Study On The Microstructure And Configuration Control Of In-situ SiC In Al-Si Matrix Composites And Its Interface Strengthening Mechanisms

With excellent mechanical,thermal and electrical properties,silicon carbide particle(SiCp)is one of the most common used reinforcement for Al matrix composites.The corresponding SiCp reinforced Al matrix(SiCp/Al)composites have several advantages including low cost,simple preparation process and excellent mechanical properties.During most of the preparation process,SiCp is externally added to the Al matrix,during which process interfacial issues,which can impair the particle reinforcement effect,are prone to generate.In-situ synthesis is an effective way to avoid the interfacial issues between the reinforcement and the matrix.However,few studies have been done on in-situ synthesis of SiCp and also the existing few studies are not complete.Based on this situation,this article has studied the in-situ synthesis of SiCp and has discussed several key problems during this process,including the in-situ coating of external SiCp,the control of the polytype and size of in-situ SiCp and the effect of in-situ SiC on the three-dimension(3D)structure of Al-Si alloy.This study has provided a novel idea to solve the interface issues between SiCp and the matrix,and has offered an effective way to fully develop the enhancement effect of SiC particles.(1)In-situ coating of external SiCA novel method,in-situ coating of ex-situ SiCp,has been developed in hypereutectic Al-Si alloys.The microstructure,growth mechanism,interface effect and strengthening behavior of in-situ SiC coating has been studied.In-situ coating,composed of many SiC lamellars,were formed by the epitaxial growth of external SiCp.The sandwich structure formed by external SiCp,in-situ coating and matrix is very stable.Close interface between SiCp and matrix,which is important to fully develop the reinforcement effect of SiCp,is realized by in-situ coating with the following three mechanisms:stress buffer,mechanical interlock and surface modification.In-situ coating SiCp can be stably added into multicomponent Al-Si alloys.With the addition of in-situ coating SiCp,the strength of Al-Si-Cu alloy was increased remarkably especially at high temperature,the wear resistance effect and the friction coefficient of the alloy was improved evidently especially at severe wear condition.(2)Control of the polytype and size of in-situ SiCpThree kinds of SiC particles,6H-SiC,3C-SiC and nano SiC have been prepared in Al-Si-C system by adjusting the raw materials ingredients and preparation parameters.The crystal morphology,internal microstructure and growth mechanism have been studied,the importance of reaction path and precursor on the preparation of SiCp has been revealed.Results show that 6H-SiC and 3C-SiC were obtained separately with liquid-solid reaction and master alloy method.The exposed faces and crystal morphologies of 6H-SiC and 3C-SiC are different,but coherent interfaces can both be observed within these two polytypes.The polytype of SiC is determined by the reaction path and precursor:6H-SiC was synthesized by one-step reaction with graphite as precursor,which provide convenience for its free growth;3C-SiC was formed by two-step reaction with Al4C3 as intermediate product and precursor,limiting its fully growth and leaving defects at the surface and within 3C-SiC.6H-SiC and 3C-SiC were grown under polynuclear multilayer growth mechanism and multi-region transition of Al4C3 mechanism,separately.Nano SiCp was obtained with liquid-solid reaction.Nano SiCp,with average size under 100 nm in diameter and thickness,has 3C polytype and show hexagonal plate morphology.To lower surface energy,nano SiC particles tend to connect with each other with coherent surface and are liable to form stacking fault at the interface.With the addition of nano SiCp,the strength of Al-Si alloy was elevated.(3)Effect of in-situ SiC on the 3D structure of Al-Si alloy.Si skeletons were directly extracted from Al-Si-SiCp composites,their microstructure,interfacial property and strengthening mechanism have been studied.Computed tomography(CT)technique was used to observe the microstructure of intermetallic micro-skeletons,the effect of in-situ SiC on the microstructure and skeleton integrity have been studied.Results show that by introducing in-situ SiC,intact self-support skeletons were formed in hypereutectic Al-Si alloy.Due to the direct and indirect connection effect of in-situ SiC,the interfacial bonding within the skeletons became closer and more effective,the microstructure of the skeletons became more consistent and regular,the pore of the skeletons became rounder and more uniform.Further research show that in-situ SiC can not only affect the nucleation and growth of Si phase,but can also improve the connection between SiC and Si.With introduction of in-situ SiC,the compressive strength of Si skeletons was increased remarkably and the average size of the pores was decreased evidently.In SiCp/Al-Si-Cu composites,in-situ SiC can affect the formation and growth of Si phase,and form micro-skeleton with Si.Meanwhile,in-situ SiC can affect the precipitation of intermetallic and improve the integrity of intermetallic micro-skeleton.Besides,the Si-in-situ SiC micro-skeleton and intermetallic micro-skeleton interweave with each other,resulting in a more complete composite skeleton.