Unraveling L12 Al3X (x=ti, Zr, Hf) Nano-Precipitate Evolution in Aluminum Alloys Via Multi-scale Diffusion Simulation

The evolution of L1(2) Al3X (X = Ti, Zr, Hf) nano-precipitates in Al-X alloys was studied by multi-scale diffusion simulation using a combination of first principles calculation and finite element method. The results show that the diffusion coefficient in Al-X solid-solution phase follows DAl>DZr>DHf>DTi, which are in excellent agreement with available experimental values. On the other hand, the mobility of X atoms in L1(2) Al3X phases is reversed, following D(Al)(Al)3(X)>D(Ti)(Al)3(Ti)>D(Hf)(Al)3(Hf)>D(Zr)(Al)3(Zr). The calculated Al and X diffusion coefficients in both Al-X solid-solution and L1(2) Al3X ordered phases were used to simulate the evolution of L1(2) nano-precipitate embedded in Al-X solid-solution matrix. It is found that only Zr could promote the formation of nano-precipitates at 650 K, the common heat-treatment temperature for advanced Al alloys. At lower temperature, the sluggish atomic movement hinder the nucleation of L1(2) phase, while at higher temperature the precipitates redissolve into the Al matrix. The multi-scale diffusion simulation underlines the indispensable role of diffusion in L1(2) Al3X phases on the nano-precipitate evolution, and also provides a feasible way to the rationally design of precipitate strengthening alloys.