Investigate On Evolution Of Microstructure And Interface Condition Of SiCp/A390 Composites

SiCp/A390 composites have become the ideal and promising new lightweight aluminum composite due to its low density,high hardness,good wear resistance,low coefficient of thermal expansion and isotropic properties,and so on.Heat treatment technology is important means to improve the performance of SiCp/A390 composites,broaden the application range of the composites.SiCp/A390 composites were fabricated by the powder metallurgy method and hot extrusion process.The microstructure evolution and the microstructure of interface and crystal degree of the composites in the process of heat treatment were observed and characterized by the metallographic microscope,scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray analyzer(XRD),high-resolution transmission electron microscopy(HRTEM).By the mean of micro-hardness tester,universal tensile tester and thermal expansion coefficient of PCY-Ⅲ tester,the mechanical properties and physical properties of the composite were measured under different heat treatment system.The best heat treatment process scheme of SiCp/A390 composites was optimized by the comparison and analysis of the heat treatment process.The researches show that there are some coarse metallic compound phases in the matrix of SiCp/A390 composites after hot extrusion,such as Al4Cu9,Al5Cu6Mg2,Al23CuFe4 phases.After the composites are subjected to the solution treatment,these coarse metallic compounds are fully dissolved into the matrix.With the increase in the solution temperature,metal compounds dissolve more adequately.When the solution treatment is at 515 ℃ for 4 hours,the metallic compounds have almost dissolved back into the matrix,with only very little amounts of the metallic compounds remaining insoluble in the matrix.When SiCp/A390 composites are aged at 180 ℃ after solution treatment at 515 ℃ for 4 hours,many nanoscale disc Al5Cu6Mg2 precipitated phases are dispersed in the matrix of the composites.With the increase of aging time,the disc the precipitated phases increase gradually;at the same time,the transformation rule of interfacial structure between the aging precipitation and Al matrix is: when the aging time is 2 hours,the interfacial structure is fully coherent interface;when the aging time is 6 hours,the interfacial structure transformes into a semi-coherent interface;when the aging time is 10 hours,the interfacial structure changes to a incoherent interface.Through the optimization of heat treatment process parameters of SiCp/A390 composites,the composites show excellent mechanical properties and physical properties.When solid-solution treatment is at 515 ℃ for 4 hours,The micro-hardness of the composites reaches maximum 223.4 HV;With the prolongation of ageing time,the hardness of SiCp/A390 composites reach “double peak” phenomenon.With the increase of ageing temperature,the hardening speed of the composites also increases,but at the same time,the hardening ability reduces.When ageing treatment is at 180 ℃ for 6 hours,the performance of the composites is optimal,and the tensile strength and elongation reach a maximum 274.6 MPa and 2.26%,respectively.The addition of SiC particles significantly reduces the thermal expansion coefficient of the composites.The effects of different heat treatment condition on the thermal expansion coefficient of the composites and the relationship between the thermal expansion coefficient of the composites and the ambient temperature are studied;with increase of ambient temperature,the thermal expansion coefficient of SiCp/A390 composites increases;the thermal expansion coefficient annealing state of the composites is greater than the extrusion state of the composite,while the thermal expansion coefficient of solid solution ageing state of the composites is lower than hot extrusion state.The SiC/Al interfaces of SiCp/A390 composites are vast majority of clean planer interfaces and tiny amorphous layer interfaces.The interface of SiC/Si and Si/Al is clean planer interface,besides it is clear,no defects,and have good combination.The interface structure between SiC/Al,SiC/Si,Si/Al is a semi-coherent interface by closely matching of atoms.