Overcoming the strength-ductility trade-off in an additively manufactured CoCrFeMnNi high entropy alloy via deep cryogenic treatment

Additive manufacturing by laser melting deposition is a powerful technology for fabricating complex-shaped alloys from powder feedstocks. However, such products typically have poor mechanical properties, which cannot be significantly improved by traditional processing means such as cold rolling or forming as these would be destructive to the fabricated shape. Here, deep cryogenic treatment (DCT) is proposed to be an easy and effective approach to enhance the mechanical performance at room temperature of the laser melting deposited CoCrFeMnNi high entropy alloy (HEA). DCT is found to induce large compressive residual stress and crystalline defects including dislocations, stacking faults, nanotwins and nanograins into the additively manufactured CoCrFeMnNi HEA. Nano-twinning plays a critical role in the strengthening, contributing 51.0-246.8 MPa to the DCT-processed CoCrFeMnNi HEA, while the compressive residual stress is thought to be responsible for the improved ductility by suppressing crack formation. The crystalline defects are evidently generated during the cryogenic soaking, which also result in the compressive residual stress upon reheating at the end of the DCT. The present work establishes DCT as an effective method to simultaneously enhance the strength and plasticity in alloys fabricated by additive manufacturing.