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_D0(22)>Al3Sc_D0(22)>TiAl3>Al3Sc_D0(23)>Al3Er_D0(23)>Al3Er_L1(2)>Al3Sc_L1(2)>Mg2Si, where MgZn2 is the most mechanically anisotropic phase. The calculated room-temperature linear thermal expansion coefficient values for the studied phases are from 10.2x10-6 K-1 to 21.2x10-6 K-1; where Al3Er_L1(2) has the highest value (17.3x10(-6) K-1), followed by Al3Sc_L1(2) (15.5x10(-6) K-1); both of which are close to that of the Al matrix, thus making the relatively lower thermal misfit.