Study On The Microstructure Regulation And Mechanical Properties Of In-situ TiB₂ Particles Or Mg₂Si Reinforced Aluminum Matrix Composites

In-situ particles reinforced aluminum matrix composites（in-situ PRAMCs）possesses many outstanding properties such as light weight,high specific strength,strong bond between reinforced particles and matrix and so on,which have promising future in academic researches and industrial applications.Among various in-situ particles,TiB₂ and Mg₂Si are the two prominent candidates that can be feasible to fabricate in industrial manufacture.However,due to low yield and corrosive environment of salt-metal reaction used to synthesize TiB₂ particles,high mass-fraction TiB₂ particles composites are excessively expensive and difficult to fabricate.Moreover,the Mg₂Si phase in composites produced by melting-casting method was extremely coarse and its morphology was acute,which weakened the mechanical properties of composites.As a results,TiB₂ and Mg₂Si hybrid reinforced aluminum matrix composites were developed in this paper to cut down the content of TiB₂particles in composites in order to reduce the cost of productivity.Meanwhile,new methods for regulating Mg₂Si phase in composites were researched to improve the malleability of composites.Not only that,attention was also paid to aging behavior of the composites that could be strengthened by heat treatment.Firstly,9%TiB₂/Al-4.5Cu composites were successfully fabricated by salt-metal method.After solution treatment,the composites were aged at 180℃to investigate their aging behavior.The precipitation ofθphase had been significantly accelerated by the presence of TiB₂ particles but also exhibited inhomogeneous distribution thatθprecipitates located mainly on vicinity of TiB₂ particles.The evolution on mechanical properties of the composites could be divided into two stages as well.When the aging time was less than 8h,the age hardening of the composites could be attribute to the fine and dispersed Al₂Cu precipitates.While the aging time surpassed 8h,coarse Al₂Cu precipitates would affect the mechanical properties adversely.Compared with the aging behavior of Al-4.5Cu matrix alloy,the time to peak-aged condition in mechanical properties of the composites come down from 20h to 8h with the effect of TiB₂ particles.Then,nickel was added into Mg₂Si/Al composites to investigate its influence on solidification behavior,microstructure,and tensile properties.The results showed that the addition of nickel could induce the formation of Al-Al₃Ni-Mg₂Si double-rod-like ternary eutectic microstructure rather than plate-like eutectic Mg₂Si.When the content of nickel reached 3%,plate-like eutectic Mg₂Si completely transformed into Al-Al₃Ni-Mg₂Si ternary eutectic.With the content of nickel increasing and surpassing 3%,primary Al₃Ni and bulk-like eutectic Al-Al₃Ni would appear in the composites.The mechanical properties increased with the increasing nickel content and reached peak value when3%nickel was added.While nickel content surpassed 3%,the tensile properties decreased because of the formation of primary Al₃Ni and eutectic Al-Al₃Ni.The fracture behavior of composites changed gradually from brittle fracture to quasi-cleavage fracture,and ductile fracture when the content of nickel was 3%.Finally,the fracture behavior of composites returned to quasi-cleavage fracture when the content of nickel was 5%.Lastly,TiB₂ and Mg₂Si hybrid reinforced aluminum matrix composites were fabricated by the combination of salt-metal reaction and melting-casting method.The production cost of this composites was greatly reduced compared with 9%TiB₂/Al-4.5Cu because Mg₂Si decreased the content of TiB₂ particles and also replaced the expensive Al-Cu matrix alloy.The suspended TiB₂particles could act as good heterogeneous nuclei of eutectic Mg₂Si due to the coherent lattice relationship between TiB₂ and Mg₂Si.Thus,coarse eutectic Mg₂Si could be refined and modified with the addition of TiB₂ particles.With 5%TiB₂ particles addition,coarse and flake-like eutectic Mg₂Si in 10%Mg₂Si/Al no longer existed.The addition of TiB₂ particles could enhance both yield strength and elongation of the composites at the same time.The yield strength and elongation of the composites increased 59%and 141%with 5%TiB₂ addition,respectively.Moreover,the modulus of composites also increased 11%with 5%TiB₂ addition.The different addition of TiB₂ particles changed gradually the fracture behavior of composites from brittle fracture to quasi-cleavage fracture,and finally to entirely ductile behavior with 5%TiB₂ addition.