Effects of Er on the Microstructure and Eutectic Phase Morphology of Directionally Solidified Al-Zn-Mg-Cu-Zr Alloys

Four alloy rods of Al-Zn-Mg-Cu-Zr-xEr (x=0, 0.1, 0.2, 0.5, wt% ) were prepared by directional solidification. The microstructure and second phase morphology of alloys were investigated by optical microscopy (OM), electron probe micro-analysis (EPMA), energy dispersive spectrometer (EDS) and other methods. The results indicate that Er element can increase the number of dendrites in the directional solidification structure of Al-Zn-Mg-Cu-Zr alloy, and reduce the primary dendrite arm spacing and secondary dendrite arm spacing of the alloy. A proper amount of Er element can reduce the content of second phase in the alloy and increase the tendency to form round second phase. The formation of Al₈Cu₄Er phase can reduce the Cu content in the T phase (AlZnMgCu phase), which plays a role of rounding the boundary of the T phase, and can change the morphology and internal structure of the T phase. The Al₈Cu₄Er phase serves as the nucleation position of the T phase, and part of the T phase grows around this phase. The addition of Er can improve the microhardness of the alloy.