Utilizing hot isostatic pressing for pore defect control and mechanical stability in direct energy deposited CoCrNi medium-entropy alloy at liquid helium temperature

Here, we investigated the effect of hot isostatic pressing (HIP) on the microstructure and mechanical stability at cryogenic temperatures down to the liquid helium temperature (4.2 K) of additively manufactured CoCrNi medium-entropy alloys (MEAs) fabricated by direct energy deposition (DED). Subsequently, HIP treatment facilitated to improve microstructural homogeneity and structural densification, particularly the pore defects in original additively manufactured state. Poorly limited ultimate tensile strength (UTS) and total elongation (T.E) were observed in the DED-processed CoCrNi MEA, whereas the HIP-treated CoCrNi exhibited excellent UTS × T.E synergy (∼76 and ∼ 69 GPa × %) at 298 K and 4.2 K. Furthermore, in-depth microstructure analyses from different viewpoints were conducted to elucidate the main deformation and/or failure mechanisms, taking into account of twinning-induced plasticity (TWIP) and intergranular fractures at cryogenic temperature. Our findings provide significant insights into the structural/mechanical stabilities of HIP treated CoCrNi MEAs at ambient to deep cryogenic temperatures near absolute zero kelvin (4.2 K). • Hot isostatic pressing recovers the anisotropic microstructure, GNDs, and pore defects in additively manufactured CoCrNi. • The HIP-treated alloys presented enhanced mechanical stability at ambient and cryogenic temperatures (298 K, 77 K, 4.2 K). • The trend of deformation-twinning by decreasing temperature, after the interrupted tensile test and failure, is different. • Decreased GB cohesive energy induced by Cr-oxide led to premature cracking under cryogenic deformation of HIP-treated CoCrNi • This study provides insightful information for the cryogenic industry and the recyclability of post-processed alloys.