Reach On Superplasticity And Microstructure Evolution With Large Grains Of WSTi3515S Burn-resistant Titanium Alloy

Titanium alloys are widely used in aviation and aerospace field due to their good heat resistance and high specific strength,but the conventional titanium alloy parts will be a cause of serious combustion accident in high temperature and high pressure and other harsh working environment.Considering the phenomenon of “titanium fire”,on the basis of Alloy C alloy,China has developed WSTi3515 S burn-resistant titanium alloy,which has good burn-resistant and comprehensive performance.However,it is difficult to meet the requirements of product performance by conventional processing means because of its coarse grain size and high temperature deformation easy to crack.Superplastic forming technology(SPF)is one of the effective methods to process materials that are difficult to deform,but the mechanical behavior and microstructural evolution of superplasticity of burn-resistant titanium alloy with coarse grains are still lack of systematic research.Therefore,in this paper,WSTi3515 S burn-resistant titanium alloys with hot-rolled annealing as the research object,the two aspects were carried out in-depth research under different deformation conditions by the superplastic tensile test of isothermal and constant strain rate.Major contents and results are summarized as follows:Superplastic mechanical behavior of WSTi3515 S burn-resistant titanium alloy and the influence of thermodynamic parameters on superplastic mechanical behavior were studied.It was found that the elongation is close to or greater than 200% in the selected experimental range(800 ℃ ~920 ℃ and 5×10-4s-1~1×10-2s-1),which proved good superplastic properties,and the maximum elongation of 556% appears at 920℃,5×10-4s-1.It revealed the law of stress-strain curve of WSTi3515 S burn-resistant titanium and the substance of flow stress curve change is the effect of the interaction between the work hardening and the dynamic softening about the WSTi3515 S titanium alloy.The strain rate sensitivity index m of WSTi3515 S titanium alloy was measured by equal strain rate stretching method and the average value of sample m was more than 0.3 in the selected strain rate range,which indicated that the alloy have good superplasticity.Strain curves were used to establish the constitutive equation of WSTi3515 S titanium alloy by using the hyperbolic sine Arrhenius model and the polynomial method respectively.The error analysis showed that the polynomial equation had higher accuracy.Based on the stress-strain curve,the constitutive equation of WSTi3515 S titanium is established by the hyperbolic sine Arrhenius model and the polynomial method respectively.It shows that the polynomial method have the higher accuracy by error analysis.The dynamic recrystallization mechanism and microstructural evolution of WSTi3515 S burn-resistant titanium alloy during superplastic tensile deformation were analyzed.It was found that the original large grains were stretched and flattened;dynamic recrystallization occurred at the grain boundary;the original microstructural was refined.With the increase of temperature or the decrease of strain rate,the recrystallized grains grow continuously and the microstructures are equiaxed and evenly distributed.The main mechanism is discontinuous dynamic recrystallization(DDRX)at high strain rate(≥5×10-3s-1)and low temperature℃(≤840℃).The main mechanism is continuous dynamic recrystallization(CDRX)at low strain rate(≤1×10-3s-1)and high temperature(≥880℃).But dynamic recrystallization process is accompanied by particle stimulation nucleation(PSN)under different conditions.It revealed the effect law of different deformation conditions on grain size and volume fraction of dynamic recrystallization.The result shows that the average grain size and volume fraction of dynamic recrystallization increased with the increase of temperature and the decrease of strain rate.Based on the Avrami model,the critical strain model and volume percentage model of dynamic recrystallization were established respectively.The critical strain model of dynamic recrystallization is and the volume percentage model of dynamic recrystallization is.