The ductile to brittle transition behavior in a Zr-based bulk metallic glass

The ductile to brittle transition (DBT) of crystalline metals and alloys has been extensively studied in scientific and engineering fields. However, the DBT of non-crystalline metals and alloys such as bulk metallic glasses (BMGs) remains poorly characterized and understood. Here, the DBT behavior of a Zr-based (Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅) BMG as a model material was systematically investigated at various temperatures and different free volume states using uniaxial tensile and compression tests. A significant DBT was confirmed by the observations of the transition of macroscopic fracture modes from shear to tension as well as microscopic fracture surface features from micron-scale vein pattern to nano-scale dimples/periodic corrugations. It is revealed that the former is determined by the intrinsic competition between the critical shear and normal stresses, and the latter is due to the competition between ductile shear banding and brittle cracking. Within the framework of the cooperative shear model of shear transition zones (STZs), a DBT map for metallic glasses is proposed in terms of STZ versus tension transition zone (TTZ), depending on free volume, temperature and strain rate.