| ?-TiAl based alloy is an ideal lightweight and high-temperature structural materials with low density, high specific strength, high specific stiffness, excellent properties at high temperature, good oxidation resistance and corrosion resistance, etc. ?-TiAl based alloy can be used at high temperature(700?1000 ?). Due to its good properties,it was widely applied in aerospace, ships and other fields. However, the ductility at room temperature of ?-TiAl based alloy was poor, and it was difficult to molding processing. What's more, ?-TiAl based alloy with poor oxidation resistance limited its application. This study is based on the original TiAl based alloys via adding higher content (6-8 at.%) of Nb element and other assistant alloying elements,such as Cr, Mo, W, B, Si, then using homogenization heat treatment and recycling heat treatment to obtain the microstructure with homogeneous fine lamellar during the solid state transformation process, in order to improving the brittle of TiAl-based alloys at room temperature and oxidation resistance at high temperature. In addition, this work has investigated the evolution of solid state transformation, mechanical properties, oxidation resistance and corrosion resistance of the alloy. The factors which influence the performance of the alloy were also studied, such as alloying elements and the structure.High Nb TiAl based alloy ingots was prepared by non-self-consumed tungsten electrode argon arc acuum furnace. The microstructure analysis results indicate that the alloy was mainly composed of ?2/? lamellar tissue, and there was a little of ? phase and B2 phase which have been precipitated along the grain boundaries and intragranular. In the structure of as-cast tissue,composition segregation and tissue inhomogeneity were existed. Through adjusting the heat temperature,reaction temperature,cooling method and the times of cycle of homogenization and cycle heat treatment, the changes of microstructure and hardness of the alloy were tested. Finally,it was found that the optimal heat treatment: Firstly, homogenization, which means to heat the specimen to 1200 ? in the furnace, holding for 24 h, then cooled in air; Secondly, cycling heat treatment, which means the specimen was rapidly heated to 1150 ?, holding for 4 h, then cooled in air, this crafts was cycled 3 times. After being heat treated, crystalline grain and lamellar structure of alloys have been significantly refined, the structure of alloys were homogenization and the content of ?-phase was increased,while the content of ?2 phase and B2 phase were decreased. The element of Cr was good to the refinement of grain and could promote the precipitation of phase. As the element of Mo which could refine the lamellar structure, however,it could not promote the precipitation of phase.The influence of composition and method of heat treatment on the mechanical properties of the alloys were investigated. The ingots was heat treated, then the hardness of alloys were decreased by about 5?10 HRC, which was good to improving the ductility and toughness of the alloys. Room temperature compression tests showed that the fracture of the materials was brittle fracture. The Alloy which was added 2 at.% Cr and 2 at.% Mo together has the best elastic modulus, moderate fracture strength, relatively low yield strength.and low compression ratio,indicating the strength, stiffness and ductility of the alloy was great. The results of high temperature compression tests showed that the materials presents certain of plasticity, and the alloy has the best elastic modulus, fracture strength and ductility. Further, the hardness of The Alloy which was added 2 at.% Cr and 2 at.% Mo together is 41.1 HRC which is in a very high level. Therefore, the mechanical properties of Ti-45Al-6Nb-2Cr-2Mo-0.5W-0.8B-0.2Si alloy was the best.The influence of composition on high temperature oxidation resistance and seawater corrosion capacity of the alloys heat treated were investigated. High temperature oxidation experiments showed that: when the heat temperature was 800 ?, the oxidation time was shorter than 200 h and under the air atmosphere, the oxidation resistance of alloy without Cr and Mo elements were better than the others; with the increase of oxidation time, the oxidation kinetic curves of alloy with 4 at.% Cr and alloy which was added 2 at.% Cr and 2 at.% Mo together showed leveling off which was good to protecting the metal from oxidatized furtherly; The oxide film of the alloy was homogeneous and formation, nodules or cracks didn't occur and there are multilayer films. The average thickness of the oxide film of alloy can achieve 3.0 ?m; The oxide film which was combined by Ti3O?TiO2?(Al0.9Cr0.1)2O3 and AlO together could be preferably protecting the material from oxidized furtherly.The results of seawater corrosion experiments indicated that: According to polarization curves and electrochemical impedance spectroscopy, the corrosion resistance of alloy was evaluated. The corrosion resistance of the alloy was greatly improved via adding 2 at.% Mo.While adding 2 at.% Cr, 2 at.% Mo to the alloy which was also good for improving the corrosion resistance. |
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