| As the transfer bridge of the load, the interfaces deeply affected the mechanical properties of the composites. In the present thesis, hot extrusion and various heat treatments were applied in order to change the interface state of the SiCp/2009 Al composit prepared by powder metallurgy and oxidized SiCp/2014 Al composites prepared by stirring casting and the evolution of the interface state was evaluated. Based on the microstructure observation of the matrix and interface of the composite, the effects of the interface state on the fracture process and crack initiation and propagation process of the composite were investigated using the in situ TEM tension and in situ SEM tension. The relationship between the interface state and interface characteristic was established, and inner link between the fracture behavior and the microstructure of the composite was explored.By powder metallurgy prepared SiCp/2009 Al composite materials, it was found that after hot extrusion, low content of secondary phases Cu Al 2 were observed in composite with clean and smooth interface. Under overaging state, precipitate free zone was found. After 50 h thermal exposure under 495℃, rod shaped Al4C3 was observed at the interface region of SiCp and the Al matrix. By stirring casting prepared SiCp/2014 Al composite materials, after hot extrusion, the Si O2 on the SiC particle surface was promoted to turn into Al2O3 and there were some secondary phases Cu Al2 in the matrix and the interface region. After aging, the size and density of the secondary phase Cu Al2 increased in both matrix and the interface region. The reaction between the SiCp and matrix was inhibited because of the oxide layer on the SiCp surface after the thermal exposure treatment. Thick and loose Al2O3 layer can be found after 20 h thermal exposure under 550℃.Based on the SEM in situ tension results, selecting oxidized SiCp/2014 Al composites prepared by stirring casting as the object, the primary crack was found to be formed due to the encounter of the initial crack and secondary c rack, and the expanding path was affected by the microstructure in front of the crack. The crack propagation process near the interface was affected by the characteristics of the SiCp, acting as crack deflection and cracking along the interface. After 10 h thermal exposure under 495℃, the strength of the matrix was moderately enhanced with excellent interface bonding strength, as a result, the interfacial debonding was inhibited with better compatibility of deformation. When extending the time to 20 h, the ductility of the matrix was improved while interfacial debonding was exhibited. When increasing the temperature to 550℃, the crack initiated at the SiCp/2014 Al oxide layer and grew along the interface.The TEM in situ tension results indicated that some ph enomena in this two different composites materials were similar: the cracks always initiated at grain boundaries, interfaces, damaged particles and particle enriched regions, and the grain boundaries, secondary phases and particles would hinder the crack propagation process. However, there still had some difference. To SiCp/2009 Al composites, due to the stress concentration, cracks can propagate along the interface between particles and the matrix. To SiCp/2014 Al composites,the load can be effectively transferred when uniformity and compact oxide layer was existed on the SiCp surface, and thus the bonding strength of the interfaces was enhanced. However, the bonding strength of the interfaces was weak when the layer was loose, and the cracks were initiated and grew along the oxide layer. |
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