Microstructure Evolution And Interface Structure Of High-silicon Aluminum Matrix Composites Prepared By Powder Metallurgy

High-silicon-content Al-Si composites are attractive candidate materials in many application areas including aerospace,electronics and automotive industries,due to their low coefficient of thermal expansion,light weight,high thermal conductivity,high strength,and good wear resistance.In addition,high-silicon-content Al-Si composites are also potential candidate electronic packaging materials.However,the complicated preparation method of high silicon aluminum matrix composites remains as a main problem to limit their wide applications.The as-cast hypereutectic Al-Si alloy matrix contains large and brittle primary silicon.Large and brittle silicon crystals may cause stress concentration in the Al-Si alloys,leading to cracks or even fracture,which will influence the properties of Al-Si alloys.Therefore,large and brittle silicon particles in high silicon aluminum matrix composites must be refined to improve the mechanical properties.High-energy ball milling can refine the microstructure of Al and Si,which may result in homogeneous Al-Si composites.In this thesis,high-energy ball milling and powder metallurgy have been combined to produce high-silicon aluminum matrix composites.Al-Si powders with different milling time and compositions were ball-milled in a planetary ball mill,followed by cold pressing and low temperature sintering,leading to the production of Al-40 wt.%Si composites and Al-50 wt.%Si composites.The ball milling mechanism of high silicon aluminum matrix composites with different compositions has been studied.The microstructure,structure evolution and interface structure of Al-Si powders during ball milling have been thoroughly analyzed.The cross-section morphologies and properties of the bulk composites have also been investigated in detail.A fracture mechanism is dominant in the early stage: the Al and Si particles are deformed and then fractured under the action of high energy ball milling and the average particle size of Al-Si powders decreases at this stage.The second stage is dominated by cold-welding: the average particle size increases,instead of decreasing,due to the agglomeration of mixed powders.In the third stage,the particle size is refined to nanoscale.In the end,there is no significant change in the average particle size of Al-Si powders.As the ball milling time increases from 2 h to 50 h,the grain sizes of Al and Si gradually refine to the nanoscale,while the microstrain gradually increases.After long time ball milling of the Al-Si powders,the silicon particles are uniformly distributed in the aluminum matrix,and a tightly bonded Al-Si interface is formed on an atomic scale.The element distribution in the sintered specimen becomes more uniform and the specimen becomes denser with increasing milling time and sintering temperatures.Correspondingly,the Vickers hardness of Al-Si composites also increases with the increase of the milling time and the sintering temperature.