Effects of solution treatment on microstructures and micro-hardness of a Sr-modified Al-Si-Mg alloy

The microstructures and micro-hardness of a solution-treated Sr-modified Al-Si-Mg alloy are investigated. The dimension and shape factor of eutectic Si particles, as well as the mean diameter and fraction of Si-containing dispersoids, are quantitatively measured. It is revealed that the best spheroidized and refined eutectic Si particles can be obtained when the solution time is about 2 h. With further raising the solution time, the eutectic Si particle grows up anisotropically and exhibits facet characteristics with sharp edges. Meanwhile, some coarsened particles appear by mutual overlapping. Moreover, the Si-rich phases in α-Al matrix facilitate the precipitation of Si-containing dispersoids. The enhanced precipitation of Si-containing dispersoids with raising solution time leads to the formation of dispersoids free zones (DFZs) between two adjoint Si-containing dispersoids regions or in the areas adjacent to eutectics. Additionally, the solution strengthening effect of Mg atoms is stronger than that of Si atoms, which contributes to the increased micro-hardness of α-Al matrix in the initial solution stage. The dispersion strengthening effect of Si-containing dispersoids first increases then remains almost unchanged, revealing the change rule of micro-hardness in α-Al matrix during the later solution treatment. Furthermore, the lower micro-hardness of (Al+Si) eutectics is strongly related with the smaller eutectic Si particles.