First-principles calculations on the crystal, electronic structures and elastic properties of Ag-rich γ′ phase approximates in Al–Ag alloys

Due to the controversy on the structure and lack of experimental data, it is difficult to understand the effects of γ′ phase on the mechanical properties of Al–Ag alloys. In this work, the bulk γ′ phase in Ag-rich solid-solution has been investigated by means of first-principles calculations within the generalized gradient approximation. The lattice parameters are determined theoretically by structural optimization of full relaxation. It is found that the Neumann phase is energetically stable from the calculated formation enthalpy and cohesive energy. The electronic structure is analyzed to understand the underlying mechanism of the structural stability and mechanical properties of the γ′ phase. The six calculated independent elastic constants indicate that the Neumann phase, a proposed γ′ structure in literature, is mechanically stable. The polycrystalline bulk modulus B, Young’s modulus E, shear modulus G, Poisson ratio ν of the γ′ phase are obtained by the Voigt–Reuss–Hill (VRH) approximation. Since there is currently no experimental measurement of elastic constants, we also calculated the elastic constants of a competing hcp structure (N.A. Zarkevich, D.D. Johnson, Phys. Rev. B 67 (2003) 064104.) at 50 at.% Ag for comparison. The results indicate that the prototype hcp phases could have large effect on the mechanical properties of the Al–Ag alloys. The results on elastic anisotropy show that the Young’s modulus E along the c axis is larger than that of the other directions.