Twinning induced strain hardening and plasticity in a γ''-precipitated medium-entropy alloy with ultrahigh yield strength

Keeping adequate strain hardening to postpone plastic instability to a larger tensile strain is a stiff challenge in alloys with high yield strength. This study showed the feasibility of activating deformation twins (DTs) to ductilize an ultra-strong medium-entropy alloy (MEA) through microstructural design. High content (∼24 %) γ'' precipitates were introduced into a Ni_49.9Fe₃₃Cr₁₀Nb₄Ta₃B_0.1 (at.%) MEA to offer high yield stress. We found that increasing the γ'' precipitate size and spacing successfully activated DTs, contributing to a sustainable strain hardening of the alloy. Accordingly, an ultrahigh tensile yield stress (YS) of 1.55 GPa and ultimate tensile stress (UTS) of 1.7 GPa, along with a fracture elongation of 14.5 % were achieved in the MEA. We further demonstrated that compared to lowering the stacking-fault energy (SFE), increasing γ'' precipitate spacing significantly reduced the critical shear stress for triggering twinning partials in a nanoscale γ/γ'' system.