Microstructure and mechanical properties of conventional cast and rheocast Mg–Sn based alloys

In the present study, microstructure and mechanical properties of conventional cast and rheocast Mg–Sn alloy systems were investigated. The microstructure of conventional cast Mg-5%Sn and Mg-8%Sn alloys showed different phases, such as primary α-Mg, β-Mg2Sn phase that precipitates at the grain boundary, and eutectic α-Mg, a dark area surrounding the precipitate phase. With the addition of misch metal in Mg-5%Sn alloy the refinement of microstructure was observed and also the interdendritic second phase changed from rod/irregular shaped to rod and feathery shaped. In Mg-8%Sn-3%Al-1%Si alloy, α-Mg, Mg2Sn, Mg17Al12 and Mg2Si phases were observed. With the addition of Al–Sc master alloy in Mg-8%Sn alloy the matrix grain size became finer. The rheocast microstructures showed that the primary α-Mg phase is non-dendritic and globular shaped. Solution heat treatment at 520°C for 48–72h leads to complete dissolution of Mg2Sn intermetallic in Mg–Sn alloys. During ageing at 200°C discontinuous precipitates were formed at the grain boundaries in the initial stage and later continuous precipitates were occurred in the remaining regions. Hardness values were significantly improved after 100h of aging for all the alloys compared to as-cast alloys. The tensile properties of rheocast alloys were significantly higher than the conventional cast alloys.