On the strength and fracture toughness of an additive manufactured CrCoNi medium-entropy alloy

An additively manufactured (nominally equiatomic) CrCoNi alloy was processed by laser powder bed fusion (LPBF). At ambient temperatures (298 K), this medium-entropy alloy displayed a yield strength, ay of-691 & PLUSMN; 9 MPa, and an ultimate tensile strength, au of-926 & PLUSMN; 15.2 MPa; at cryogenic temperatures (77 K), yield and tensile strengths increased respectively to ay -944 & PLUSMN; 6 MPa and au -1382 & PLUSMN; 11 MPa. These strength levels are 57 and 44% higher than that of the wrought alloy, due to strengthening from the solidification cellular structures intertwined with dislocations in the LPBF CrCoNi. The crack-initiation fracture toughness, KJIc was measured to be-183.7 & PLUSMN; 28 MPa & RADIC;m at 298 K; this value marginally decreased by-4% to-176 & PLUSMN; 11 MPa & RADIC;m at 77 K. These KJIc values of the LPBF CrCoNi were 11% and 35% lower than the wrought CrCoNi alloy at 298 K and 77 K, respectively. The resistance to crack growth of the LPBF CrCoNi from its hierarchical micro-and meso-structures was evaluated using nonlinear-elastic fracture mechanics by measuring R-curve behavior in the form of the J -integral as a function of crack extension. The specific features of the hierarchical microstructures at different length-scales provide a basis for the strengthening and toughening properties of this additively manufactured medium-entropy alloy. This correlation between the deformation and the hierarchical microstructures at different length-scales may provide future guidance for improving the fracture toughness properties of medium -entropy alloys.