Al–Mg–Zn alloys reinforced by T–Mg₃₂(Al,Zn)₄₉ phase had higher structure stability and strength than Al–Zn–Mg–(Cu) alloys reinforced by MgZn₂ phase, but the reasons for these two kind of alloys was not well-known. To reveal the discrepancy between T phase and MgZn₂ phase, the lattice parameters, cohesive energy, and electronic structure as well as the elastic properties were investigated based on density functional theory. Four types of T phase unit cell were employed according to symmetry of space group. The calculated lattice constants well-agreed with experimental data. Compared to MgZn₂ phase, T phases obtained lower cohesive energy owing to their partial covalent bond, which may result in a higher structure stability. The elastic modulus E of T phase depended on the occupation of Al atom, and the effect of the occupation of Al atom on the structure and properties of T phase was also discussed.

High-performance Sc-containing aluminum alloys are limited in their industrial application due to the high cost of Sc elements. Er, Zr, and Y elements are candidates for replacing Sc elements. Combined with the first-principles thermodynamic calculation and the classical nucleation theory, the nucleation of L1₂-Al₃M (M = Sc, Er, Y, Zr) nanophases in dilutealuminum alloys were investigated to reveal their structural stability. The calculated results showed that the critical radius and nucleation energy of the L1₂-Al₃M phases were as follows: Al₃Er > Al₃Y > Al₃Sc > Al₃Zr. The Al₃Zr phase was the easiest to nucleate in thermodynamics, while the nucleation of the Al₃Y and Al₃Er phases were relatively difficult in thermodynamics. Various structures of Al₃(Y, Zr) phases with the radius r < 1 nm can coexist in Al-Y-Zr alloys. At a precipitate’s radius of 1–10 nanometers, the core–shelled Al₃Zr(Y) phase illustrated the highest nucleation energy, while the separated structure Al₃Zr/Al₃Y obtained the lowest one, and had thermodynamic advantages in the nucleation process. Moreover, the core–shelled Al₃Zr(Y) phase obtained a higher nucleation energy than Al₃Zr(Sc) and Al₃Zr(Er). Core–doubleshelled Al₃Zr/Er(Y) obtained a lower nucleation energy than that of Al₃Zr(Y) due to the negative ΔG_chem of Al₃Er and the negative Al₃Er/Al₃Y interfacial energy, and was preferentially precipitated in thermodynamics stability.

Atomic diffusion by the vacancy defect of L1₂-Al₃M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (V_Al) were, respectively, the preferred diffusion paths in Al₃M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-V_Al) was Al₃Zr < Al₃Y < Al₃Er < Al₃Sc. The Al₃Sc phase had high stability with a high self-diffusion activation barrier, while the Al₃Zr and Al₃Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L1₂-Al₃M was also further investigated. Zr atoms were prone to diffusion into the Al₃Y core layer, resulting in no stable core-shelled Al₃(Y,Zr), which well agreed with experimental observation.