Effect Of Superplasticity Deformation And Heat Treatment On Microstructure Evolution Of TC4-DT Alloy

TC4-DT with moderate strength and high damage tolerance is a kind of new a+β alloy which is developed from China. And it is widely used in the field of aerospace, chemical and so on. The aggregation and growth of microstructure has influence on the performance under the high temperature and low strain rate during superplastic forming process. Therefore, the research of TC4-DT superplastic forming and heat treatment on the microstructure evolution has a very important significance to the isothermal forging. This subject adopt the constant strain rate superplastic tensile deformation experiment, the influence of deformation conditions on the flow stress is analyzed, the microstructure evolution of alloys is researched by metallographic microscope, then expect to receive basketweave structure with much better comprehensive performance through the double annealing heat treatment to deformed structure. The main research content as follows:By the constant strain rate superplastic tensile deformation experiment, hot deformation behavior of TC4-DT titanium alloy is investigated at range of880～980℃and strain rate range of5.6×10-5～5.6×10-2s-1with the SANS-CMT4104thermo-simulation machine. The change law of flow stress and microstructure are also studied.The thermal activation energy of307.84kJ/mol is evaluated by analyzing the stress-strain curves. Constitutive equations of the maximum flow stress are established.Based on the double annealing heat treatment to deformed structure under the different deformation condition, the effect of solution temperature and aging temperature are studied. The results show that:(1) Under the same aging temperature, the content of secondary phase in the β phase region is much than α+β phase region and the aggregation growth is occurred between secondary phase and primary phase when the deformation temperature is a constant. It’s because the saturation of metastable phase is much more in the higher temperature. When the solution temperature is a constant, the higher deformation temperature, the worse stability of the matrix, and the much metastable phase of β is obtained during solution. So, much secondary phase of a is obtained during aging which present fine acicular and dispersively distributed in the matrix, and the equiaxed structure changes into the basketweave structure.(2) When at the same solution temperature of β phase region, the higher aging temperature, the stronger driving force of precipitation, and the much content of secondary phase and tend to growth. Because composition segregation causes by the uneven diffusion of every alloy element and the enrichment of a stable element, much of secondary phase precipitates along the β phase boundary and aggregates and grows up into a big plate.The superplastic forming of TC4-DT alloy sample adopted at980℃and5.6×10-4s-1, and then the basketweave structure is observed in the deformed structure by solution treatment at930℃for1h and aging at540℃for4h.