Research On The Microstructure And Properties Of Multiple-reinforced Titanium Matrix Composites

With the developing of science and technology and the broadening of titanium matrix composites’application field, titanium matrix composites are requied to own not only high unitary property but also good combination properties, such as high specific modulus, high specific strength, creep resistance, size stability, wear resistance, oxidation resistance, good heat conduction and so on. In order to obtain such good combination properties, it is necessary to use multiple reinforcements with different features and various sizes. Taking use of the interaction, couple effect, size effect of multiple reinforcements, titanium matrix composites are promising to obtain structure-function integrated characteristics for satisfying the increasing demand on the materials.At present, the most important and the most potential application field of titanium matrix composites is serving as structure materials at ambient and elevated temperature in the aviation and space field. Under this serving condition, titanium matrix composites are expected to obtain high strength, high modulus as well as good oxidation-resistant ability, good heat-resistant ability and so on. To this objective, Titanium matrix composites reinforced with multiple TiB, TiC and RE₂O₃ particles were in situ synthesized utilizing the vaccum arc remelting technology through designing and selecting suitable in situ reactions. In the composites, the RE₂O₃ reinforcements mainly exhibited nano-size by controlling the addition method of rare earth elements. In this research, the main work was done as following:1. The thermodynamics were calculated to predict both the feasibility of the in situ reactions and the effects of alloying elements on the in situ reactions. The results showed that the Gibbs free energies （?G） of in situ reactions were negative, which indicated that the in situ reactions were liable to take place. In addition, since the matrix of practical titanium matrix composites was usually not pure titanium but titanium alloys, in which there existed many alloying elements, the effects of alloying elements on the thermodynamics of in situ reactions were calculated utilizing the Miedia model and Wilson equations. The calculatin results provided theoretical and regularty basis for controling and optimizing the shape and size of reinforcements.2. Unitary TiC/Ti composites, binary （TiB+TiC）/Ti composites and multiple reinforced （TiB+TiC+RE₂O₃）/Ti composites were in situ synthesized utilizing the vacuum arc remelting technology through selecting and designing suitable in situ reactions according to thermodynamical theory. In the composites, the phases were identified by X-ray diffraction （XRD）, the microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. It was found there were mainly three kinds of reinforcements: TiB whisker, TiC near-equiaxed particle and dendritic crystal and RE₂O₃ spherical particle. The RE₂O₃ reinforcements were divided into coarse primary phases and nano-sized second-precipitated phases. The reinforcements all homogeneously distributed in the matrix.3. The effects of rare earth elements on the microstructures of titanium matrix composites were studied by adding Nd, Al2Nd and LaB6, respectively. It was found that addition of rare earth elements could coarsen the TiC reinforcements. With the addition amount of rare earth element increasing, TiC reinforcements turned bigger and more dendritic TiC reinforcements occurred in the composites. Furthermore, both the amount and the size of primary rare earth oxide RE₂O₃ increased with the addition amount of rare earth elment increasing. Addition of Al could reduce the size of TiC reinforcements. Addition of pure Nd metal could lead to the formation of coarse primary RE₂O₃ particles, which were harmful to mechanical properties, due to the high chemical activity at room temperature and the low melting point of pure Nd metal. Addition of Al2Nd or LaB6 instead of Nd could effectively supress the formation of coarse primary RE₂O₃ particles because of their stable chemical activity at room temperature and their higher melting point than Ti, particulary LaB6 which could greatly decrease the size of primary RE₂O₃ particles and could make the RE₂O₃ reinforcements mainly exhibite nano-sized particles.4. The interfacial bonding characteristics between the reinforcement and matrix titanium were examined by transmission electron microscopy （TEM） and high-resolution transmission electron microscopy （HREM）. The interfaces were found to be very clean and there is no interfacial phases caused by interfacial reaction. Furthermore, Nd₂O₃, La₂O₃ and TiB were found to have certain crystal orientation relationships with the matrix titanium. The relationships were as following: 5. The mechanical properties of the multiple reinforced titanium matrix composite were studied. With the incorporation of TiB, TiC and RE₂O₃ reinforcements, the modulus, yield strength and ultimate strength of the matrix alloy were significantly improved while the plasticity decreased. The strength and plasticity of multiple reinforced （TiB+TiC+RE₂O₃）/Ti composites were both higher than those of hybrid reinforced （TiB+TiC）/Ti composites with the same volume fraction of reinforcements. The strengthen mechanism of multiple reinforced composites at room temperature mainly attributes to the load undertaking of reinforcements, refinement of matrix grain sizes and dispersion strengthening of nano-sized RE₂O₃ particles. The strengthen mechanism of multiple reinforced composites at high temperature mainly attributes to the impediment effect of reinforcements on the motion of dislocations and the dispersion strengthening of nano-sized RE₂O₃ particles. The improvement on plasticity attributes to the decreasing of oxygen content in the matrix resulting in formation of nano-sized RE₂O₃ particles.6. The oxidation behavior of multiple reinforced （TiB+TiC+Nd₂O₃）/Ti composites at elevated temperature were studied. The oxidation kinetics of the composites were found to obey parabolic rule. Ti, TiB and TiC were found to react with the oxygen at elevated temperature while Nd₂O₃ didn’t react with the oxygen. The in situ incorporation of multiple reinforcements greatly increased the oxidation resistant ability of the composites. The （TiB+TiC+Nd₂O₃）/Ti composites were found to possess higher oxidation activation energy, lower weight gain, thinner oxidation scale and higher oxidation-resistant ability than the unitary TiC/Ti composites with the same volume fraction of reinforcements.The in situ technology used in this research could effectively avoid interfacial contamination and obtain excellent interfacial bonding, which is benefical to mechanical properties. Furthermore, this in situ technology can utilizing the equipments and processing parth for producing titanium alloys with only adding some reactants in the raw materials and adjusting some processing parameters, which could greatly decrease the cost of titanium matrix composite and was beneficial to industrial mass production. In this research, some probing work was done on multiple reinforced titanium matrix composites facing the aviation and space application field of titanium matrix composite. Titanium matrix composites reinforced with multiple reinforcements with different shapes and sizes were found to own excellent mechanical properties, which provides theoretical and regularity guide for further developing titanium matrix composites with high heat-resistant property and excellent combination properties. This work is also significative to the exploration and utilization of abundant rare earth resources in our country.