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.
Influence of Ge content on the microstructure, phase formation and microhardness of hypereutectic Al–Si alloys
Abstract In this study, composition of the phases in ternary Al–18Si– x Ge 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– x Ge 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– x Ge 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.
Gas-chromatographic analysis of carbides in aluminium and magnesium
ZusammenfassungZur Bestimmung von Al4C3 in Aluminium und Magnesium wurde eine quantitative gas-chromatographische Methode (MolekularsiebSäule, FID) entwickelt. Der Einfluß von Methan aus äußeren Quellen, wurde untersucht und ausgeschaltet. Die metallographischen Untersuchungen von Carbiden an polierten Proben stimmten mit den Ergebnissen der gas-chromatographischen Messungen mit einem Faktor von 2 überein. Der Unterschied kann durch die großen Unsicherheiten bei der lichtmikroskopischen Analyse der kleinen Carbide erklärt werden. Die Analyse des technisch reinen Magnesiums zeigt, daß das Metall ca. 2–9 ppm Al4C3 enthält. Die Konzentration von Carbiden in elektrolytisch raffiniertem Aluminium liegt im Bereich 10–35 ppm und im Endprodukt bei ca. 2–12 ppm Al4C3. CaC2 wurde in einigen Magnesium- und Aluminiumproben mit Hilfe von gas-chromatographischen und Mikrosondenuntersuchungen festgestellt.SummaryA quantitative gas-chromatographic method (molecular sieve column, FID) for the determination of Al4C3 in aluminium and magnesium has been developed. The effect of methane from external sources has been investigated and eliminated. Metallographic analysis of carbides in polished specimens were consistent with the gas-chromatographic measurements within a factor of 2. The discrepancy can be explained by the large uncertainties in light microscopy analysis of the small carbides. Analysis of commercial magnesium shows that the metal contains ca. 2–9 ppm of Al4C3. The concentration of carbides in aluminium from the electrolysis cell is in the range of 10–35 ppm and in the final product of ca. 2–12 ppm Al4C3. CaC2 has been detected in a few magnesium and aluminium samples by means of gas-chromatography and microprobe analysis.