Effects of alloying elements with positive enthalpy of mixing in Mg65Cu25Gd10 bulk-forming metallic glasses

Abstract

The present study investigates the effects of a minor addition (MA) of alloying elements to Mg65Cu25Gd10 bulk-forming metallic glass (BMG). First, we show that a MA of Y or Ca that has singular positive enthalpy of mixing relationship (PEMR) with a constituent element in Mg65Cu25Gd10 BMG leads to an improvement in plasticity as well as glass-forming ability (GFA) in a limited composition range. Second, we show that a MA of Co with plural PEMRs and a large negative enthalpy of mixing relationship (NEMR) to Mg65Cu25Gd10 BMG drastically deteriorates both GFA and plasticity due to large variation of matrix composition. Finally, the paper demonstrates that a MA of Be that has unfavorable mixing relationships with all constituent elements has no adverse effect on GFA and precipitates Be-rich secondary nanocrystals in the amorphous matrix, which leads to improved plasticity up to ɛf = 3.41 ± 0.10% in (Mg65Cu25Gd10)95(Be)5 BMG. Consequently, our results suggest that a MA of a 4th alloying element with PEMR to Mg65Cu25Gd10 BMG can result in different degrees of heterogeneity in the amorphous matrix depending on the overall combination of mixing enthalpies among atomic pairs, which significantly influences the plasticity as well as GFA of the BMGs.