Abstract:The evolution of the eutectic structures in the as-cast and homogenized 7X50 aluminum alloys was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), differential scanning calorimetry(DSC), X-ray diffraction(XRD) and tensile test. The results show that the main phases are S(Al₂CuMg), T(Al₂Mg₃Zn₃) and MgZn₂, with a small amount of Al₇Cu₂Fe and Al₃Zr in the as-cast 7X50 alloy. The volume fraction of the dendritic-network structure and residual phase decreases gradually during the homogenization. After homogenization at 470 °C for 24 h and then 482 °C for 12 h, the T(Al₂Mg₃Zn₃) phase disappears and minimal S(Al₂CuMg) phase remains, while almost no change has happened for Al₇Cu₂Fe. There is a strong endothermic peak at 477.8 °C in the DSC curve of as-cast alloy. A new endothermic peak appears at 487.5 °C for the sample homogenized at 470 °C for 1 h. However, this endothermic peak disappears after being homogenized at 482 °C for 24 h. The T(Al₂Mg₃Zn₃) phase cannot be observed by XRD, which is consistent with that T phase is the associated one of S(Al₂CuMg) phase and MgZn₂ phase. The volume fraction of recrystallized grains is substantially less in the plate with pre-homogenization treatment. The strength and fracture toughness of the plate with pre-homogenization treatment are about 15 MPa and 3.3 MPa·m^1/2 higher than those of the material with conventional homogenization treatment.

Key words: 7X50 aluminum alloy; microstructural evolution; homogenization; residual phase; recrystallization