Effect of residual stresses on the hardness of bulk metallic glasses

Nanoindentation experiments were conducted on Zr-based metallic glass samples, which were elastically and plastically bent in order to investigate the effect of residual stresses on hardness. It was found that tensile residual stress reduced the hardness significantly, while compressive residual stress produced only a small effect on the hardness. These observations are consistent with three-dimensional continuum-plasticity-based finite-element simulations. The hardness was also found to vary more significantly with residual stresses, in particular in tension, than that caused by shear-banding-induced softening, suggesting hardness measurement is a practical method for the evaluation of tensile residual stresses in a metallic glass. Hardness variation in the bent sample was correlated with the residual-stress-induced volume dilatation through a free-volume-based model. In this paper, we also present a detailed stress analysis based on yield asymmetry under tension and compression to describe the distribution of residual stresses in bent metallic glass specimens. The calculations agree well with the hardness variations measured experimentally.