| TB8 titanium alloy is a kind of ultra-high strength metastable ?-type titanium alloy,commonly used in the preparation of aerospace,aerospace structural parts.But the high content of beta stable element makes the deformation resistance of the alloy higher,which leads to the limitation in the application of plastic forming technology.So the alloy superplastic forming technology becomes the key to this problem.In this paper,the effect of thermal deformation parameters on the dynamic recrystallization and aging precipitation behavior of TB8 alloy is taken as the starting point.The superplastic behavior of TB8 titanium alloy and the microstructural evolution during superplastic deformation were studied from the aspects of recrystallization mechanism and aging behavior in the process of thermal deformation.And introduce the deformation induced superplastic stretch method,through the deformation induced by the secondary ? phase and recrystallization tissue optimization of the dual means to effectively improve its properties of superplastic.The microstructure evolution of superplastic deformation and the effect of deformation conditions during superplastic deformation behavior of TB8 alloy were investigated.The results showed that from 690~840? deformation temperature and 10-4~10-3 s-1 strain rate,TB8 alloy had the superplastic deformation property.Under deformation condition of 750? and 10-4 s-1,it had the biggest elongation of 524.9%.During deformation,the deformation softening and the work-hardening canceled out each other,and performed the traditional steady-state superplastic flow characteristic;Meanwhile,the deformation temperature,strain-rate and deformation degree had significant influences on the microstructure after superplastic deformation,the equiaxed ? phase grain size increased with the strain-rate decreasing;With the increase of the deformation temperature,the grain size of the ? phase increases,and ? phase transform to ? phase,while the grains still maintain a certain isometric degree.With the degree of deformation increasing,the content of ? phase separated out dispersedly by the grain boundary and matrix of ? phase increased and the tiny and dispersed ? phase can limit the excessive growth of the grains.The superplastic tensile test of TB8 titanium alloy was carried out by means of deformation induced method,The effects of deformation temperature,pre-deformation amount and intermediate holding time on superplastic properties and microstructure evolution of the alloy were studied.The results show that the superplasticity of the alloy is greatly improved compared with the constant strain rate method.When the deformation temperature is 750?,pre-deformation is 50% and holding time is 20 min,the superplastic properties of the alloy is the best,elongation is 796.1%.Predeformation stage,the double optimization of recrystallization and exsolution precipitation makes the metastable ? phase change into fine and uniform recrystallized structure.In the subsequent deformation process,the small dispersion of ? phase not only can inhibit the recrystallization of grain growth,but also to a certain extent,make the recrystallized microstructure concentrated and broken.The microstructure of the alloy after superplastic deformation remains well equiaxed,with significant superplastic deformation characteristics.Based on the study of the hot deformation behavior of TB8 titanium alloy,the dynamic recrystallization model of alloy was established.The results show that as the deformation temperature increases and the strain rate decreases,the dynamic recrystallization volume fraction increases and the recrystallized grain size increases.However,low temperature and high strain rate environment will delay the occurrence of dynamic recrystallization.Through the rational control of thermal deformation parameters,uniform and fine dynamic recrystallization structure can be obtained,and the plastic deformation ability of the alloy can be improved.The conclusions are in agreement with the experimental results. |
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