Role of pre-existing shear band morphology in controlling the fracture behavior of a Zr–Ti–Cu–Ni–Al bulk metallic glass

Shear bands were introduced into a Zr–Ti–Cu–Ni–Al bulk metallic glass (BMG) by one-directional cold rolling and the effect on fracture toughness anisotropy was examined. Two different rolling and shear band orientations relative to the crack plane were used, with the rolling force oriented along either the width (CR-W) or thickness (CR-T) direction of single edge notched bend specimens. The results showed the CR-W samples demonstrated a 50% reduction in KQ relative to the as-cast material while the CR-T samples demonstrated the highest fracture toughness. The low fracture toughness of the CR-W samples was attributed to easy crack propagation paths formed by the shear bands. In contrast, the higher fracture toughness of the CR-T samples was attributed to the difficulty of crack twisting out of the mode I precrack plane onto the shear band planes and the high energy absorption of mode III tearing between the parallel crack planes. Overall, when cold rolling BMGs for structural applications, care must be taken to design the shear band morphologies to achieve the desired fracture toughness properties rather than solely focusing on increasing the average relaxation enthalpy/free volume.