Application And Properties Of Rare Earth Yb And Al₂O_3?p?in Al-Si Alloys

The Al-Si cast aluminium alloys have been widely used in aviation,electronics and automotive industries,owing to their low thermal expansion coefficient,good wear resistance and low density.However,in Al-Si cast aluminium alloys,there are a large number of coarse slab-like primary silicon and needle-like eutectic silicon in the as-cast microstructure,and the coarse dendrites of?-Al seriously,which significantly affect the machinability and mechanical properties of the castings.In addition,particle reinforced aluminium matrix composite is a new type of high performance material.However,the wettability between ceramic particles and aluminium matrix is poor and can not be uniformly dispersed in the matrix alloy.Improving the wettability between ceramic particles and aluminium matrix is important to improving the properties of composite materials.In this paper,rare earth Yb and the wettability of Al₂O₃ was studied.The effects of Yb addition on the solidification behavior,microstructure and silicon twinning of Al-10Si alloys were investigated by thermal analysis,optical microscopy,X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The results indicate that with the increase of Yb content,the three characteristic temperatures:nucleation temperature,minimum temperature and growth temperature of Al-10Si alloys decreased,and the cooling curve of Yb modified alloy shows obvious recalescence phenomenon.When Yb content is 0.7wt.%,the maximum recalescence value of the modified alloy is 2.3?.The three-dimensional morphology of eutectic silicon in Yb modified alloy has changed from coarse plate-like structure to honeycomb structure with many fine fibers structure.The Al-Si-Yb intermetallic compounds were observed in the 1.0 wt.%Yb modified alloy.Meanwhile,the results of X-ray diffraction and transmission electron microscopy indicate that the average twin spacing of Yb modified alloy was 18-46.2nm.The average twin spacing of eutectic silicon increased with the increase of Yb content.The average twin spacing of eutectic silicon reaches 18 nm when Yb content increases to 0.7wt%,which promoted the formation of twins and refinement of eutectic Si.The surface of Al₂O₃ powder was treated by electroless plating.The composite powder was analyzed by X-ray diffraction and scanning electron microscopy.The effect of coated particles on the wettability of Al₂O₃/Al-Si alloy was studied by high temperature and high vacuum contact angle measuring instrument,and the mechanism of improving the wettability was discussed.The results show that the surface of Al₂O₃ powder is successfully coated with a dense metallic Ni layer.SEM and EDS analysis show that the Ni layer is tightly wrapped on the surface of Al₂O₃powder.The X-Ray diffraction analysis of the composite powders after nickel plating shows that only the diffraction peaks of Al₂O₃ and Ni exist,and no diffraction peaks of other materials occur.By analyzing the contact process between Al-Si alloy and Al₂O₃ ceramic substrate,it can be concluded that after electroless Ni plating,the contact between Al-Si alloy and Al₂O₃ ceramic substrate is wetting.At high temperature,a nickel aluminate spinel?NiAl₂O₄?compound is formed by solid-state reaction between nickel and oxide film on the alloy surface,which breaks the oxide film on the alloy surface and promotes the flow of molten alloy.The interfacial tension of the system was reduced and the wettability of the Al₂O₃-Ni/Al-Si system was improved.The holding time at 1373K?1100??has a significant effect on the wettability of Al₂O₃ and molten Al-Si droplets after Ni plating.The experimental results indicate that the contact angle between Al₂O₃ and molten Al-Si droplets before and after Ni plating decreases rapidly with the increase of holding time.With the increase of holding time,the wetting angle of molten droplets continues to decrease,and the contact angle remains basically unchanged.It shows that holding in high temperature environment is conducive to the rupture of oxide film on alloy surface and the decrease of contact angle.In addition,because nickel exists in ceramic particles,the solid/liquid interface can be reduced to a certain extent,and the wettability of ceramic particles to Al-Si alloy can be improved.Al₂O₃ particles treated by electroless plating are evenly dispersed in the composites,which means that Al₂O₃ particles treated by electroless nickel plating have good dispersion in the composites.Compressive stress-strain curves show that the compressive strengths of the three composites are 417.2 MPa,474.4 MPa and514.2 MPa,and the elongation is 32%,34%and 35%respectively.This shows that the addition of Al₂O₃ particles makes the plastic deformation of the matrix alloy well coordinated under the same strain conditions,thus improving the compressive properties.