Corrosion mechanism of T1 phase in Al-Cu-Li alloy: First-principles calculations

First principle calculations in the framework of density functional theory are performed to calculate the T1 phase (Al₆Cu₄Li₃), which is the main precipitation in Al-Cu-Li alloy. In this paper, the surface energy values and surface electron work functions of different termination surfaces in T1 phase are calculated. Meanwhile, the effects of stress and common alloying elements on the T1 phase are also discussed. There are 10 different termination surfaces for T1 phase. The surface energy varies between 0.59 and 1.28 J·m^–2. It is found that the surface energy is dependent on the atomic configuration of the surface. The relaxation of the surficial atoms leads to low surface energy. For work function, it is controlled by the surficial atomic species. When a surface contains Li atoms, low work function is expected, which can be attributed to the low electronegativity of Li atom. The (010) T1 surface with Li termination has a minimum work function, 3.40 eV. In addition, as is different from pure metal, work function of some T1 surfaces shows unique behavior under stress state. The (010) T1 surface with Al and Cu termination has an increasing work function under the action of tensile strain. In fact, tensile strain induces the first and second surface layer to merge, which can improve the surface electronic density and raise work function. As a result, the corrosion resistance can be enhanced. Finally, the effect of alloying elements on the precipitation of T1 phase is studied. Al(111)/T1(010) interface is built and the substitution energy of Mg, Zn and Ag are calculated. Comparing with Mg and Zn atom, the energy of Ag atom to substitute the interfacial one is low, meaning that Ag can relax the strain in the interface. Ag atom has the closest atomic radius to Al atom, and the same chemical valence as Li atom. Therefore, Ag atom is more likely to promote the precipitation of T1 phase, which is also in agreement with the experimental result.