Effects of extrusion and Ag, Zn addition on the age-hardening response and microstructure of a Mg-7Sn alloy

The effects of extrusion and macro-alloying of Ag and Zn on the age-hardening behavior and microstructure of a Mg-7Sn alloy were investigated. The results show that the extrusion process and extrusion combined with 2 wt%Ag or 2 wt%Ag+1 wt%Zn addition can significantly improve the hardness of the unextruded Mg-7Sn alloy from 46 HV to 52 HV, 72.5 HV and 77.5 HV, respectively at the unaged state. The strengthening effect is mainly attributed to the dynamic precipitation and much refined grain size obtained by the extrusion process. Due to the depletion of the alloying elements by dynamic precipitation, the hardness increments resulted from artificial ageing at 160 °C are almost equal (around 10 HV) for the base and macro-alloyed Mg-7Sn extruded alloys. The extruded alloys exhibit a bimodal grain size distribution containing major dynamically recrystallized fine grains and minor severely deformed coarse grains. There are two types of Mg2Sn precipitates. One is the spherical particles distributed randomly and the other is the laths formed within the matrix. Ag or Ag+Zn addition to the extruded Mg-7Sn alloy can significantly refine the size of Mg2Sn laths. Blocky ε'-Mg54Ag17 phase is also formed along grain boundaries in the extruded Mg-7Sn-2Ag-1Zn alloy.