Study On Superplastic Deformation Behavior Of Ti₂AlNb Alloy

Ti₂AlNb alloy is a kind of titanium aluminide alloy with high strength-to-weight ratio and excellent oxidation and creep resistances at high temperature.But its poor plasticity and machinability at room temperature limit its wide application.The superplastic forming technology at high temperature has the advantages of low deformation resistance,forming parts with complicated shapes and no springback and residual stress.Titanium and titanium aluminide alloys exhibit excellent superplasticity at certain temperature and strain rates.In this paper,the influencing factors and deformation mechanism of superplasticity of Ti₂AlNb alloy were studied in depth through the mechanical property tests and microstructural analysis.The strain rates and temperature conditions of superplastic deformation of Ti₂AlNb alloy were studied by superplastic uniaxial tensile tests.The results showed that the optimal superplastic deformation temperature of Ti₂AlNb alloy is 990°C,and the initial strain rate is 6×10^-4s^-1.Correspondingly,the peak stress and elongation are 49.2MPa and 299%,respectively.During the tensile deformation,a large amount of dislocations in the hard brittle phase?₂ hinder material flow and cause stress concentration and work hardening.Superplastic deformation at high temperature and low strain rates results from the softening process caused by dynamic recrystallization and hardening process caused by dislocation pinning.Through the tensile tests and microscopic analysis of Ti₂AlNb alloy with different hydrogen contents,the influence law and mechanism of different hydrogen contents on superplasticity were investigated.The results showed that hydrogen addition can significantly increase the elongation and reduce the peak stress.At 910°C and initial strain rate of 10^-3s^-1,the peak stress and elongation of the hydrogenated alloy with 0.18wt%H are 142.3MPa and 238%,respectively.Compared with unhydrogenated alloy,the peak stress is reduced by 22%while the elongation is increased by 71%.This is mainly due to the grain refinement of Ti₂AlNb alloy and the transformation of B2 phase and hard brittle phase?₂ phase to O phase by hydrogen addition.The dynamic recrystallization process accompanied by phase transition recrystallization improves superplasticity.Based on tensile test data,the constitutive equations for superplastic deformation of Ti₂AlNb alloy with different hydrogen cotents were established by the linear fitting method.The relative error analysis for different models was carried out in comparison with experimental data.It was found that the optimal constitutive models of Ti₂AlNb alloy with different hydrogen contents are different.The relative error of the hyperbolic sine function model is below3%for both alloys,which applicability is the best.The finite element simulations of the superplastic forming/diffusion bonding?SPF/DB?process of Ti₂AlNb/TA15 four-layer structures were carried out by MSC.MARC software.The synergistic deformation process of dissimilar alloys of four-layer structure was explored.The simulation results showed that face sheet of Ti₂AlNb alloy thickness reduction is lighter and the thickness of TA15 alloy core sheet is seriously reduced.The height and width of the hollow cavity,the bonding zone width determined the final thickness distribution of core sheet.The results of process tests showed that the bonding quality and bonding resist thickness affect the forming process and quality of four-layer structure.