Effects of cryogenic temperature on tensile and impact properties in a medium-entropy VCoNi alloy

Multi-principal element alloys usually exhibit outstanding strength and toughness at cryogenic temperatures, especially in CrMnFeCoNi and CrCoNi alloys. These remarkable cryogenic properties are attributed to the occurrence of deformation twins, and it is envisaged that a reduced stacking fault energy (SFE) transforms the deformation mechanisms into advantageous properties at cryogenic temperatures. A recently reported high-strength VCoNi alloy is expected to exhibit further notable cryogenic properties. However, no attempt has been made to investigate the cryogenic properties in detail as well as the underlying deformation mechanisms. Here, the effects of cryogenic temperature on the tensile and impact properties are investigated, and the underlying mechanisms determining those properties are revealed in terms of the temperature dependence of the yield strength and deformation mechanism. Both the strength and ductility were enhanced at 77 K compared to 298 K, while the Charpy impact toughness gradually decreased with temperature. The planar dislocation glides remained unchanged at 77 K in contrast to the CrMnFeCoNi and CrCoNi alloys resulting in a relatively constant and slightly increasing SFE as the temperature decreased, which is confirmed via ab initio simulations. However, the deformation localization near the grain boundaries at 298 K changed into a homogeneous distribution throughout the whole grains at 77 K, leading to a highly sustained strain hardening rate. The reduced impact toughness is directly related to the decreased plastic zone size, which is due to the reduced dislocation width and significant temperature dependence of the yield strength.