Study On Microstructure And Superplasticity In TA32 Titanium Alloy Plate

With the development of engineering application of TA32 titanium alloy plate in aerospace field,it is necessary to carry out in-depth and systematic research on the characteristics and evolution of the microstructure and texture,the relationship between texture and uniformity of tensile properties,and the superplastic deformation behavior of plate.In this thesis,TEM and EBSD were applied to study the characterization and evolution of the microstructure and texture of TA32 titanium alloy plate.On this basis,we systematically studied the high temperature deformation behavior of the plate.The superplastic deformation constitutive equation was established and the superplastic deformation mechanism was explored.It is expected to provide theoretical basis and experimental basis for further promoting the engineering application of TA32 titanium alloy plate.Due to the difference of rolling process,the original microstructure of thick plate and thin plate was different.The microstructure of thick plate was mainly composed of lamellar a phase with few retained ? phase.The microstructure of RD(rolling direction)-ND(normal direction)plane and TD(transverse direction)-ND plane were not significantly different from the surface to the center along the thickness direction.The morphology of a grains presented either straight or wavy depending on their orientations with respect to the principal rolling directions.The microstructure of thin plate was consisted of equiaxed a phase,a small amount of a lath and residual ? phase.The texture types of thick plate and thin plate were typical T-type texture and B-type texture in this thesis,respectively.The evolution of microstructure and texture with heat treatment of thin plate were studied.With increase of heat treatment temperature,the microstructure was gradually transformed from equiaxed microstructure to bimodal microstructure and finally to fully lamellar microstructure.The change of a phase texture was closely related to the change of microstructure.The texture type kept unchanged when the microstructure kept equiaxed at low heat treatment temperatures(550?850?).However,the texture strength decreased slightly,which may be related to the rotation of the sub-grains in the heat treatment process.When the heat treatment temperature was high(900?990?),the texture changed obviously due to the ????? phase transition.The newly formed R-type texture was likely to be related to the texture components of<110>//RD of the residual ? phase and<0001>//RD of the primary ? phase.With the increase of heat treatment temperature,the B-type texture strength increased firstly and then decreased,while the R-type texture strength gradually increased,which was mainly determined by the volume fraction of primary ? phase and transformed ? phase.When the heat treatment temperature increased to ? phase region(1000?1020?),the texture was composed of strong B-type texture and R-type texture.Besides,other texture components were also formed.For the thick plate,there was no obvious anisotropy in the tensile strength,which was mainly due to the existence of a large number of intragranular substructures.However,from the surface to the center along the thickness direction of the plate,the decrease of the proportion of intragranular substructure could lead to a gradual decrease in tensile strength.In addition,the c-axis of a phase gradually deviated from the TD direction,resulting the gradual increase of Schmidt factor,which was another important factor leading to the gradual decrease of tensile strength.The lower tensile strength and plasticity along the thickness direction(ND)were mainly related to the geometric orientation of lamellar a phase.Thus,it could be concluded that under the condition of no obvious difference in microstructure,the intragranular substructure and texture were the main factors affecting the tensile strength of thick plates at different positions;After the double annealing treatment,microstructure was the main factor affecting the tensile strength,and the texture could lead to the anisotropy of tensile strength.For the thin plate,the change of tensile strength at room temperature with the heat treatment temperature was mainly affected by microstructure and texture.When there was no significant change of microstructure(550?850 ?),elimination of intragranular substructure,precipitation of solid solution atoms and the grains growth were the main reasons that result in the decrease of tensile strength.When the microstructure had an obvious change(900?1020?),interface strengthening was the main reason for the improvement of tensile strength,but fully lamellar microstructure could lead to deterioration of tensile strength.The anisotropy of the tensile strength was mainly affected by the texture.When the texture was a single B-type texture(550?850?),there was no anisotropy of tensile strength.When the R-type texture was formed(900?1020?),strong anisotropy of tensile strength appeared.The microstructure has a significant effect on the superplastic deformation behavior of TA32 titanium alloy plate.The superplastic deformation behavior of equiaxed fine grain plate under vacuum environment was systematacially studied.The maximum superplastic elongation was obtained to be 702%at 940? with a strain rate of 3.33×10-3 s-1.The superplastic deformation constitutive equation was established and the average apparent activation energy(Q)and strain rate sensitivity(m)were calculated to be 316.3 kJ/mol and 0.46,respectively.After superplastic deformation,the microstructure changed significantly.From the grip region to the tip region,the spheroidization degree of primary ? increased.The spheroidizing process of a lath was accelerated by dynamic globularization.Active dislocation motion was observed and developed an attractive interaction with silicides((TiZr)6Si3).Moreover,the growth of grain was inhibited as pinning effect of the silicides((TiZr)6Si3)precipitated at grain boundary.Transformation of low angle grain boundaries(LAGBs,2°<?<15°)to high angle boundaries(HAGBs,?>15°)was promoted by dynamic recrystallization.Significant change in texture intensity of B-type and R-type texture was observed after superplastic deformation,from the grip region to the tip region,the strength of B-type texture gradually decreases,while the strength of R-type texture gradually increases,which was attributed to grain rotation.Superplastic deformation of TA32 plate was mainly controlled by grain boundary sliding along with kinds of accommodation processes,including grain boundary diffusion,dynamic spheroidization,dynamic recrystallization,dislocation movement,grain rotation and grain growth.The superplastic deformation behavior and fracture mechanism of thin plates with three kinds of typical microstructure were studied.With microstructure changes from equiaxed to bimodal to fully lamellar microstructure,the peak flow stress gradually increased,but the fracture elongation gradually decreased.The strain rate sensitive factor m value of the equiaxial microstructure was higher than those of the bimodal and lamellar microstructure.The high flow stress of the bimodal and lamellar microstructure was mainly attributed to the coarse primary ?-phase grains and the ?-lamellar clusters with various geometrical orientations in the transformed ? phase.Such?-lamellar clusters can effectively hinder the dislocation movement and prevent grain boundary sliding.The superplastic softening mechanisms of bimodal and lamellar microstructure were mainly dynamic recovery,dynamic recrystallization,dynamic spheroidization of lamellar a phase and formation of cavities.The spheroidization model lamellar a phase was mainly boundary separation model.The fracture modes of three kinds of microstructures were all combinations of ductile fracture and cleavage fracture.The equiaxed and bimodal microstructures were dominated by ductile fracture,while the fully lamellar microstructure was dominated by cleavage fracture.