Influence of Ge content on the microstructure, phase formation and microhardness of hypereutectic Al-Si alloys

In this study, composition of the phases in ternary Al-18Si-xGe alloys was investigated. It was found that except α-Al, there may form primary Si, primary Ge, eutectic Si and eutectic Ge, depending on different composition of the alloys. The influence of Ge content on the microstructure, phase formation and microhardness in Al-18Si alloy was investigated. Experimental results show that the introduction of Ge in Al-18Si alloy can modify the microstructures, and Ge and Si can replace with each other to some extent in both primary and eutectic phases. The increase of Ge addition level in Al-18Si-xGe alloys leads to an increase of Ge content in primary Si. There also exist continuous composition transition zones for Si and Ge in eutectic structure. Combining with the microstructures, the solidification process of the Al-18Si-xGe alloys was analyzed by differential scanning calorimeter (DSC). Besides, with the increase of Ge content in the alloys, the microhardness of primary Si decreases first and then increases, while the microhardness of the eutectics and macrohardness of the bulk alloys exhibit monotone increase.