Effect of homogenization treatment on microstructure and properties of novel Al-Mg-Zn-Sc-Zr alloys with different Mg/Zn ratios

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This study investigated the effect of homogenization treatment on microstructure and properties of Al-Mg-Zn-Sc-Zr alloys with different Mg/Zn ratios using optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), hardness, and conductivity tests. The results show severe dendrite segregation in Al-Mg-Zn-Sc-Zr ingots produced by water-cooled copper mold casting, with the as-cast microstructure mainly composed of an α-Al matrix, non-equilibrium T-Mg32(Al,Zn)49 phase, and a small amount of coarse primary Al3(Sc, Zr) phase. As Mg/Zn ratio decreases, the volume fraction of T-phase increases from 4.92 % to 5.41 %, and the grain size increases from 15.41 μm to 20.43 μm. DSC curves show the first endothermic peak at 468–475 °C, corresponding to the melting temperature of non-equilibrium second phase, setting the homogenization temperature to 460 °C. As homogenization time increases, non-equilibrium phase gradually dissolves. When Mg/Zn ratio is 1.71, the over-burning phenomenon occurs at 24 h; With the reduction of Mg/Zn ratio to 1.33, the over-burning occurred at 16 h. Two-stage homogenization significantly increased the amount of the second phase Al3(Sc, Zr) and refined grain size. After two-stage homogenization, the hardness and conductivity of the studied alloys with Mg/Zn ratio of 1.77 and 1.31 reach maximum values of 135.53HV and 144.22HV, and the conductivity is 30.10 % IACS and 28.71 % IACS, respectively.