Study On Preparation And Microstructure And Mechanical Properties Of C_f/Ti-Al Composites

Three methods were used to fabricate C_f/Ti-Al composites for application of continuous fiber reinforced titanium aluminide matrix composites in the fields of aeronautics and astronautics: pressure infiltration, pressure infiltration-homogenizing treatment and mechanical alloying-pressure infiltration. By using a series of equipment and techniques, such as scanning electron microscope(SEM), transmission electron microscope(TEM), tensile test, three-point bending test and homogenizing treatment, the microstructure, tensile properties, three-point bending properties were studied in this paper.In the matrix of the composites fabricated by pressure infiltration method, reaction between Ti and Al is weak. Addition of rare-earth element can be conductive to the progress of the reactions between Ti and Al. TEM observations show that addition of No.1 solvent the reactant between Ti particles and the aluminum is TiAl₃, while addition of No.2 solvent the reactant between Ti particles and the aluminum is lamellar TiAl. The reactant between rare-earth element and Al is XAl₃.The reaction layers don't increase after high-temperature diffusion for fifteen minutes at 800℃. Under some kinds of homogenizing technology such as 550℃/20h, 550℃/30h, 550℃/50h, 600℃/3h, and so on. Diffusion occurs noticeably. Circular structure that diffusion encircles titanium cores is formed. The reactants in the diffusion layer are TiAl₃. But pores are entrapped in the composites.The kinetics model that aluminum diffusion and reaction with titanium is entablished. The theoretical calculations show that theoretic values are lower than the practical ones. There are two main causes. Firstly, It is difficult to aquire the diffusion coefficient; Secondly, the actual homogenizing time is longer than the theoretical homogenizing time.In the fabrication process by three-step method, Ti₃Al gained through mechanical alloying reacts with Al and lead to the formation of TiAl₃.The strength of the composites fabricated by three-step method and two-step method are lower than that of the composites fabricated by pressure infiltration owing to the existence of the brittle phase TiAl₃. Studies in this paper show that the reduction of Ti particle sizes can enhance the strength of the composites; Once rare-earth elements added to the composites the tensile strength and flexural strength increase by 60% and 12.6%, respectively; When more content of Ti particles is added strength of the composites increase 108%; The strength of the unidirectional fiber reinforeced composites almost double that of the fiber fabric reinforced composites. The fracture mode is brittle fracture.For the purpose of the fabrication of carbon fiber reinforced titanium aluminide matrix composites, the mechanical alloying-mechanical method is more valuable in practice.