Superplasticity of bulk metallic glasses (BMGs): A review

Under specific deformation conditions, thermoforming in the supercooled liquid region (SLR) might lead to Newtonian flow characterized by high strain-rate-sensitivity-index (m) of ∼1 and achieving superplastic ductilities. Both incipient deformation-induced crystallization and rapid increase in the stress-assisted free volume lead to decreased m-values and transition to non-Newtonian flow. The maximum elongations can be achieved near the transition strain rate from Newtonian to non-Newtonian flow. The effects of heating rate to the thermoforming temperature, total processing time, and the extent of the SLR are significant for structural stability against the in-situ crystallization during elevated-temperature hot deformation. Increasing the deformation temperature normally accentuates the superplastic behavior, but high temperatures might promote crystallization and loss of superplastic ductility. Future prospects for research have been recognized as optimization of the volume fraction of the crystalline (reinforcement) phase in BMG composites, superplasticity of high-entropy BMGs, thermoplastic micro-formability, ultrasonic-assisted forming, improved thermoplastic formability by alloying method, and superplasticity of BMG parts fabricated by additive manufacturing processes. © 2022 Elsevier B.V.