, and highlight limitations in the use of fictive temperature to characterize glassy states. Rejuvenation of the Au-based MG decreases its elastic modulus and hardness, extending trends reported for other MGs. • Fast-scanning calorimetry permits extended thermal rejuvenation of a metallic glass. • The cooling-rate ratio giving rejuvenation, 10 2 in earlier work, is extended to 10 7 . • Rejuvenation is in two regimes, limited by anneal temperature or cooling rate. • Thermal rejuvenation gives extended decoupling of vitrification from α relaxation. • Greater cooling-rate ratio extends reductions in elastic modulus and hardness.

Highlights

On annealing, as-cast metallic glasses (MGs) undergo structural relaxation, reaching states of lower volume and lower enthalpy
We review the literature on thermal rejuvenation (TR) of MGs, and we show that, using fast differ ential scanning calorimetry (FDSC), the reported effects can be far exceeded
The heat-flow trace of the Au-based ribbon aged for eight years at room temperature (RT), measured at 100 K s− 1 by FDSC, shows a large overshoot overlapping with the glass transition (Fig. 1a)

Summary

Introduction

As-cast metallic glasses (MGs) undergo structural relaxation, reaching states of lower volume and lower enthalpy. It has long been known that the relaxation-induced property changes, notably embrittlement, can be partially reversed by a second anneal at higher temperature [1,2,3]. A variant technique involves not an anneal above Tg, but a brief excursion enabled by ultrafast Joule heating [14] We consider all these effects as examples of thermal rejuvenation [7], taking the BMG to a higher-energy state (we do not favour the term recovery annealing [4], as that is more widely used for treatments that take polycrystalline metals and ceramics to lowerenergy less-defective states). The atomistic aspects of thermal rejuvenation have been examined in molecular-dynamics simulations [14,16]

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