Deformable κ phase induced deformation twins in a CoNiV medium entropy alloy

The degradation of ductility is obvious in low-temperature annealed face centered cubic (FCC) based alloys under bulk quasi-static tensile experiments due to intermetallic phase embrittlement. Here, we demonstrate a novel approach to overcome this loss of deformability by realizing brittle intermetallic phase (κ) shearing and deformation twins in an equiatomic CoNiV medium entropy alloy (MEA) with high stacking fault energy. The brittle κ phase not only contributes to high true ultimate tensile strengths (∼1800→2000 MPa) by shearing, but also enhances the flow stress to approach the critical values for the onset of deformation twins (∼1660-1750 MPa) in this MEA. Such shearing and twins in turn assist further work hardening and strengthening mechanisms that improve the deformability of MEA (uniform elongation ∼25→27%). As a result, the degradation of ductility caused by intermetallic phase embrittlement in this MEA can be recovered. The combination of deformable intermetallic phase and high stress deformation twins proposes a so far untapped strengthening mechanism, for enabling the design of FCC based alloys with improved mechanical properties.