Study On Superplastic Deformation Behavior And Microstructure Coordination Mechanism Of Coarse-grained Ti40 Burn-resistant Titanium Alloy

Ti40 alloy is a kind of burn resistant titanium alloy which was developed to solve the problem of"titanium fire"and it has excellent comprehensive performance.However,it is difficult to form through traditional hot processing method by easy cracking and strong resistance to deformation at elevated temperature.Superplastic forming technology（SPF）is one of the effective ways to process titanium alloys which are hard to deform.However,Ti40alloy cannot meet the microstructure requirement of fine-grained superplasticity because of its coarse grains.Therefore coarse-grained Ti40 alloy was used in this paper to study its mechanical behaviors,evolution of microstructure,main deformation mechanism during superplastic deformation systematically.The results can provide theoretical basis for coarse-grained superplastic deformation and then reduce the preprocessing cost.The contents and results are as follows:The mechanical behavior of superplastic deformation of Ti40 alloy was analyzed.The results have shown that Ti40 alloy with original grain size of 135μm has good superplasticity in the range of temperatures 800^oC⁸80^oC and strain rates 5×10^-4s^-11×10^-2s^-1.The maximum elongation of 436%was obtained at condition of 840^oC and 1×10^-3s^-1.The true stress-strain curves of Ti40 alloy can be divided into four stages.The strain rate sensitivity index m values increased first and then remained unchanged with increase of temperatures,and the maximum m value of 0.41 was obtained at condition of 840^oC.The activation energy of superplastic deformation is 263.3KJ/mol,which is close to its self-diffusion activation energy.Compared with the constitutive model constructed by the Arrhenius equation,the constitutive model constructed by the BP neural network has higher precision and it can satisfy the precision requirement of numerical simulation.The evolution of microstructure of Ti40 alloy during superplastic deformation was studied.It was found that original grains was elongated during superplastic deformation and the original grains boundaries was protruded and widened,significant dynamic recrystallization was observed at the grain boundaries.Dynamic recovery is the mainly softening mechanism at the temperature of 760^oC.The nucleation of dynamic recrystallization is the mainly softening mechanism in a range of 800^oC⁸40^oC and 1×10^-3s^-11×10^-2s^-1 and the growth of recrystallized grains is the mainly softening mechanism in a range of 840^oC⁸80^oC and 5×10^-4s^-1.Based on Sellars model and KM equation respectively,the critical strain model and dislocation density evolution model of Ti40 alloy for superplastic deformation were established.The coordinated law of microstructures of Ti40 alloy during superplastic deformation was investigated.It was found that the way of recrystallization was mainly continuous dynamic recrystallization,and the discontinuous dynamic recrystallization was supplemented.The number of low angle grain boundaries was changed as the deformation conditions changes.A large number of low angle grain boundaries were generated near the original grain boundary when dynamic recovery occurs.And when the dynamic recrystallization occurs,on the one hand,the orientation difference of low angle grain boundaries increases gradually to the large angle grain boundaries through the low angle grain boundaries absorbs dislocations,On the other hand,the dynamic recovery of the original microstructures produces new small angle grain boundaries,resulting in the relative stability of low angle grain boundaries under different deformation conditions.The superplastic deformation mechanism of Ti40 alloy was analyzed.It was found that dislocation cells and subgrains were formed successively during the deformation of Ti40 alloy under the drive of deformation storage energy.Subgrains grow up and develop into the core of recrystallization through the migration of subgrains boundaries.The main mechanism of deformation at initial stage is the migration of dislocations such as cross-slip and climb,therefore dynamic recovery was observed at initial stage.The main mechanism of deformation at later stage is continuous dynamic recovery and recrystallization,the auxiliary mechanism is grain boundaries sliding of fine recrystallized grains.