Unveiling the Strengthening and Ductility Mechanisms of a CoCr0.4NiSi0.3 Medium-Entropy Alloy at Cryogenic Temperatures

Materials utilized in extreme environments, such as those necessitating protection and impact resistance at cryogenic temperatures, must exhibit high strength, ductility, and structural stability. However, most alloys fail to maintain adequate toughness at cryogenic temperatures, thereby compromising their safety during cryogenic temperature service. This study investigates the quasi-static mechanical properties of a CoCr0.4NiSi0.3 medium-entropy alloy (MEA) at room temperature, −75 °C, and −150 °C. The deformation behavior and mechanisms responsible for strengthening and toughening at reduced cryogenic temperatures are analyzed, revealing that decreasing cryogenic temperature enhances the strength of the as-cast MEA. Specifically, both the yield strength (YS) and ultimate tensile strength (UTS) of the MEA increase significantly with decreasing temperature during cryogenic tensile testing. Under tensile testing at −150 °C, the YS reaches 617.5 MPa, the UTS is 1055.0 MPa, and the elongation to fracture remains approximately 21.0% at both −150 °C and −75 °C. After cryogenic temperature tensile deformation, the matrix exhibits a dispersed distribution of nanoscaled tetragonal and orthorhombic phases, a coherent hexagonal close-packed phase, L12 phase and layered long-period stacking ordered (LPSO) structures, which are rarely observed in the cryogenic deformation of metals and alloys. The metastable phase evolution path of this MEA at cryogenic temperatures is closely associated with the decomposition of perfect dislocations into a/6<112> Shockley partial dislocations and their subsequent evolution at reduced cryogenic temperatures. At −75 °C, the a/6<112> Shockley partial dislocation interacts with the L12 phase to form antiphase boundaries (APBs) approximately 3 nm thick. At −150 °C, two phase transition paths from stacking faults (SFs) to nanotwins and LPSO occur, leading to the formation of layered LPSO structures and deformation-induced nanotwins. The dispersion of these coherent nanophases and nanotwins induced by the reduced stacking fault energy under cryogenic temperatures is the key factor contributing to the excellent balance of strength and plasticity in the as-cast MEA, providing an important basis for research on the cryogenic mechanical properties of CoCrNi-based MEAs.