| Particulate reinforced aluminum matrix composites(AMCs)have been paid much attention by researchers and are widely used in aerospace fields in recent years,due to their combinative advantages of both Al matrix and the reinforcements,such as low density,high strength and good wear resistance.In general,the composites are prepared by powder metallurgy,spray forming or casting methods,and then the deformation process is carried out,such as extrusion,rolling or compression.Especially hot extrusion,which is beneficial to eliminate voids and improve the density of material,is an important method for Al alloy processing.After elaborate researches over the last decades,the strength potential of particulate reinforced aluminum matrix composites has been continuously enhanced.However,the strength-ductility trade-off is a long-standing problem and the ductility of AMCs is generally low,which limit subsequent processing and application of this kind of composites.Therefore,preparation of AMCs with great strength and ductility simultaneously is very important.In addition,due to the expanding of application fields of aluminum alloys,especially in aerospace and automotive pistons,the requirements for low density and heat resistance of materials are increasing.Due to the reinforcements of Al matrix composites having high melting point and thermal stability,Al matrix composites show outstanding application potential.Based on the preparation of AlN/Al composites by the research group and the micro-interface reaction of Si3N4 with Al during the liquid-solid reaction process,the nano-AIN and sub-micron Si3N4 particle reinforced aluminum matrix composites were prepared.Firstly,the interfacial reaction of Si3N4 in aluminum melt was systematically studied by XRD,DSC,SEM and TEM.At the same time,the influence of T4 heat treatment on the microstructure and properties of extruded composites was analyzed.In addition,by changing the preparation process to achieve the spatial distribution of particles,the relationship between the material microstructures and properties was studied and analyzed.The main research work of this thesis is as follows:(1)Preparation of AlN+Si3N4/Al composite and influence of particle contentsFirstly,a variety of analytical methods were used to study the transformation process of Si3N4 in aluminum melt:at lower temperatures,Si3N4 and Al have a slight interfacial reaction,and many nano-sized and differently oriented AIN particles are distributed on the surface of Si3N4,resulting in forming a nanoscale cladding layer formed.At this time,AlN with Si3N4 and AlN with Al have clean interfaces and good interface combination.With the increase of temperature,the reaction degree of Si3N4 and A1 increases continuously.After reaching a higher temperature,Si3N4 will completely react with A1 to form Si and AlN.In addition,Gibbs free energy calculation,DSC thermal analysis,XRD,SEM and TEM observations shows that the reaction between Si3N4 with A1 was not only related to thermodynamic energy,but also affected by reaction kinetics.Based on the micro-interface reaction of Si3N4 with Al and the preparation of in-situ AlN/Al composites,the nano-AlN and sub-micron Si3N4 particles reinforced Al matrix composite was obtained by liquid-solid reaction.And then we studied the effect of particle content on the properties of composites and the results show that with the reinforcement content increasing,the strength of the composites obviously increases and the plasticity decreases.However,when the particle content reaches a relatively high level,the strength of the composites moderately increases and the plasticity sharply decreases.(2)Effect of heat treatment on strength-ductility at room temperature and heat resistance at high temperatureThe extruded Al-7.7%AlN-5%Si3N4 composite was used as the research object,and it was subjected to T4 heat treatment which was water quenching after being held at 540 ? for different time.The test results show that the static recrystallization and grain growth occur in the extruded composite during the heat treatment,leading to the changes of composite microstructure,including the texture,grain orientation,grain size,dislocation density of the matrix andthe grain boundary angle orientation distribution,which ultimately affects the mechanical properties of the composites.Under the condition of water quenching at 540? for 3 h,the strength-ductility at room temperature and strength at high temperature reached the maximum,which is closely related to the microstructure of the material,and the relationship between the microstructure and performance was systematically studied.(3)The effect of spatial distribution of particles on the propertiesThe extruded Al-7.7%AlN-5%Si3N4 composite was used as the research object.By changing the preparation parameters(extrusion parameters and gradation of different size A1 powders),the distribution of the particles was control and the comprehensive properties of the composites were affected.Taking different extrusion parameters as an example:for the low extrusion deformation degree,more particle-free regions in the as-cast composite are retained in extruded composite.And after extrusion,strip-like structures are formed along the extrusion direction;However,the high extrusion deformation degree causes large deformation of the matrix and particles,resulting in the disappearance of the particle-free zone and the more uniform distribution of particles.The tensile test results show that the uniformly distributed particles are beneficial to the composite strength,especially the strength and stability at high temperature,while the existence of the particle-free region is beneficial to the plastic deformation and toughness of the material.Similarly,the gradation of coarse and fine aluminium powders can also change the distribution of particles,thus making the distribution of particles more uniform and reducing the area without particles,which is conducive to improving the strength properties of composites both at room and high temperature. |
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