Improvement of mechanical properties and reduction of yield asymmetry of extruded Mg-Al-Zn alloy through Sn addition

The effects of Sn addition on the microstructure and mechanical properties of AZ62 alloy were investigated via optical microscopy, field-emission scanning electron microscopy, X-ray diffraction analysis, electron probe microanalysis, and electron backscatter diffraction analysis and by means of tensile and compressive tests of indirectly extruded AZ62 alloys with Sn contents of 2 wt%, 4 wt%, and 8 wt% (AZT62x alloy). All the extruded alloys exhibit a fully recrystallized grain structure with abundant fine precipitates. However, only Mg17Al12 precipitates are present in the AZ62 alloy, whereas Mg2Sn precipitates as well as Mg17Al12 precipitates are present in the Sn-containing AZT alloys. With an increase in the Sn content, more abundant Mg2Sn precipitates are dynamically formed during extrusion, which results in a reduction in the size of recrystallized grains through effective inhibition of their growth via the grain boundary pinning effect. The tensile and compressive yield strengths of the extruded alloy improve gradually with an increase in the Sn content, which is attributed mainly to the enhancement of the grain boundary hardening and precipitate hardening effects. The tensile elongation decreases slightly up until the addition of 4 wt% Sn and then deteriorates considerably upon the addition of 8 wt% Sn because of the presence of large undissolved Mg2Sn particles in the AZT628 alloy, which act as crack sources during tensile deformation. The tension-compression yield asymmetry reduces significantly with increasing Sn content; this is because the activation stress for {10-12} twinning increases to a greater extent than does that for basal slip owing to the refined grain size and additionally formed Mg2Sn precipitates.