Suppression of annealing-induced embrittlement in bulk metallic glass by surface crystalline coating

• Free volume (FV) in a Zr-based bulk metallic glass (BMG) could be preserved by crystalline W coating upon annealing.
• The maintained FV was redistributed in the crystalline W-coated BMG upon annealing.
• The FV formation energy in the Zr-based BMG was much less than the vacancy formation energy in crystalline W.
• Both enhanced strength and plasticity were achieved in BMG after coating and annealing.

Annealing-induced embrittlement in metallic glasses (MGs) due to the running off of free volume (FV) is detrimental to their practical applications. By adding crystalline W coatings, we demonstrated that the majority of free volume (FV) could be preserved and redistributed in a Zr50.7Cu28Ni9Al12.3 bulk metallic glass (BMG) upon annealing as, thermodynamically, it was unfavorable for the FV to migrate from BMG into crystalline W. Correspondingly, the strength and ductility of the BMG were both enhanced relative to as-cast ones, due likely to decomposition processes such as the formation of Zr- and Cu-rich clusters in the BMG. The results of this study may be applied to avoid annealing-induced embrittlement in BMGs and construct a simple yet effective method to elevate both the strength and ductility of BMGs.

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