Effect of the strain rate on the intermediate temperature brittleness in Zr-based bulk metallic glasses

In this work, the effect of strain rate on the ductility, intermediate temperature brittleness and fracture surface of Zr-based BMG over a wide range of temperatures from 0.1 Tg to near Tg, has been systematically investigated. The results showed two remarkable ductile to brittle transition at low cryogenic and intermediate temperatures. At low temperatures, below 0.4 Tg, the activation of diffusion mediated phenomena is negligible and the combined effects of strain rate and temperature contribute to the plasticity of material by changing the STZ volume, hence the possibility of forming multiple shear bands. At the temperature range from 0.4 Tg to the temperature at which the intermediate brittleness happens, the activation of fewer STZs takes place during inelastic shuffling of caged atoms by nearly constant loss (NCL). At higher temperatures, with the onset of Johari-Goldstein β-relaxation, which is accompanied by the NCL caged dynamics termination, more atoms can leave the cages and cooperate together, thus facilitate the activation of STZs. Furthermore, increase of strain rate shifts the intermediate temperatures brittleness to higher temperatures. Moreover, an intriguing correlation between the shear offset and the width of stable crack region with ductility is reported by studying the fracture surface topography.