Study On Microstructure And Mechanical Properties Of In-situ Prepared TiB₂ Particles Or Hybrid Reinforced Aluminum Matrix Composites

The particulates reinforced aluminum matrix composites?PRAMCs?have outstanding combination of mechanical properties,such as high specific strength,specific modulus,hardness and low thermal expansion coefficient.They are promising materials for application in structure,transportation,aerospace and the military.Among various potential particulate reinforcements,TiB₂ phase is portrayed to be an outstanding reinforcements in aluminum because of its good thermodynamic stability,high hardness,high melting point,high modulus,high corrosion resistance and low density.TiB₂ also has no interface reaction with aluminum.The salt-metal reaction route is an in-situ process for preparing TiB₂ particulate reinforced aluminum matrix composites with unique advantages.Compared with external addition processes,TiB₂ particles formed by the salt-metal reaction route are much well bonded with aluminum matrix,thus the particle-matrix system has clearer interface and better interfacial thermodynamic stability.And the in-situ process has no wetting problem between TiB₂ particles and molten aluminum.Compared with other in-situ processes,the salt-metal reaction route has much lower reaction temperature which means the reactions are more controllable,meanwhile TiB₂ particles which synthesized by the saltmetal reaction are smaller and can be submicron in size.But the salt-metal reaction route also has some shortages such as it is difficulty to fabricate high TiB₂ particulate volume percentage aluminum matrix composites,intermediate phases created by the salt-metal reaction may stay in the final composites and the agglomerations of TiB₂ particles increase with the increasing of particulate volume percentage.Thus the reaction process needs improvements.This thesis includes study of the microstructures of TiB_2p/Al-4.5Cu composites prepared by the salt-metal reaction route and analyzing of the formation mechanism of all kinds of the agglomerations in the composites.Meanwhile the study of using mechanical stirring and ultrasonic vibration to improve the particulate distribution of TiB_2p/Al-4.5Cu composites is achieved with analyzing the mechanism of the improvements and the mechanical properties of the composites.This thesis also contains the preparing of TiB₂+Mg₂Si/Al composites and the study of the microstructures and mechanical properties of the composites.Firstly,the 2.5vol% and 5vol%?nominal?TiB_2p/Al-4.5Cu composites are successfully prepared by the salt-metal reaction route.The intermediate phases Al3 Ti and AlB₂ are well controlled by adjusting the technological parameters of the salt-metal reaction route such as control the ratio of Ti/B in reactants,avoiding the fluctuation of reaction temperature and so on.The agglomerations of TiB₂ particles in the matrix are studied and classified.Part kind of the agglomerations can be eliminated by adjusting the technological parameters of the salt-metal reaction route such like the agglomeration created by the uncompleted reactions,but the most common agglomerations of TiB₂ particles need some assistant processes to eliminate.Then the mechanical stirring is successfully applied to improve the particulate distribution of 5vol% TiB_2p/Al-4.5Cu composites which have more agglomerations.The large agglomerations are effectively reduced by mechanical stirring,agglomerations are gradually reduced,refined and broken with the increasing of stirring speed.Finally,the agglomerations over 50 ?m in size are eliminated when the composites treated by mechanical stirring with speed at 540 rpm.TiB₂ particles are uniformly distributed along the grain boundary regions.The grains of the matrix are also effectively refined.Some dispersed TiB₂ particles under 400 nm in size are observed in the matrix,and some nanoparticles are observed in the gaps of macron particles in the gathers along the grain boundary.The composite treated by mechanical stirring with speed at 540 rpm has the best overall mechanical properties,the yield stress is improved to 120 MPa and the ultimate tensile strength is improved to 243 MPa.For further improving the particulate distribution,the ultrasonic vibration is successfully applied in preparing 5vol% TiB_2p/Al-4.5Cu composites.Large agglomerations in the melt are eliminated at the very early stage of ultrasonic vibration treatment.With extension of ultrasonic vibration treatment time,small agglomerations under 50 ?m are also eliminated.Nanoparticles are also effectively dispersed by ultrasonic vibration treatment.Finally,the ideal composite melt with dispersed and uniform particulate distribution is achieved after ultrasonic vibration treatment for 240 s.TiB₂ particles are uniformly distributed along grain boundary throughout the bulk matrix.The grains are refined and the refinement increases with extension of ultrasonic vibration treatment time.The composite treated by ultrasonic vibration for 240 s has the best overall mechanical properties,the yield stress is improved to 137 MPa and the ultimate tensile strength is improved to 245 MPa.Yield strength and ultimate tensile strength are improved by 114% and 50% comparing with base metal,respectively.The ultrasonic vibration treatment also can effectively improve the particulate distribution of 2.5vol% TiB_2p/Al-4.5Cu composites.It only needs 120 s for ultrasonic vibration treatment to achieve the ideal 2.5vol% TiB_2p/Al-4.5Cu composite melt with dispersed and uniform particulate distribution.Compared with eliminating the particles agglomerations,the ultrasonic vibration treatment has more significant effect on making the TiB₂ particles distributing more uniformly throughout the bulk matrix.The mechanical properties of 2.5vol% TiB_2p/Al-4.5Cu composites are also improved after treated by ultrasonic vibration.In this thesis,the salt-metal reaction route and melting process are firstly combined to fabricating 5vol%TiB₂+10vol%Mg₂Si/Al composites.The TiB₂ and Mg₂ Si particles are uniformly distributed along grain boundary in the composite.The size of TiB₂ particles is under 400 nm in diameter.And the grains of the composites are effectively refined.The mutual refinement of TiB₂ particles and Mg₂ Si phase is discovered,TiB₂ gathers along the grain boundary are broken up by Mg₂ Si phase and the eutectic cells of Mg₂ Si are refined.The Mg₂ Si eutectic phase is also refined from over 50 ?m to under 10 ?m.The mutual refinement mechanism is discussed.The mechanical properties of the composites have a great improvement by the mixture of TiB₂ particles and Mg₂ Si phase.Compared with TiB₂/Al composite,the yield stress and ultimate tensile strength of TiB₂+Mg₂Si/Al composite are improved by 91 % and 48 %,respectively.