Hardness fluctuations caused by annealing in Zr-based bulk metallic glasses via nanoindentation

The micromechanical response of a Zr-based bulk metallic glass (BMG) annealed in air below and above its glass transition temperature was studied. Nano-sized particles in different states at different annealing temperatures were found on the surface of the samples, which were identified by energy-dispersive spectroscopy (EDS) as being CuNi elemental composition. The surface microhardness and nanoindentation creep behavior of the BMGs after annealed exhibited interesting results, at low annealing temperatures, the microhardness of the samples showed softening accompanied by poor creep resistance, while at high annealing temperatures, the hardness of the samples shows hardening and creep resistance was enhanced. The consequences of such a temperature-dependent dichotomy were intricately linked to element segregation, which led to microstructural fluctuations. A competing mechanism between the hardening effect of zirconia at the surface and the softening effect of elemental segregation in the formation of microporous structures inside the samples is proposed to account for the divergent mechanical properties due to the annealing temperature. This work presents helpful insights into microstructural changes and micromechanical properties of BMGs during annealing, as well as an experimental support for structural and property modulation of BMGs.