800^oC-stable D0₂₂ superlattice in a NiCrFe-based medium entropy alloy

The metastable-stable transition and rapid coarsening of metastable D0₂₂ phase remains its Achilles hell, restricting the application of D0₂₂-strengthened alloys at service temperatures higher than 650°C. Hence, it is crucial to improve thermal stability of D0₂₂ phase. In this work, we report an 800°C-stable D0₂₂-Ni₃(W_0.24Ti_0.38Ta_0.19Nb_0.19) superlattice in the Ni₅₈Cr₂₃Fe₁₀W₅Ti₂Ta₁Nb₁ MEA. Experimental results and theoretical analysis showed that the exceptional thermal stability is due to Nb-sublattice occupying elements, low diffusivity of W, and extremely small lattice misfit between the D0₂₂ phase and FCC matrix. These findings shed lights on developing high-performance D0₂₂-hardened alloys capable of operating at temperatures above 650°C.