Microstructure And Texture Of Ti-6Al-4V Titanium Alloy Deformed By Multidirectional Isothermal Compression Cyclic Deformation

Ti-6A1-4V titanium alloy has been widely applied in aviation,aerospace,biomedicine and alternative fields due to excellent comprehensive mechanical properties,corrosion resistance,heat resistance and high biocompatibility.Nevertheless,it is difficult to control the structure,performance uniformity and stability of titanium alloy due to high deformation resistance under low-temperature deformation conditions,easy oxidation caused by high deformation temperature under high-temperature deformation conditions,high strain rate sensitivity and narrow window period for machinability,etc.Multi-directional compression and severe plastic deformation technology in three-directional compressive stress state can obtain relatively large cumulative strain and refine the crystal grains without changing the appearance,shape and size,thereby achieving the goal of regulating the uniformity of material structure and improving comprehensive properties.In this article,Ti-6A1-4V titanium alloy with an equiaxed starting structure(α+β)was utilized as the study material.Ti-6A1-4V titanium alloy was subject to multi-directional isothermal compression deformation in the warm machining range by using isothermal deformation method.The microscopic structure,microstructure,grain boundary character distribution and recrystallization under different multi-directional compression deformation conditions and duplex annealing conditions were characterized by OM,SEM,TEM and XRD analyses.The evolution and rules of evolution of recrystallization were analyzed.The main research contents and results were reported as follows:(1)To determine the optimal deformation temperature,the microstructure,grain boundary character distribution and evolution and rules of evolution of recrystallization of Ti-6A1-4V titanium alloy under different deformation temperatures were investigated within the warm machining range.The results demonstrated that along with the increasing deformation temperature,the average grain size of α phase was decreased,the proportion of small-angle grain boundary was declined,the proportion of large-angle grain boundary was increased(48%,45%and 66%,respectively),and the proportion of fine recrystallized structure was elevated.These results suggest that fine recrystallized grains dominated by large-angle grain boundary can be obtained under the deformation temperature of 650℃ in the warm machining range from 550℃ to 650℃.(2)To obtain ultrafine-grained Ti-6A1-4V titanium alloy with uniform overall structure,the microstructures of each part of the sample was systematically studied under different cumulative strain conditions.The experiment indicated that after the first cycle of deformation,α" martensite with crystal plane diffraction peak of(004)appeared.Along with the increase of cumulative strain,the grains in each part of the sample were refined to different degrees.The morphology of β phase in the easily-deformable region was distributed on the matrix α phase in a fibrous or dot-like pattern.After four cycles of deformation,the overall structure of the sample was nearly uniform.The average grain size of α phase in the easily-deformable region was refined from the 4.8 μm to 0.14 μm.The microstructure was mainly consisted of fine equiaxed grains.The results suggested that along with the increasing cycles of deformation,the refinement effect on the grains in the Ti-6Al-4V titanium alloy was significantly enhanced and the uniformity of the overall structure of the sample was improved.The experimental results provided data reference for preparing large Ti-6Al-4V titanium alloy with uniform overall structure.(3)To unravel the grain boundary character distribution and the rules of recrystallization evolution of Ti-6Al-4V titanium alloy under different cumulative strain and duplex annealing conditions,the grain boundary character distribution and recrystallization were analyzed.The experimental results indicated that under the warm machining condition,the proportion of small-angle grain boundary was decreased,the proportion of large-angle grain boundary tended to increase and the proportion of fine recrystallized structures was elevated along with the increase of cumulative strain.After annealing,the proportion of large-angle grain boundary in the sample was above 74%.A large quantity of special twin grain boundaries existed.High peaks appeared surrounding the grain boundaries at an angle of 3°,10°,60° and 90°.In addition,the number of grain boundaries around 60° tended to increase along with the increasing cumulative strain,whereas the number of grain boundaries around an angle of 10° and 90° did not significantly change.Under the warm machining condition,fine recrystallized grains dominated by large-angle grain boundaries can be obtained along with the increase of cumulative strain.After duplex annealing and deformation with different accumulated strain,the microstructure was mainly composed of grains with a proportion of large-angle grain boundaries>74%surrounding the angle of 60°.(4)To reveal the intrinsic relationship between microstructure,texture and deformation behavior under multi-directional isothermal compression deformation conditions,the evolution of α-phase texture in the easily-deformable region of Ti-6Al-4V titanium alloy was explicitly investigated under different isothermal compression deformation conditions.The experiment indicated that the texture density did not significantly change along with the increasing deformation temperature,which formed the main texture components including {(?)010}<1(?)02>,(0002)[10(?)0]and {(?)010}<1(?)02>.In the first cycle of multi-directional isothermal compression deformation,along with the transformation of the applied load axis,the texture density was initially decreased and subsequently increased,forming {0002} basal texture mainly composed of(0002)[1(?)02],(0002)[10(?)0]and(0002)[10(?)0].After multiple cycles of multi-directional isothermal compression deformation,the texture density was decreased and the degree of texture diffusion was increased along with the increasing cycle of deformation,which formed non-basal texture with the main components of {(?)010}<0001>,{(?)010}<1(?)10>and {(?)010}<1(?)10>.After deformation and annealing with different cumulative strain,the degree of texture diffusion was declined and the types of texture components were altered,forming the main texture components of(0002)[1(?)02],((?)011)[1(?)02],(((?)010)[0001]+((?)010)[0(?)11]),((?)112)[2(?)(?)3],(((?)010)[1(?)12]+((?)012)[2(?)(?)2])and(((?)010)[1(?)12]+((?)113)[0(?)10]).The experimental results demonstrated that during the first cycle of decompression,along with the transformation of applied load axis,{0002} basal texture was weakened.Following multiple cycles of decompression,{(?)010} non-basal texture was formed,whereas the basal texture was absent.After duplex annealing,the texture components were mainly consisted of non-basal texture.These experimental results provide evidence for improving the properties of Ti-6Al-4V titanium alloy by reasonable application of texture hardening or softening.