The Evolution And Control Of SI(GE) Phases In AL-SI-GE Alloys

In this work, the influence of Ge addition on microstructure and performance of Al-18Si alloy was investigated. The phase composition and evolution of Si(Ge) phases in the ternary Al-18Si-xGe alloys were conducted, and the relationship between morphology and hardness was also discussed. By grain refinement and rapid solidification, morphology of phases in the alloys can be controlled. In addition, the influence of Sn element on solidification characteristics, microstructure and hardness of Al-Si-Ge alloy was studied, and the feasibility of the alloys used as an aluminum solder was also explored.The major research results are as follows:(1) The evolution of microstructure and hardness in Al-18Si-xGe alloyThe phase compositions in ternary Al-18Si-xGe alloys were investigated.It forms a-Al, eutectic Si and eutectic Ge in the alloys when Ge content is lower than 10%, while it includes Si(Ge) solid solution besides a-Al and eutectic Ge when the addition of Ge is 60% or higher. An appropriate amount of Ge can refine eutectic Si and improve microstructures of the alloys. No matter primary or eutectic phases, both Si and Ge form a continuous solid solution. With the increasing addition of Ge, precipitation temperature of primary phase increases and the eutectic reaction temperature of Al-Si decreases. When Ge content increases from 0 to 10% in the alloys, the microhardness of the eutectics and macrohardness of the bulk alloys exhibit monotone increasing, while the microhardness of primary Si decreases firstly and then increases owing to the lower hardness of Ge and the change of Si crystal structure with Ge introduction.(2) Refining behaviors and mechanism of A1P on primary phase in Al-Ge and Al-Ge-Si alloyIn binary hypereutectic Al-70Ge alloy, the addition of 0.2% Al-3P master alloy leads to a refinement performance for primary Ge from 230μm to 40μm.While for Al-Ge-Si alloy, Si and Ge form continuous solid solution, with Si distributing in the center of the primary particles and Ge the outer layer. The primary solid solutions can also be significantly refined by AIR Its mechanism is that AlP can act as the nucleation sites for primary Ge particles and primary Si(Ge) solid solutions, and the lattice mismatch between them is less than 3.85%.(3) The study on the microstructure of rapidly solidified Al-Si-Ge alloyRapid solidification significantly changes the morphology of Al-Si-Ge alloys. Eutectic Si and eutectic Ge evolve to the strip or globular from the acicular or fishbone-like, and nanoscale size from micronmeter scale. In particular, the surface microstructures of the rapidly solidified Al-18Si-10Ge alloy were investigated. It exhibits the obvious separation between primary and eutectic Si. The enrichment of primary Si happens at the thinner oxide film area, while the enrichment of eutectic Si happens at the thicker oxide film area. There is no Ge phase in rapidly solidified Al-18Si-xGe alloys which is different from the usual as-cast alloys, and all of Ge element dissolves into the alloys. It is attributed to two factors, on one hand, Si-Si and Si-Ge atoms clusters are more stable than Ge-Ge atom cluster, on the other hand, due to the solid-liquid interface moving so quickly, Ge atoms have no enough time to spread and more Ge atoms are stayed in the solid phase.(4) The influence of Sn element on microstructures and properties Al-Si-Ge alloysIn this study, a serious of Al-9Si-15Ge-xSn (x=0,2,5,10) alloys were prepared. With the addition of Sn, eutectic Sn phase is formed in the alloys, distributing at the periphery of eutectic Si and eutectic Ge, and the size of eutectic Si decreases firstly and then increases. A small amount of Sn dissolves into Ge, little in Si, and the precipitation process is eutectic Si â†' eutectic Ge â†' eutectic Sn during the eutectic reaction. With the increasing of Sn addition, the liquidus temperature of the alloy decreases at first and increases later while the solidus temperature gradually decreases. The hardness of the alloy decreases, but its wettability for 6061 alloy increases. As in all things, Al-9Si-15Ge-5Sn alloy may be applied to study and development of the middle-temperature aluminum solder.