| The Mg2Si intermetallic compound, which exhibits high melting point, low density, high hardness, low thermal expansion coefficient and reasonably high elastic modulus, can be widely used in the aerospace industry, military industry and transportation industry. But Mg2Si has very serious intergranular embrittlement tendency, moreover it has intrinsic brittleness below450℃, so that the elongation at room temperature is almost zero. This weakness restricts its wide application. Therefore, toughening of Mg2Si has become a hot research issue in recent years. As the Mg2Si phase is thermodynamically stable, and has good compatibility with matrixes, as well as advantages including simple process and low cost, composites combined with Mg2Si phase and ductile matrixes such as Mg and Al can be a potential way to improve the toughness of Mg2Si. So, in this paper, the Mg2Si/Al composite materials were prepared by in situ melt reaction method, and the microstructure and mechanical properties were systemically studied to discuss the mechanisms for strengthening and toughening. The work has both important theoretical meaning and practical value for developing Mg2Si phase based materials with high strength and low density which will have great application potential.The effect of the morphology and distribution of primary Mg2Si phase on the microstructure of Mg2Si/Al(Al-30%Mg2Si和Al-40%Mg2Si) composite materials was systemically studied. By adding proper amount of alloy elements, rare-earth elements, AlTi-X grain refiners and heat treatments, the microstructure of Mg2Si/Al composites is controlled effectively, and the mechanisms are then discussed. Meanwhile, the mechanical properties of Mg2Si/Al composites are also studied, and the fracture mechanism is analyzed by fractography. The Detailed results are as follows:The proper alloying elements can improve the microstructure and mechanical properties of Mg2Si/Al composites. The Ca, P-Cu and Bi which are selected as modifications of Mg2Si/Al composite materials in the experiments are found to refine the primary Mg2Si, improve its morphology and distribution, and also inhibit anisotropic growth of the eutectic phase. The tensile strength can be enhanced by20-35%and elongation by14-39%with optimum addition of0.4%-0.8%Ca, P-Cu and about3.0%Bi.The addition of rare-earth elements improves the microstructure and mechanical properties of Mg2Si/Al composites. Several representative rare earth elements are selected for experiments including Sc, Y, La and mixed rare earth (La&Y) and (Sc&La). The addition of0.2%rare earth elements could inhibit growth of Mg2Si dendrites to a certain extent and improve the distribution uniformity of primary Mg2Si phases. When the addition of rare earth elements reaches0.6%-0.8%, the most uniformity distribution of primary Mg2Si phases and finest microstructure could be achieved. And the corresponding tensile strength and elongation could be increased by17-30%and27-38%respectively.Adding different AlTi-X refiners could obviously refine the microstructure and improve the mechanical properties of Mg2Si/Al composites. The selected three kinds of grain refiners including AlTiC, AlTiP and AlTiZr all could reduce the size and wedges of Mg2Si phase. In addition, the AlTiZr can get the most effective results, especially for Mg2Si/Al composite with higher Mg2Si content. By adding0.6-0.8%refiners, the tensile strength and elongation of Mg2Si/Al composites could be increased by11-20%and10-24%respectively.Solid solution plus aging heat treatment can reduce cast defects such as segregation and internal stress of Mg2Si/Al composites, and improve the hardness and tensile strength of the composites by ensuring certain ductility. The optimized heat treatment process is found to be545℃,5h solid solution plus175℃,3h aging. Under this process, the optimum microstructure could be achieved with fine primary Mg2Si phase and uniform, dispersed round-like eutectic Mg2Si without lamellar Mg2Si phase. The corresponding hardness and tensile strength increased by13.7%and11.9%respectively, with slight decrease of elongation. |
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