Refined microstructure and improved mechanical properties of high-ratio extruded AZ91−xSn magnesium alloy

To improve the mechanical properties, including the ductility, of cast AZ91 magnesium alloy (Mg-9Al-Zn) to provide high workability, small amounts of Sn were added to AZ91, forming AZ91−xSn (x = 1-3) alloys; following homogenization/aging treatments, high-ratio extrusion was conducted on the alloys. The correlation between the microstructural refinement and the improvement in mechanical performance was studied. In the as-cast AZ91−xSn alloys, small Mg2Sn precipitates formed in an Al-enriched α-Mg matrix, and the formation of detrimental, large eutectic β-Mg17Al12 was suppressed, resulting in an increase in hardness from Hv 68 to 77. The high-ratio extrusion greatly refined the microstructure and increased the hardness further to Hv 101. A higher Sn content resulted in the formation of more small Mg2Sn precipitates, and reduced the size of the coarse grains to 5 μm. The wrought AZ91−xSn alloys with added 2-3 wt.% Sn and extrusion had a markedly better tensile strength, 380 MPa, than that of a cast AZ91 alloy, and exhibited an elongations of 8-9%.