Composition and microhardness of Si-Ge solid solution precipitates in Al-Si-Ge alloys solidified during centrifugation

The chemical composition of precipitates, the Knoop microhardness of the precipitates and of the matrix of Al-Si-Ge alloys solidified during centrifugation at 7 × g were investigated. The initial content of (Si + Ge) was (10 + 10), (20 + 20) or (25 + 25) at.%. The Knoop microhardness of the ingots varied with height in the ingot. This variation is attributed to sedimentation of Si-Ge during solidification. Chemical analysis showed that the maximum Si:Ge ratio in the precipitates for Al-10 at.% Si-10 at.% Ge alloy was about 92:8, and decreased with increasing Si and Ge content in ingots. The Si:Ge ratio and Knoop micro hardness also varied throughout each precipitate. The composition was constant along precipitates of alloy solidified without centrifugation. A variation in Knoop microhardness along the precipitates was attributed to this composition variation. The variation of the maximum Si:Ge ratio in the precipitates in ingots of different composition solidified with centrifugation is attributed to nonequilibrium conditions of solidification induced by decreased mass transport in the melt adjacent to the growing precipitates. The variation of Si and Ge along the Si -Ge precipitates is created by convection of the melt during solidification with centrifugation.