Effect of hot isostatic pressing on the cryogenic mechanical properties of CrCoNi medium entropy alloy processed by direct energy deposition

In the present study, we have investigated the effect of hot isostatic pressing (HIP) on the cryogenic mechanical properties of CrCoNi medium entropy alloy (MEA) fabricated by direct energy deposition (DED). The microstructure and mechanical properties of as-built and HIP treated CrCoNi MEA-DED were characterized by electron backscatter diffraction (EBSD) and uniaxial tensile test at room (298 K) and cryogenic (77 K) temperatures. The CrCoNi MEA-DED after HIP exhibits a decrease in strength due to the reduction of dislocation density. At 77 K, the as-built CrCoNi MEA-DED shows a typical trade-off tensile behavior of increased strength and reduced elongation to fracture. The HIP treated CrCoNi MEA-DED reveals an excellent combination of high strength and preserved elongation to fracture at 77 K, resulting in significant increase of the strain energy density by 56% compared to room temperature. The excellent elongation to fracture with high strength at 77 K is attributed to a significant decrease of micropores after HIP. Furthermore, the earlier activation of deformation twinning at low strains in the HIP treated CrCoNi MEA-DED can provide steady work hardening behavior, which postpones the onset of necking instability. It reveals that the CrCoNi MEA-DED with HIP possesses the exceptional mechanical properties at cryogenic temperature in comparison with the other CrCoNi MEAs prepared by different processes.