Multiple deformation scheme in direct energy deposited CoCrNi medium entropy alloy at 210K

Abstract

CoCrNi medium entropy alloy (MEA) and stainless steel 316L (SS316L) were manufactured by direct energy deposition of additive manufacturing (DED-AM). Exceptional mechanical properties of DED CoCrNi at 210K were achieved by the activities of a multiple deformation scheme that changed from dislocation slip to twinning-induced plasticity followed by transformation-induced plasticity. While SS316L at room temperature has micro-twins, CoCrNi at 210K exhibited nano-twins, resulting from lower stacking fault energy. Moreover, transformed hexagonal close-packed (HCP) phases were found near the face-centered cubic (FCC) {111} grain boundaries, where remarkable stacking faults and severe lattice distortion were measured. The formation of HCP substructure was understood from stacking fault probability, mean square strain, and lower temperature, which increased the strain hardening rate of several orientations in the FCC matrix, finally improving the strength of CoCrNi at 210K. A combination of CoCrNi MEA and DED-AM achieves a synergistic effect of strength, ductility and toughness for low temperature applications.