Microscopic phase-field simulation of competition mechanism between L1₂ and D0₂₂ structure in Ni-Cr-Al alloy

Simulations are performed on temporal evolution of atom morphology and ordering parameters of Ni-14.5 Cr-16.5 Al alloy during early precipitation process at different temperatures based on microscopic phase-field theory; the relationship between precipitation sequence and mechanism of L1₂ and D0₂₂ structure and precipitation temperature are illuminated. The nonstoichiometric ordered L1₂ phases appear first with congruent ordering+spinodal decomposition mechanism which is then followed by precipitation of D0₂₂ phases at ordering domain boundaries of L1₂ phases by spinodal decomposition mechanism at 1073 K and 1223 K. The nonstoichiometric L1₂ phases transform to stoichiometric ordering phases gradually. The incubation period of L1₂ and D0₂₂ phases is shorter at 1073 K than that at 1223 K, and growth speed is higher at 1073 K. At 1373 K, L1₂ and D0₂₂ phases appear simultaneously by non-classical nucleation and growth mechanism. After that the particles of D0₂₂ phases diminish and disappear gradually; L1₂ phases grow and single L1₂ phases are remained at last.