Research On The Preparation Of AlN Particles Reinforced Mg Matrix Composite And Its Microstructures And Properties

Magnesium(Mg)alloy is light in weight,which exhibits prominent advantages in energy saving and material lightweight development.However,as an industrial structural material,Mg alloy has insufficient mechanical properties such as strength and stiffness,which limits its further application.Introducing AlN reinforced particles with high melting point and high elastic modulus into Mg alloy is an effective way to improve its performances.In addition,the theoretical research and preparation technology of AlN particles reinforced Mg matrix composites are still in the exploration stage,and the microstructures and mechanical properties of the composites need to be optimized.In view of that,this work utilized crystal seed alloy pretreatment process and liquid-solid reaction in-situ synthesis technology to prepare composites,which exhibit excellent mechanical properties.The problem of excessive agglomeration of nano AlN particles due to the poor wettability between AlN particles and Mg matrixis,was also effectively solved.By adjusting the preparation process,the distribution and configuration of AlN particles could be controlled,and then optimized the microstructures and properties of the AlNp/Mg-8AI composites Moreover,the effect of AlN particles on grain refinement and second phase morphology improvement of Mg-8Al alloy was researched,and the interfacial microstructures between AlN particles and Mg matrix were also characterized.The strengthening effect of AlN particles on Mg-Al based alloys was revealed by comparing the mechanical properties between the matrix alloy and the prepared AlNp/Mg-8Al composites.The main research works are as follows:(1)Pretreatment process of Al-AlN crystal seed alloy and its application in preparation of AlNp/Mg-8Al compositesIt was found that the Al-AlN crystal seed alloy was added directly into Mg melt,AlN particles were aggregated into coarse agglomerations which were not easily dispersed,thereby affecting the microstructure uniformity and mechanical properties of the composite.In contrast,the Al-AlN crystal seed alloy was pretreated and then introduced into Mg melt,the dispersibility of AlN particles and interfacial bonding were significantly improved in the prepared AlNp/Mg-8Al composite.This is due to the formation of Mg-containing metal coatings around AlN particles during the pretreatment process,which improves the wettability between AlN particles and Mg matrix.In addition,AlN particles could refine ?-Mg grains,regulate the morphology of brittle ?-Mg17A1l12 phase,and pin grain boundaries,therefore the mechanical properties of Mg-8Al alloy were significantly improved.The tensile strengths of 0.8AlNp/Mg-8Al composite prepared by the pretreatment process is up to 225 MPa at room temperature and 148 MPa at 200?,and the elongation at room temperature is also increased by 213%compared with the matrix alloy.(2)Microstructure evolution behavior of diffusion couple in Mg/Al-Si-AlN crystal seed alloyAn diffusion layer rich in AlN particles would be formed after solid/solid diffusion of the Mg/Al-Si-AlN couple.AlNp/Mg-8Al composite was prepared successfully by adding the diffusion layer as a carrier of AlN particles into Mg melt,which exhibites excellent microstructure uniformity and mechanical properties.The reason is that the method eliminates the strong bonding between AlN particles and original Al matrix,and forms Mg17Al12 coating layers around AlN particles,thus improving the distribution of AlN particles in the Mg matrix and enhancing mechanical properties of the composite.(3)Design of liquid-solid reaction path and in-situ synthesis of AlN particles in Mg-Al alloyUsing Mg-Al-N as the reaction system,nano-sized AlN particles were in-situ synthesized in Mg-Al melt by liquid-solid reaction method.The in-situ AlN particles with sizes ranging from 10 to 100 nm are uniformly distributed in the composite matrix in interconnected chains structure.There are good interfacial compatibility and matching relationship between AlN particles and a-Mg matrix.Among them,the(0002)crystal plane of AlN is parallel to the(10-11)crystal plane of Mg,showing the bonding characteristics of near-coherent interface.The 8.2AlNp/Mg-8AI composite prepared by the liquid-solid reaction method is characterized in that the distribution of nano AlN particles is generally dispersed and partially connected to network configuration supporting the Mg matrix.The analysis shows that a large number of dislocations are introduced due to the thermal mismatch and strain gradient of AlN particles and Mg matrix.On the other hand,AlN particles can also effectively hinder the dislocation motion,pin the grain boundary slip,and withstand the external load.Therefore,compared with the Mg-8Al matrix alloy,the mechanical properties such as room temperature hardness,high temperature hardness and compressive strength of the 8.2AlNp/Mg-8Al composite are obviously improved.