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 alpha-Mg, beta-Mg2Sn phase that precipitates at the grain boundary, and eutectic alpha-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, alpha-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 alpha-Mg phase is non-dendritic and globular shaped. Solution heat treatment at 520 degrees C for 48-72 h leads to complete dissolution of Mg2Sn intermetallic in Mg-Sn alloys. During ageing at 200 degrees 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 100 h 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. (c) 2012 Elsevier B.V. All rights reserved.