Controlling the relaxation versus rejuvenation behavior in Zr-based bulk metallic glasses induced by elastostatic compression

Abstract

Elastostatic compression (ESC) has received considerable research attention as a tool to study rejuvenation and relaxation processes for bulk metallic glasses (BMGs). However, little is understood about the conditions that control whether rejuvenation or relaxation will occur, and whether conditions exist that can give structural stability. We address these questions by applying ESC at 90% of the yield stress to both cast and laser powder bed fusion (LPBF) manufactured Zr-based BMG samples in the as-cast, as-built, and different annealed states. The structural state and mechanical properties for each material condition were characterized by differential scanning calorimetry and microhardness, respectively, and two representative groups were also used for compression testing. Initial relaxation or rejuvenation was observed for elastostatically compressed as-cast samples, and the behavior reversed over 72 h of ESC. In contrast, no ESC effect was observed for the as-built LPBF samples. It was found that the onset of either relaxation or rejuvenation by ESC could be better predicted if samples were annealed into a controlled initial state. Five different types of initial response to ESC were observed, corresponding to different initial energy state ranges. Materials in the highest and lowest initial energy states were stable against structural changes by ESC. Close to the highest energy state, rejuvenation was dominant, while relaxation took place close to the lowest energy state. At intermediate initial energy states, both relaxation and rejuvenation were observed after ESC loading, suggesting that the glass structure easily finds different local minima in the potential energy landscape. In all cases, relaxation was associated with BMG hardening and rejuvenation was associated with softening. Overall, the results of this study provide new insights into how ESC impacts the structural state and mechanical properties of BMGs.