Research On The Microyield Behavior And Strengthening Mechanism Of SiC/Al Composites

The Si C/Al composites have excellent micro-yield strength, specific stiffness and temperate thermal expansion coefficient, which guaranteed their wide application in parts of precision instruments. However, there are still some fundamental questions without clear theory. For instance, the mechanism of impact of reinforcement sizes on micro-yield strength, and the problem that the growth of precipitates in matrix will decrease stability of the composites during the long-term service, etc. In order to design the properties of Si C/Al composites that have excellent dimensional stability, research on size effect of micro-yield strength and optimal selection of matrix alloy were carried out in this thesis. Strain gradient theory was utilized to study the mechanism of influence of particle size on micro-yield strength of composites, then finite element method was used to create the prediction model of micro-yield strength with particle size term, which provides guidance for particle size selection. Then optimal meth od of alloy elements selection that have high micro-yield strength and appropriate for long-term service was proposed, aiming at the problem of the growth of precipitates in matrix, Sc, Zr and Mg were chose as alloy elements, and composites were fabricated by pressure infiltration technique correspondingly. Microstructure and micro-yield strength of Si C/Al composites with optimal selected alloy elements were studied systematically, mechanism of the influence of alloy elements on micro-yield strength of Si C/Al composites were discussed, providing basic knowledge to optimization of alloy elements.Si C/Al composites with four different particle sizes were fabricated by pressure infiltration technique, influence of Si C particle size on micro-yield strength of Si C/Al composites was studied. According to the analysis, the micro-yield strength increases as particle sizes decrease. Through the microscopic strain gradient model built, it has been proved that particle size affects the density of geometrically necessary dislocation of matrix by changing distance between particles, and then affects the micro-yield strength of the composites. Further studies show that geometrically necessary dislocation produced during the casting cooling process is the main cause of strengthening of micro-yield strength of composites by particle size.According to the mechanism of impact of particle size on micro-yield strength, correction has been made to the elasticity part of the constitutive relation of matrix, based on which prediction model that describes influence of particle size on micro-yield strength of Si C/Al composites was built by finite elements method. With different geometric models or constitutive relations that have impacts on micro-yield strength of the composites compared, multi-particle model and “CTE+EM” constitutive relation have been shown to be suitable for Si C/Al composites. Micro-yield strengths of Si Cp/pure Al composites with different particle sizes were predicted by this model, experimental verification by composites with 5μm particle size showed a relative error of 11.7% of predicted result, which means that this model is reliable to some extent.Aiming at the problem of poor stability of long-term service of Si C/Al composites caused by the growth of precipita tes, Ostwald coarsening theory was utilized to obtain the selection method of alloy elements added into Al alloy: i) that could form intermetallic compound with Al; ii) that have large maximum solid solubility and small solid solubility at room temperature; iii) that have low diffusion coefficient; iv) that have precipitates with coherent lattice with Al matrix; v) that could be fabricated by traditional casting technique. Sc, Zr and Mg were selected as alloy elements according to the selection method, then the mechanism of impact of alloy elements on microstructure and micro-yield strength of Si C/Al composites were studied.Microstructure and microyield strength of the Si C/Al-Sc-Zr composites added Sc,Zr was studied under different heat treatments, and the action mechanism of Sc,Zr on microyield strength was revealed.. The results shows that, after adding Sc-Zr, supernano grains with a size of 300 nm were produced in the matrix of the composites as cast condition. The precipitate of Al 3(Sc,Zr) nucleated and grew inside the grain, and the relationship changed from coherency to semicoherency. The peak value of precipication density was 16.7×1013m-2 at 350°C. The results showed that, after adding Sc-Zr, the main strengthening mechanism of the Si C/Al composites as cast condition turned from dislocation strengthening into refined crystalline strengthening. At the peak aging treatment, the effect of precipitations worked by making dislocations engendering Orowan mechanism, and the microyield strength of the composites reached 166.8MPa, at the peak aging.The effect of Mg on the microstructure and microyield strength of the Si C/Al-Sc-Zr-Mg was further studied. The adding of Mg increased the nucleation rate of Al3(Sc,Zr) phase, which leading the average particle size of Al3(Sc,Zr) phase under the same heat treatment more small and tiny, and higher areal density of precipitations. Besides the solid solution strengthening of itself, the adding of Mg made contributions to the Orowan strengthening effect by refining Al 3(Sc,Zr) phase. The microyield strength of the Si C/Al-Sc-Zr-Mg composites was 250 MPa at the peak aging, which improved 120% compared to the Si C/pure Al composites.