Combined strengthening from nanoprecipitates, stacking faults and nano-twins enables a strong and ductile medium-entropy alloy

Abstract

Single-phase face-centered cubic (fcc) alloys typically exhibit remarkable strain-hardening capability and large ductility but possess low yield strength. Conventional strengthening strategies inevitably lead to the loss of ductility. Here, an Nb alloyed CrCoNi-based medium-entropy alloy with a superb yield strength of ∼1.82 GPa, an ultimate tensile strength of ∼1.94 GPa, and good ductility of ∼24 % at 93 K is designed through the combined strengthening from nanoprecipitates, stacking faults, and nano-twins. The cold-rolling is employed to introduce high-density stacking faults and nano-twins, while subsequent low-temperature annealing generates numerous Nb-rich γ″ nanoprecipitates in the fcc matrix. The combined effects of precipitation strengthening, stacking fault- and nanotwin-induced strengthening contribute to an increase in flow stress to surpass the critical stress for the formation of stacking faults and deformation twinning. Moreover, the precipitation of high-density Nb-rich γ″ nanoprecipitates reduces the stacking fault energy of the fcc matrix. Therefore, the precipitation of γ″ nanoprecipitates produces more stacking faults and nano-twins during tensile deformation. The formation of stacking faults and nano-twins in turn, by dynamically introducing high-density interfaces for reducing the mean free path of dislocations, generates a high strain-hardening rate, thus high ultimate tensile strength and large ductility. More importantly, the coherent interfaces between γ′′ nanoprecipitates and the fcc matrix facilitate the transmission of stacking faults and nano-twins across the γ′′ nanoprecipitates. This mechanism could relieve the stress/strain concentrations at interfaces that could otherwise lead to premature crack initiation. This kind of combined strengthening from nanoprecipitates, stacking faults, and nano-twins provides a new avenue to further enhance the strength of nano-twinned materials or precipitation-strengthened materials while maintaining good ductility.