Research On The Superplasticity Of Tial-Based Alloy

TiAl-based alloys have high specific strength, specific stiffness, excellent oxidation and creep resistance, which have been paid broad attentions to for years. However, the major factors limiting structural applications in this material are their ambient temperature brittleness and poor workability. It is said that superplastic forming is a novel process for fabricating titanium aluminides. So, a research on the superplasticity of the alloy is done in the paper. First, a fine-grained TiAl-based alloy is obtained by a quickly canned-forging thermo-mechanical processing or by crystallization of mechanically alloyed amorphous Ti-50at%Al. Then the tensile superplasticity of the IM (ingot metallurgy) TiAl alloy with deformed microstructure or with duplex microstructure obtained by the quickly canned-forging thermo-mechanical processing and the compress superplasticity of the PM (powder metallurgy) TiAl alloy obtained by crystallization of amorphous Ti-50at%Al powders are investigated systemically and deeply. The superplastic mechanism of the alloy is discussed. The cavitation behavior and the fracture mechanism under superplastic deformation are analyzed too.Quickly canned-forging thermo-mechanical processing is an efficient and economical IM technology. By this technology, a sub-microstructure titanium aluminide with mean grain size of about 0.6μm is obtained after 3-step forging with the final forging temperature of 950℃. For a cast TiAl-based alloy, lamellar globularization is the mechanism of grain refining. By analyzing the deformation behaviors and the microstructure evolution of lamellas during forging and the following heat treatment, the lamellar globularization mechanism of the alloy is discussed. The technology of crystallization of mechanically alloyed amorphous Ti-50at%Al powders is a novel process. By this technology, a titanium aluminide with mean grain size of about 0.15*m is obtained under 40Mpa, 1000℃,lh hot-pressing. During mechanical alloying, the amorphous phase formation process can be described by: Ti+Al→Ti (Al) supersaturated solid solution (h.c.p)→morphous phase. In our research, the Ti-Al system has a large and negative heat of mixing, the?IV ?ABSTRACTdiffusivities of the two components are great different, so, the formation of the amorphous phase obeys SSRA (solid state amorphisation reaction) mechanism. Crystallization of the amorphous phase occurs at 660-750, the process can be described by: Amorphous phase disordered a phase (.c.p) ordered y phase, during the process, the disordered a phase is a metastable phase.The fine-grained TiAl-based alloys obtained by above technologies exhibit good tensile superplastic characters. An elongation of 566% is obtained at lOSO'C^SxlO'V1 in the TiAl-based alloy with deformed microstructure, and an elongation of 350% is obtained at 1050,lxloV in the alloy with duplex microstructure. It worth especially noting that the alloy with deformed microstructure exhibits excellent low temperature superplasticity. The alloy shows superplastic characters at very low temperature- 800 (the elongation is 313%). A maximal elongation of 413% is obtained at 900,5xlO~V in air. Even at a higher strain rate of 1x1 0'V1 and in air, the elongation also reaches 350%. In addition, under compress condition, the alloy also exhibits superplastic characters.Mechanically properties of the superplastic deformation are investigated by constant strain rate tests and incremental strain rate tests. These properties include: elongation, curves of flow stress-true strain, strain rate sensitivity (m), and apparent activation energy. Results show that these mechanically properties are affect by deforming conditions, microstructure, grain size, pre-deformation, and grain boundary structure. In the research, a large amount of work is done in TiAl-based alloy with deformed microstructure. A phenomenon of strain hardening resulting from pre-deformation is observed in the alloy. And the phenomenological relationship of superplastic flow is investigated in the alloy. It is found...