Abstract
In this study, low-weight, high-strength and ductile wrought Mg-9Al-Zn-xSn (x =1-3) Mg alloys were developed, using a conventional cast AZ91 base alloy specially with the addition of small amounts of Sn and the application of high-ratio extrusion. In the as-cast state, by Sn alloyed, the additional precipitation of Mg2Sn suppressed the homogeneous nucleation of detrimental eutectic -Mg17Al12 but drove the dissolution of more Al into primary -Mg, as verified from the shift of XRD peaks and a lowered liquidus temperature. In the high-ratio extruded Mg-9-Al-Zn-xSn alloys particularly with 2-3 wt.% Sn and with either pre-homogenization or pre-homogenization/pre-aging, the size of coarse grains was very effectively reduced to only 5 m in consequence of multiplied recrystallization and retarded boundary migration by a large amount of small Mg2Sn precipitates. Compared with a cast or homogenized/aged cast AZ91 alloy, the intense precipitation hardening and the effective microstructure refinement accordingly yielded markedly improved mechanical properties including a high strength of about 379 MPa and elongation of about 7.7%; a slight decrease in ductility was found only in the extruded Mg-9Al-Zn-3Sn alloy with pr-aging due to detrimental precipitate coarsening. These small Mg2Sn particles effectively retarded α-Mg grain growth, which would assist the alloys in retaining fine microstructures and developing high superplasticity. Small particles, in particular Mg2Sn as clearly seen on the fractured surfaces, impeded grain boundary sliding, which would detrimentally induce cavity formation and result in relatively low elongation at high strain rates.