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%.