| SiC particle(SiCp)reinforced Aluminum Matrix Composites(AMCs)have excellent mechanical properties and thermal properties.Low volume fraction SiCp/AMCs(SiCvol%<20%)get better wear resistance,fatigue resistance and high temperature mechanical properties,which can be widely used in aerospace and automotive industries.High volume fraction SiCp/AMCs(SiCvol%>40%)can be used as electronic packaging materials due to its good thermal conductivity(>180w/mK),low thermal expansion(<4×10-6K-1)and lower density.At present,the methods of preparing SiCp/AMCs mainly include ex-situ synthesis and in-situ synthesis.Compared with the ex-situ synthesis,in-situ synthesis has got a lot of attentions due to its obvious advantages in well interface bonding.However,the chemical reaction during the in-situ synthesize is hard to control.In this thesis,different volume fraction SiCp reinforced aluminum matrix composites(AM Cs)were successfully synthesized in Al-Si-C alloy via in-situ reaction.The reaction mechanism and the microstructure of composites was studied.On this foundation,in order to improve the properties of the composites,an idea of combining ex-situ SiCp and in-situ SiC.p in AMCs to get a synergy strengthen effect is proposed by adding 5wt%ex-situ SiCp cleaned in mixed acid into Al-Si-C system and their effects on the properties of in-situ SiCp reinforced AMCs has investigated.The Al-Si-C mixed powder was prepared by planetary ball milling(PBM)through a wet milling process and then pre-formed in a steel mold.The pre-formed specimens were firstly sintered in a vacuum tube furnace at 1000? without pressure.In order to improve the relative density,the specimens were subsequently re-sintered in Spark plasma sintering(SPS)at 570?for 30min with 30MPa.By analyzing the reaction mechanism and performance of in-situ synthesized SiCp reinforced AMCs,we have got following conclusions:(1)The in-situ SiCp are mainly synthesized by indirect reaction,which is divided into two steps:4Al+3C ? Al4C3 and Al4C3+3 Si ? 3 SiC+4A1.SiCp reinforced AMCs without brittle phase Al4C3 can be in-situ synthesized only if the proportion ratio of raw materials in L+ SiC phase zone in Al-Si-C ternary phase diagram,in which the atom proportion of Si must be larger than 14 at%,and also larger than the content of C.Compared with ex-situ SiCp/AMCs,the in-situ SiCp are much smaller.Besides,the distribution of in-situ SiCp are more uniform and the edges of in-situ SiCp are much more rounded than ex-situ SiCp,which can lead to a better combination with matrix.(2)A core-shell structure of SiCp was found in SEM photos by adding 5wt%ex-situ SiCp(be cleaned by mixed acid)into the Al-Si-C system.In-situ SiC layer with 1?5?m thickness was synthesized around ex-situ SiCp,which is called SiC core-shell structure.The SiC core-shell structure leads to a better interface combination between reinforcements and Al matrix,which therefore causes a big increase of the strength of the composites.What's more,the synthesis of in-situ SiC layer of SiC core-shell structure can also reduce the interfacial energy of the in-situ SiC particles,which benefits the motion of free electros and leads to an improvement of thermal conductivity.(3)The fracture mechanism of in-situ SiCp reinforced AMCs is mainly based on the combination of ductile fracture of A1 matrix and brittle fracture of Al-Si eutectic structures by analyzing the SEM photos of the fracture surface of tensile specimen.Which indicates that the increase of Si can decrease the plasticity of the material.While the fracture mechanism of SiC core-shell structure reinforced AMCs consists of the ductile fracture of Al matrix and cleavage fracture of SiC core-shell structure.Some in-situ SiCp with small size were found in the dimples,which indicates a classical particle-induced ductile fracture behavior. |
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