First-principles calculations of elastic and thermodynamic properties of the four main intermetallic phases in Al–Zn–Mg–Cu alloys

Abstract

First-principles calculations are performed to investigate the structural, elastic, and thermodynamic properties of the four main intermetallic phases, namely, MgZn2, Al2CuMg, Al2Cu, and Al3Zr, in Al–Zn–Mg–Cu alloys. The calculated lattice parameters agree with experimental and theoretical values. The formation enthalpy (ΔH) decreases in the order Al3Zr>MgZn2>Al2CuMg>Al2Cu, whereas the binding energy (Eb) decreases in the order Al3Zr>Al2Cu>Al2CuMg>MgZn2. The calculated result is in accordance with the experimental phenomena. The elastic constants Cij, aggregate elastic modulus (B, G, E), Poisson’s ratio, elastic anisotropy, and Debye temperature have been calculated. The calculated intermetallic phases exhibit elastic anisotropy on the basis of the universal elastic anisotropy index AU. According to the critical values for B/G and Poisson’s ratio, MgZn2 is a ductile phase, whereas Al2CuMg, Al2Cu, and Al3Zr are brittle phases. The Debye temperature decreases in the order Al3Zr>Al2CuMg>Al2Cu>MgZn2. The electronic structures have been investigated based on band structures and density of states. Metallic bonding mode coexists with a fractional ionic interaction in MgZn2, Al2CuMg, and Al2Cu. In Al3Zr, metallic bond coexists with covalent interaction. The temperature and pressure dependencies of volume, heat capacity, and thermal expansion coefficient are investigated systematically in pressures ranging from 0GPa to 20GPa and at temperatures ranging from 0K to 900K.