Effect of deep cryogenic cycling treatment on the microstructure and mechanical properties of Ti-based bulk metallic glass

This study investigates the effect of deep cryogenic cycle treatment (DCT) on the microstructure and mechanical behaviors of Ti33Zr30Cu9Ni5.5Be22.5 bulk metallic glass (BMG). The results revealed that after the DCT, the relaxation enthalpy increased, whereas the Ti33Zr30Cu9Ni5.5Be22.5 BMG still maintained a complete amorphous nature. As the cycle numbers increased, the room-temperature plasticity of the Ti33Zr30Cu9Ni5.5Be22.5 BMG represented a tendency to increase first and then decrease. When the cycle numbers increased to 30, the Ti33Zr30Cu9Ni5.5Be22.5 BMG obtained the maximum compressive plasticity of 7.8%, which is 5.6 times that of the as-cast specimen, accompanied by a higher strength. Moreover, the nanoindentation results demonstrated that the Ti33Zr30Cu9Ni5.5Be22.5 BMG that underwent the DCT possessed a larger pop-in size, a lower initial pop-in force and a lower hardness. The shear transformation zone (STZ) theory was employed to elucidate the plasticising mechanism induced by the DCT for the Ti33Zr30Cu9Ni5.5Be22.5 BMG. The larger STZ volumes and the lower shear band formation energy induced through the DCT promoted the densification of multiple shear bands, which is the principal reason for the excellent plasticity after the DCT.