Aluminum matrix composites(AMCs)have broad application prospects in the fields of aerospace,automobile manufacturing and electronic packaging.Compared with other reinforcements,hexagonal boron nitride(h-BN)has many excellent properties such as high melting point(>3000 K),high thermal conductivity(600 W/m·k)and good high temperature stability.So,it becomes one of the choices of aluminum-based composite material reinforcement.However,due to the chemical inertness,soft texture and lubricating properties of h-BN,the strengthening effect on AMCs is not obvious.At a certain temperature,BN and Al will react in situ to form AlN and AlB2.However,the current research on the in-situ reaction mechanism and the effect of the reaction products on the properties of aluminum matrix composites are not well studied.In the research of preparing Al composites with BN as a reinforcement,the following problems restrict the development and application of the composites.(1)The wettability of BN and Al matrix is poor,the interface bonding force is week,the effect of in-situ reaction on its enhancement effect is worthy of in-depth study;(2)The in-situ reaction mechanism of BN and Al is not clear;(3)The control of in-situ reaction between BN and Al and the effect of the products on the properties of AMCs.In view of the above problems,this thesis adopts powder metallurgy method,through high-energy ball milling,SPS sintering,tube furnace reaction heat treatment and hot extrusion,etc.,proposes the idea of first dense sintering followed by reaction treatment,and analyzes the hexagonal boron nitride at different temperatures.The thermodynamic conditions of the in-situ reaction between BNNSs and Al matrix,the reaction process and the formation of product phases,explore the influence of the in-situ reaction on the structure and performance of AMCs,through the characterization of the microstructure,interface structure,room temperature mechanical properties and electrical and thermal conductivity of AMCs,the strengthening mechanism of AMCs by the addition of BNNSs and its in-situ reaction products was clarified.The results of thermodynamic calculation and DSC analysis show that there is a slow exothermic peak between 740℃ and 1000℃.After heat treatment under different temperature conditions,it was found that Al and BN reacted slowly locally at 750℃,and the Al and BN completely reacted at 950℃.Analysis of the reaction mechanism found that the diffusion ability of B and N atoms in Al matrix is different,and AlN and AlB2 phases are formed at the grain boundaries and surface defects of the matrix through diffusion and nucleation.Microstructure observations found that with the increase of the reaction temperature,more and more second phases were formed at the interface between the matrix and BN,and the size showed an increasing trend.Performance test analysis results show that after heat treatment at 750℃,the material has a tensile strength of 166 MPa,a yield strength of 131 MPa,and maintains a good elongation of 20.4%and electrical conductivity of 50.8%IACS.After the reaction treatment at 950℃,the strength did not change much,but the elongation decreased.And with the increase of the reaction temperature,the thermal conductivity of AMCs increased,from 13 1 W/m·k at 750℃to 173 W/m·k at 950℃.Observation of the composite material interface shows that a nano-scale reaction transition layer is distributed on the interface to form a stable Al/AlN interface,which makes the AMCs have good interface bonding.In the analysis of the tensile fracture of the composite material,compared with the presence of more tearing edges in the Al-BNNSs composite before the reaction,the dimple size of the fracture is smaller and relatively uniform after the reaction treatment.In addition,comprehensive analysis of the strengthening mechanism of AMCs shows that the comprehensive properties of the composites are significantly improved due to the synergistic effect of the in-situ reaction generated AlN and AlB2 generated Orowan strengthening,fine grain strengthening and load transfer strengthening. |