Mechanical and thermodynamic behaviors of the second phases in Al-Zn-Mg-Cu alloys

The mechanical and thermodynamic behaviors of intermetallics in Al-Zn-Mg-Cu alloys are studied by first-principles calculations. All studied second phases have negative values of formation enthalpy and cohesive energy indicating their excellent thermodynamic stability. Al₃Er_D022 has the most significant metallic nature, while Mg₂Si shows the least metallicity. TiAl₃ shows the highest bulk, shear, and Young's moduli. All Al₃M polymorphs, Mg₂Si and TiAl₃ phases show covalent/metallic hybrid bonding. The mechanical anisotropic behaviors obey the trend of: MgZn2>Al3Er_D022>Al3Sc_D022>TiAl3>Al3Sc_D023>Al3Er_D023>Al3Er_L12>Al3Sc_L12>Mg2Si, where MgZn₂ is the most mechanically anisotropic phase. The calculated room-temperature linear thermal expansion coefficient values for the studied phases are from 10.2×10-6 K-1 to 21.2×10-6 K-1; where Al₃Er_L1₂ has the highest value (17.3×10-6 K-1), followed by Al₃Sc_L1₂ (15.5×10-6 K-1); both of which are close to that of the Al matrix, thus making the relatively lower thermal misfit.