Balancing κ-formation and γ-recrystallization towards exceptional mechanical properties in a NiCoV0.9 medium entropy alloy

Abstract

In this work, by lowering the V amount, two thermodynamic processes, i.e., κ-formation and γ-recrystallization, are near-concurrently activated in a NiCoV0.9 medium entropy alloy (MEA) during 900 °C annealing. The NiCoV0.9 MEA with only 2 min annealing inherits the high strength from the cold-rolling structure while regains apparent work-hardening, endowing the alloy at this state with excellent strength-ductility synergy. By utilizing correlated observation methods, it is found that in the very beginning of annealing, γ-recrystallization occurs closely prior to κ transformation, which in turn enables elemental redistribution needed for κ growth. Further in-situ synchrotron high-energy X-ray diffraction reveals the synchronous onset of plastic deformation in both κ and γ, suggesting the good deformability of κ regardless of its hard nature. With prolonged annealing at 900 °C, the yield strength drops sharply yet the ductility is not further enhanced. This is likely due to the fast increment in both size and volume fraction of the κ phase that significantly raises the cracking sensitivity.