Abstract:The billets of Al-Zn-Mg-Cu-Zr and Al-Zn-Mg-Cu-Zr-0.5Er alloys were prepared by semi-continuous direct chill casting (DCC). The effects of trace Er on microstructure of Al-Zn-Mg-Cu-Zr alloy under as-cast and homogenization conditions were studied. The results show that the grain morphology is large dendritic structure and the grain size increases obviously by the addition of 0.5% Er. Moreover, most of Er element in the alloy segregates at grain boundary during solidification, resulting in ternary Al₈Cu₄Er phase. After homogenization, most of the MgZn₂ phase at grain boundary has dissolved back to Al matrix in the two alloys. In the Er-containing alloy, the dissolution temperature of Al₈Cu₄Er phase is about 575 °C. Therefore, the homogenization treatment cannot eliminate Al₈Cu₄Er phase validity.

Key words: Al-Zn-Mg-Cu-Zr alloy; Er; Al₈Cu₄Er phase; homogenization; microstructure