Effects of deformation–induced BCC martensitic transformation and twinning on impact toughness and dynamic tensile response in metastable VCrFeCoNi high–entropy alloy

The metastable high- and medium entropy alloys (HEAs or MEAs) have drawn many attentions regarding deformation mechanisms and mechanical properties. Most of their studies have conducted under quasi–static or uniaxial tensile/compressive loading conditions. For cryogenic applications, however, the fracture or impact toughness should be carefully evaluated because it is one of the most important indices for the low–temperature performance. In this study, quasi–static and dynamic tensile properties of a metastable VCrFeCoNi HEA were investigated at room and cryogenic temperatures, and they were systematically correlated with the Charpy impact toughness. Under the quasi–static tensile loading, the Twinning Induced Plasticity (TWIP) mechanism occurred at room temperature, while the Transformation Induced Plasticity (TRIP) from FCC to BCC phases via an intermediate HCP phase occurred at cryogenic temperature. Under the dynamic loading, more deformation twins were formed at room temperature, and the amount of martensite reduced at cryogenic temperature. These variations of twinning and martensitic transformation were elucidated by the raised flow stress and by the adiabatic heating effect, respectively. They were confirmed by combining with ab–initio calculations, leading to the strong dependency of the energetic stability of BCC and HCP phases relative to the FCC phase. As a result, a plenty of deformation twins under the dynamic loading resulted in the high impact toughness of 112.6 J at room temperature. The martensitic transformation and consequently refined network structure played key roles in sustaining the remarkable toughness and in preventing the DBT phenomenon as the test temperature decreased.