Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition. Published under license by AIP Publishing.