Influence of solution heat treatment and ageing on microstructure and properties of in-situ Mg2Si/Mg-Al matrix composite

In-situ Mg2Si/Mg-Al matrix composite was prepared by permanent mold casting. The effects of heat treatment on microstructure and properties of in-situ Mg2Si/Mg-Al matrix composite were investigated. The results show that the morphology and distribution of Mg2Si in the Mg2Si/Mg-Al matrix composite are changed during T4 heat treatment. With increasing holding time, the roughly prismatic dendritic Mg2Si phase tends to be dissolved and broken and spheroidized and finally transformed into particles with size of 10-30 μm; at the same time, the β-Mg17Al12 phase is dissolved into the matrix to form supersaturated solid solution of α-Mg, then precipitates during subsequent ageing heat treatment. Due to solution of β-Mg17Al12 phase, the hardness of the composite decreases after T4 treatment, and then increases during subsequent ageing treatment. The hardness is improved by 14.9% after 415°C × 12 h solution heat treatment and then aging at 175°C for 16h(T6 treatment). The spheroidization of roughly prismatic dendritic Mg2Si phase during heat treatment can be explained by Gibbs-Thomson's theorem.