| With the rapid development of transportation, aerospace, electronic communications and other fields, the requirements for functional and structural materials are higher. The metal matrix composites, because of its high specific strength, specific modulus, thermal stability and so on, have become an important research direction in the materials field. At present, a large number of nano/sub-micron-sized ceramic particles reinforced Al and Mg matrix composites are studied and the research results show that the reduction of the ceramic particles size can significantly improve the mechanical properties. But there are few reports on the nano/sub-micron-sized ceramic particles reinforced iron matrix composites. Due to the high melting point, high density of Fe and density difference between Fe alloy melt and ceramic particles, nano/sub-micron-sized ceramic particles will cause suspension and partial agglomerate in the traditional casting method. In addition, because of the higher melt temperature, the size of the ceramic particles generated in the melt is also larger. By using in situ method to prepare particles reinforced metal matrix composites, not only the size of ceramic particles is smaller and well-distributed, but also the interface of matrix and ceramic particles is better, and the effect is obvious. Therefore, in this thesis, the in situ method will be used to fabricate nano/sub-micron-sized ceramic particles reinforced iron matrix composites. It has the important scientific significance and practical value in the research.In this paper, the nano/sub-micron-sized TiC/Fe composites were fabricated by the combination of combustion synthesis and hot pressing reaction synthesis in the Fe-Ti-C system. The effect of different ceramics contents, carbon sources, C/Ti molar ratios and alloy elements (Cr, Mo, Ni) on the microstructures, ceramic particles size, morphology and compression performance of the composites was investigated, meanwhile, we revealed its mechanism. The main results are as follows:(1) It reveals the influence of different carbon sources (carbon black, carbon nanotubes) on microstructures and TiC ceramic particles size of the TiC/Fe composites with different ceramic contents. Using carbon black as the carbon source, the matrix is mainly lamellar pearlite, while using carbon nanotubes as the carbon source, the matrix is mainly iron matrix, and the TiC particle size is smaller. Compared with carbon black, carbon nanotubes have smaller size, larger contact area and higher activity, which promotes the dissolution rate in the Fe-Ti melt. Thus, the reaction temperature is lower and the reaction is more fully, which make the number of TiC particles increase and the size smaller. The average size of TiC particles is 133nm in the 10 vol.%TiC/Fe composites.(2) It reveals the effect of different carbon sources (carbon black, carbon nanotubes) on mechanical properties of the TiC/Fe composites with different ceramic contents. The compressive strength and microhardness of the composites prepared by carbon black and carbon nanotubes increase first and then decrease with increasing TiC content. When using carbon black as carbon source, the compressive strength and microhardness are higher than carbon nanotubes as carbon source. They all get the maximum compression strength in the 30vol.%TiC/Fe composites.2203MPa (carbon black as carbon source) and 1768MPa (carbon nanotubes as carbon source), respectively, and their difference is 435MPa. For the following reasons:using carbon black as carbon source, the matrix is lamellar pearlite matrix, while using carbon nanotubes as carbon source, the matrix is mainly iron matrix. When in plastic deformation, the dislocation slip could be limited by the existence of Fe3C phase and lamellar microstructure, which can increase the strength and hardness of the composites.(3) It reveals the influence of different C/Ti molar ratios (0.8,0.9,1.0,1.1 and 1.2) on the microstructures and compression performance of 30vol.% TiCx/Fe composites. With the increase of C/Ti molar ratios, matrix transforms from iron matrix to pearlite matrix, but ceramic particle size changes little. When the C/Ti molar ratio is less than 1.0, the morphology of the ceramic particles changes from polyhedron to near spherical with increasing C/Ti molar ratios; When the C/Ti molar ratio is higher than 1.0, the degree of the ball decreases. With increasing C/Ti molar ratios, the yield strength, maximum compressive strength and microhardness of TiCx/Fe composites first increase then decrease, and it gets the optimal compression performance at the C/Ti molar ratio of 1.0. At the same time, spheroidal TiCx particles disperse better in the lamellar pearlite matrix, and can significantly reduce the stress concentrated, so it gets the best compression performance at the C/Ti molar ratio of 1.0. Its yield strength, maximum compression strength, fracture strain and microhardness are 1525MPa,2203MPa, 5.5%,and 691.5HV, respectively.(4) It reveals that using TiFe powder instead of elemental Ti powder, the combustion temperature increases with increasing ceramic content, and it leads to the ceramic particles size bigger. When using TiFe powder, the TiC particles size is slightly larger and has more smooth surface than using pure Ti powder at the same volume fraction. With the increase of ceramic content, the compressive strength increases gradually, and the optimal performance can be gotten when TiC content is 30vol.%. Its yield strength, maximum compression strength, fracture strain and microhardness are 1484MPa, 2102MPa,6.03% and 672.7HV, respectively. Compared with the pure Ti powder, the strength and hardness are slightly lower than using TiFe powder preparation. The reasons are as follows:using TiFe powder as Ti source, the matrix is mainly iron, the resistance ability to plastic deformation of which is not as lamellar pearlite, so the strength is slightly lower than using elemental Ti powder.(5) It reveals the effect of alloy elements (Cr, Mo, Ni) on the microstructures, TiC ceramic particles size and compression properties of 30 vol.% TiC/Fe composites.i) It reveals that with the addition of Cr, Mo and Ni in Fe-Ti-C system, the ceramic particles can be significantly refined. The reason is that alloy elements has high melting point, which can be as diluent to reduce the reaction temperature. The refinement effect is as follows:Cr>Ni>Mo.ii) It reveals that the compressive strength and microhardness of the composites can be significantly improved with the addition of Cr, Mo and Ni. Cr and Ni can improve the strength effectively compared with Mo. The composites with 4wt.% Cr possess the best comprehensive properties:the yield strength, maximum compression strength and microhardness are 1946MPa,2652MPa and 845.6HV, respectively, resulting in an overall increase by 462MPa,550MPa and 172.9HV, respectively. The composites with 2wt.% Ni possess the best comprehensive properties:the yield strength, maximum compression strength and microhardness are 2032MPa, 2815MPa and 889.2HV, respectively, resulting in an overall increase by 548MPa, 713MPa and 216.5HV, respectively.iii) It reveals the effect mechanism of Cr, Mo and Ni on the mechanical properties of 30 vol.% TiC/Fe composites. Firstly, the addition of alloy elements make TiC ceramic particles size decrease. If the particles size is smaller and the amount of particles increases, it will improve the strength significantly. Secondly, the addition of alloy elements can improve the wettability between Fe matrix and ceramic particles, and improve the interfacial bonding strength. Thirdly, the addition of alloy elements can generate solid solution strengthen, and improve the matrix strength.In this paper, it provides a new way for the preparation of nano/sub-micron-sized TiC reinforced iron matrix composites, as well as providing a certain experimental basis and application for the performance optimization of the iron matrix composites. |
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