| As more stringent demands,such as specific strength,specific stiffness,have been put forward along with the rapidly developing aviation and aerospace fields,and it is difficult for conventional aluminum alloy to meet the design requirements of the future aircraft.Particle reinforced aluminum matrix composites with high specific strength,high specific stiffness and well wearing resistance,have been put forward as one of the hot issues in aviation and aerospace fields.However,the integration of the reinforcement particles often severely degrades the machinability of the matrix as they are in micrometer-scale,which makes it difficult to processing precision components.The in-situ method can refine the particles to the nano-scale in the aluminum matrix composites.And by modifying the microstructure through severe plastic deformation,the aluminum matrix composites with superplasticity are obtained.Application of the superplastic deformation to the particle reinforced aluminum matrix composites can realize the one time forming of complex components,and benefit to the application and development of the aluminum matrix composites in the fields of aerospace materials.This thesis obtained the in-situ TiB2/7050 composite with fine-grain structure by severe plastic deformation（including friction stir processing and accumulative orthogonal extrusion）and analyzed microstructure of the material and its relevance with the superplasticity.The range of the temperature and initial strain rate in which the superplasticity occurred in the in-situ TiB2/7050 composite was determined through tensile test at high temperature.Besides,the true stress-strain curves during superplastic deformation were acquired.This thesis also studied the superplastic mechanism in the in-situ TiB2/7050 composite and discussed the effect of the TiB2 particles during the deformation by calculating the value of strain rate sensitivity m and activation energy Q,and analyzing the microstructure of the tensile fractures.Firstly,improve the microstructure of the in-situ TiB2/7050 composite by friction stir process of which the processing is simple and short.Discuss the possibility of achieving superplasticity from the microstructural aspects in the composite.The research shows that the material obtains a fine-grain structure with dispersed TiB2 particles.The composites are thermal stable due to the efficient pinning effect of TiB2 particles on the grain boundaries at high temperature and its inhibition of the abnormal grain growth.As a result,the composites exhibit good superplasticity at a temperature of450℃ and a strain rate of 10-3 s-1.The maximum elongation is 619.2%.Considering the impossibility of preparing the large size sample by friction stir processing,the theoretical guidance for the industrial application of superplastic forming is limited.Secondly,improve the microstructure of the composites by accumulative orthogonal extrusion which is suitable for large size sample preparation.Discuss the superplastic behavior of the in-situ TiB2/7050 composite and provide the theoretical foundation for the superplastic forming.The study shows that the superplasticity was observed at 350-500℃ and 2×10-4-10-2 s-1.The maximum elongation is 557.0%occurred at a temperature 450℃ and a strain rate of 10-3 s-1.The peak stress under that test condition is 19.4 MPa.The analysis of mechanical properties shows the value of m is in the range of 0.20-0.34,and the value of Q is in the range of 134.66-223.04kJ/mol.The microstructural study shows that the obvious grain boundary sliding is observed on the surface of the tensile sample.While the nano-sized TiB2 particles are existed on the grain and at the grain boundary.Besides,the fibers in the nano-scale are overlapped around the grains.The analysis of mechanical properties and observation of the microstructure reveal that the dominant mechanism is grain boundary sliding,which is accommodated by dislocation motion,lattice diffusion and liquid phase. |
PDF Full Text Request