Rheological Solidification Behavior and Mechanical Properties of AZ91-Sn Alloys

The solidification and tensile deformation behaviors of rheo-cast AZ91-Sn alloys were revealed to study the effects of Sn alloying on improvement of AZ91 alloy's mechanical properties. Two kinds of Mg17Al12 phases precipitated from the supersaturated magnesium matrix during rheo-solidification were observed: coarse discontinuous precipitates (DP) at grain boundaries and small-sized continuous precipitates (CP) inside grains. With increasing Sn content, the amount of Mg17Al12 phases was increased whilst the amount of Al atoms in the matrix was decreased. Due to the higher melting point of Mg2Sn than Mg17Al12, Mg2Sn precipitated earlier from the melt, and therefore provided heterogeneous nuclei for Mg17Al12 during the eutectic reaction. Due to grain refinement and solid solution strengthening, AZ91-2.4Sn (mass%) gained 52% increase in tensile strength and 93% increase in elongation compared with pure AZ91 alloy. The higher-density twins and microcracks induced by Sn alloying relaxed stress concentration during plastic deformation, so the fracture mode was transformed from cleavage fracture of pure AZ91 alloy to ductile fracture of AZ91-Sn alloys.