Study On The Superplastic Behavior And Two Stage Superplasticity Of TB8 Alloy

TB8 alloy has excellent high temperature strength, creep resistance, cold workability, deep hardenability, high oxidation resistance, and good corrosion resistance. It has moderate strength high plasticity after solid solution and high strength fracture toughness after aging. TB8 alloy can obtain an ultrahigh tensile strength of more than 1450 MPa after aging. It is an ideal structural material of aircraft. Studies of TB8 alloy have focused on the sheet metal forming and the hot compression deformation. The contents mainly are the structure property. However, the superplasticity and deformation mechanism of TB8 is not clear and the researches of two stage superplastic behavior need to be carried out.The superplastic deformation, superplastic deformation after heat treatment and two stage superplastic tests of TB8 titanium alloy have been taken in this article. The superplastic deformation behavior and microstructural evolution in the process of the tests have been researched. The main research contents and conclusions are as follows:(1) Superplastic tensile test of TB8 alloy was carried out with constant strain rate. The influence of different process parameters for the superplasticity and deformation behavior has been studied to analyse the microstructural evolution and establish the stress-strain constitutive model.The superplasticity of TB8 alloy is sensitive to temperature and strain rate. The elongation of superplastic deformation is firstly increased and then decreased with the increase of deformation temperature at a constant strain rate, while it is increased with the decrease of strain rate at a constant deformation temperature. The elongation reaches the largest of 356% at a deformation temperature of 840℃ and strain rate of 10-4s-1.The flow stress is increased with the increase of deformation temperature or the decrease of strain rate. Dynamic recystallization occurred obviously in the deformation process. And the α-phase content is decreased gradually and the grains aggregated and grew up. The superplastic deformation activation energy and the corresponding stress exponent are 251.25kJ/mol、2.3895. And the stress-strain constitutive model had been established.(2) The superplastic deformation tests of TB8 alloy after solid solution or aging treatment were carried out. The influence of heat treatment parameters for the elongation, flow stress and microstructure of TB8 alloy had been studied.Within the scope of the study, the flow stress and the α-phase content of TB8 alloy after solid solution treatment is decreased with the increase of deformation temperature. The microstructure turned to β-phase single organization when the temperature exceeds the phase transition point. The elongation of TB8 alloy after solid solution treatment is decreased. And the dynamic recystallization occurred in the process of deformation. With the increase of aging temperature, the flow stress of TB8 alloy after aging is firstly increased and then decreased, while the elongation is firstly decreased and then increased. The elongation reached the largest of 358% when the temperature of aging is 520℃ and part of the grains refined.(3) Two-stage superplastic behavior tests of TB8 titanium alloy had been taken. The research of two-stage constant strain rate method and the strain-induced of maximum m-value superplasticity method had been carried out.The flow stress of the second stage and the elongation of TB8 alloy is decreased in the process of two-stage superplastic deformation. The influence of strain rate and holding time for the microstructure in the first stage is obvious. The microstructure is uniform and fine when the speed is 1.0mm/min, the deformation is 0.5 and the holding time is 15 min or when the speed is 1.5mm/min, the deformation is 0.5 and the holding time is 5min. The reason why the elongation of the strain-induced of maximum m-value superplasticity method cannot be increased may be that the grains haven’t been refined in the first stage...