Microstructure and tensile properties of as-cast and as-aged Mg–6Al–4Zn alloys with Sn addition

• The Sn addition effectively refines the α-Mg dendrite by solute segregation.
• The Sn addition is benefical to modify the precipitation process at 175 °C.
• The Sn addition remarkably improves the as-cast and as-aged tensile properties.

The effect of 0–1.8 wt.% Sn addition on the microstructure and tensile properties of Mg–6Al–4Zn alloys was investigated. The results indicate that α-Mg dendrite is gradually refined with increasing Sn content and interdendritic Mg21(Al,Zn)17 and Mg17Al12 intermetallics can also be refined with 0.6 wt.% Sn addition. However, higher Sn addition leads to the coarsening and continuous distribution of the interdendritic intermetallics due to the stronger interdendritic segregation of Al and Zn. With the modified casting structure, the as-cast Mg–6Al–4Zn–0.6Sn alloy shows optimal ultimate tensile strength and elongation. After ageing treatment, the density of nano-scaled continuous precipitates into α-Mg matrix in all the Sn-containing alloys is enhanced markedly, which results in higher tensile strength due to enhanced ageing strengthening. Meanwhile, based on the microstructural analyses, alloy with 1.2 wt.% Sn can restrict forming discontinuous precipitates at grain boundary effectively. The as-aged Mg–6Al–4Zn–1.2Sn alloy possesses the excellent combination of strength and ductility.