Intermetallic phase formation and evolution during homogenization and solution in Al-Zn-Mg-Cu alloys

The effects of major alloy element contents of Zn, Mg, Cu in Al-Zn-Mg-Cu alloys on the formation and evolution of intermetallic phases during casting, homogenization and solution treatment have been investigated through using X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Experimental results showed that a relatively higher Zn content with lower Mg and Cu contents was beneficial to the formation of MgZn 2 phase instead of the Al 2 CuMg phase, which resulted in the unicity of the intermetallics in the Al matrix, and that the MgZn 2 phase was easier for diffusion and dissolution during homogenization and solution than the Al 2 CuMg phase. Additionally, the results of the first-principles calculations gave support for explaining the experimental phenomena. A larger absolute value of formation enthalpy and a smaller value of binding energy of the MgZn 2 phase, as compared with the Al 2 CuMg phase, give it priority to precipitate during casting and make it easier to re-dissolve during homogenization and solution treatment. What's more, higher elastic constants with severe anisotropy of Young's modulus make undissolved blocks of Al 2 CuMg phase act as crack initiation, which degrade the performance of the materials.