Composition and microhardness of Si–Ge solid solution precipitates in Al–Si–Ge alloys solidified during centrifugation
Abstract 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.
High-temperature centrifugation: a tool for finding eutectic compositions in multicomponent alloys
A metallic melt of composition Al52.6Cu13.4Ge28Si6 was processed for 2 h in a centrifuge at a temperature of 530 °C and inertial acceleration of 60 000 g (g=gravitational acceleration), and then slowly cooled to room temperature during continuous centrifugation. Primary phases forming in the melt during cooling were sequentially separated from the remaining liquid, changing the composition of the liquid, until it solidified in a ternary eutectic microstructure of composition Al64Cu3Ge33. Presenting scanning electron microscope images, we demonstrate that this method of high-temperature centrifugation is a efficient tool to isolate and identify multiphase eutectic compositions and the sequence of crystallization in multicomponent alloys. The latter is particularly useful for the discovery of new bulk metallic glasses.
The morphology of precipitates in an AlGe Alloy—I. experimental observations
Abstract The morphology and crystallography of Ge precipitates formed in a quenched and aged Al-4 wt% Ge alloy have been characterized using transmission electron microscopy and electron microdiffraction. Five different precipitate forms have been distinguished in a sample aged 3 h at 200°C: (i) triangular plates on {111} Al , (ii) laths parallel 〈100〈 Al , (iii) rods parallel 〈110〉 Al , (iv) hexagonal plates on {111} Al , and (v) small regular tetrahedra with {111} Al facets. The cube-cube orientation relationship, (111) Ge //( 1 11) Al and [110] Ge //[110] Al , has been assocaited with three separate forms of precipitate: the triangular and hexagonal plates on ( 1 11) Al andthe tetrahedra. A second type of twinned hexagonal plate has been observed on ( 1 11) Al with an orientation relationships deviating from cube-cube by a rotation of 30° about the habit plane normal, [ 1 11] Ge . The 〈100〉 Al laths have [110] Ge //[100] Al along their long axis and ( 1 11) Ge //(001) Al ; the long axis is an approximately invariant line. Rods parallel to 〈110〉 Al have in common a rod axis [110] Ge //[110] Al , but several orientation relationships have been detected, differing by a rotation about this axis. A summary is presented of all precipitate forms and associated orientation relationships observed in the AlGe system.