| Eutectic Al-Si alloy is widely used as preferred material for internal combustion engine pistons because of its low thermal expansion coefficient, high wear resistance, excellent castability and machinability for a long time. However, high speed and high power development requirement of internal combustion engine make conventional eutectic Al-Si alloy pistons unsatisfactory to this development requirement, and so hypereutectic Al-Si alloy (Si=16~ 26wt.%) is developed. Nevertheless, in the hypereutectic Al-Si alloy, primary silicon becomes extreme coarse with the increase of Si content, seriously affecting comprehensive properties and machinability of the alloy.Since the first discovery of icosahedral quasi-crystal phase, it has been a research focus in materials science area all along. Quasi-crystal possesses some unique properties such as high hardness, heat-resistant and low surface energy due to its unusual quasi-periodic lattice structure, which quite favors for its application as a strengthening phase in toughness matrix materials. Aluminum matrix composites reinforced by quasi-crystal have been a research focus and is an important generalization in field of quasi-crystal application. It is of great importance for expanding its application to do the research in this field.This research is based on the idea of quasi-crystal strengthening aluminum -based composite materials and designs a kind of new aluminum matrix quasi-crystal master alloy. And it is added to Al-25%Si alloy to improve the microstructure of eutectic aluminum-silicon alloy. Al-Mn-Ti ternary master alloy was prepared using vacuum induction melting and water-cooled copper mold solidification by adding Ti element into Al-Mn binary quasi-crystal alloy. The microstructure evolvement law of Al-Mn-Ti alloy has been detailedly discussed by using X-ray diffraction, optical microscope, scanning electron microscope and energy diffraction spectrometer. A new kind of ternary quasi-crystal is discovered and its formation has been studied. At the same time, Al-Mn-Ti quasi-crystal master alloy is added to Al-25%Si hypereutectic aluminum-silicon alloy and its effect on morphology of primary silicon is observed.The main research conclusions can be summarized as follows:1) There is no quasi-crystal phase and no ternary phase found in Al (86-X)Mn14TiX master alloys. Because the cooling rate of the experiment is much lower than that in rapidly quenched solidification condition, the as-cast microstructure of Ti-free alloy is composed ofα-Al phase and Al6Mn phase, while the microstructure of Ti-added alloy consists ofα-Al phase, Al6Mn phase and Al3Ti phase. And with the Ti content increases,α-Al phase decreased,and Al3Ti phase increased. 2) In Al (77.5-X)Mn22.5TiX master alloy, decagonal T quasi-crystal phase exist in the Ti-free alloy. However, some crystalline approximants Al4Mn phase exist in the alloy besides the quasi-crystal phase. Ti addition can refine the decagonal T quasi-crystal phase particles and restrain the formation of crystalline approximant(Al4Mn).When Ti addition surpasses 3.5at.%, petal-like phase appears in the alloy. Combination of XRD and EDS analysis identify it as Al67Mn13Ti20 quasi-crystal phase and its volume increases with increase of Ti addition.3) Analysis of Al67Mn13Ti20 quasi-crystal phase formation indicates that Mn2Ti phase first precipitate during the solidification and subsequent peritectic reaction: L + Mn2 Ti→Al67Mn13Ti20 finally result in the formation of ternary Al67Mn13Ti20 phase. Formation of ternary phase cause increase of Mn content in the remaining liquid and promote the formation of quasi-crystal T phase.4) Formation of Al67Mn13Ti20 quasi-crystal phase follows law of nucleation and growth. The growth interface of quasi-crystal is rough interface and its growth obey continuous growth model. Because the alloy is cooled by water-cooled copper mold, its cooling rate is much faster than that in the equilibrium solidification. At the same time, constitutional undercooling formed at ahead of the growth interface caused by solute atoms makes instability of growth interface increase and cause quasi-crystal phase grow in form of six-petal shape. The end of petal-like appear tact.5) Al73.5Mn22.5Ti4 quasi-crystal master alloy addition to Al-25%Si alloy results in obvious change of its microstructure. Primary silicon change from coarse five-petal asteroid shape to small particles, and the average size of primary silicon change from 300μm to 30μm when quasi-crystal master alloy addition is 4wt.%. Meanwhile, the macrohardness of Al-25%Si alloy slightly increases with increasing master alloy addition.
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