Effects of Fe content on plane-stress fracture toughness of Fex(CoCrMnNi)100-x complex concentrated alloys

Fe addition is considered a cost-effective strategy to replace expensive Co and Ni in CCAs while maintaining mechanical performance. In this study, the plane-stress fracture toughness of Fe_x(CoCrMnNi)_100-x (x = 20–60 at.%) alloys was investigated through J-integral tests at room (298 K) and cryogenic (123 K) temperatures. Twinning was the dominant deformation mechanism at room temperature, while both twinning and ε-martensitic transformation were active at cryogenic temperature. Crack propagation was facilitated by twin boundaries and ε-martensites aligned parallel to the crack path. In contrast, fine α′-martensites located near the crack tip effectively hindered crack growth, thereby increasing the resistance to crack extension as measured by the J-integral. Under cryogenic conditions, the high-volume fraction of α′-martensite induced significant crack tip blunting and localized compressive stress, thereby suppressing crack propagation. Contrary to conventional expectations, strain-induced α′-martensite was found to enhance the fracture resistance under plane-stress conditions via transformation-induced toughening mechanisms.