Investigation On The Effect Of Rolling And Annealing On Mechanical Properties And Microstructure Evolution Mechanism Of Near-α CT20 Titanium Alloy

Titanium alloy has excellent properties such as low density,high specific strength,corrosion resistance and good weldability,and has become extremely important metal structural material in modern industry.Rolling is extensively applied to manufacture titanium alloy profiles for low cost and convenience.By adjusting the rolling process,not only the size of profile can be changed,but also the mechanical properties can be optimized.At present,domestic and foreign investigation on the evolution of microstructure and mechanical properties of near-alpha titanium alloys during cold rolling is quite limited,and intrinsic mechanisms between them are inadequate,which results in the lack of theoretical guidance for the improvement of the property of this type of alloy.Meanwhile,significant distortion energy formed in the cold rolled alloy can be released by heat treatment,and this will promote dramatic change in the microstructure,which is beneficial to optimize the comprehensive mechanical properties of the alloy.Therefore,applying appropriate rolling and heat treatment process to regulate the microstructure,enhance the comprehensive mechanical properties of alloy,and establish the process-microstructure-property relationship is significantly vital for the development of precise processing technology and application of near-alpha titanium alloy.In this paper,a near-alpha titanium alloy CT20 developed for extreme low-temperature service conditions was used as the investigation object,and the sheets were prepared by different rolling processes.By various characterization techniques,the effects of cold rolling reduction on microstructure and texture of alloy were investigated,and to analyze respective evolution mechanism.By testing the mechanical properties of alloy with different rolling reductions,the relationship was discussed between microstructure and properties,and the deformation behavior and mechanism during the stretching process at room temperature were deeply investigated.Subsequently,part of the rolled alloys were annealed,and the effect of annealing temperature on the microstructure and mechanical properties was studied to reveal the triggering mechanism of excellent strength/ductility match of annealed alloy.The main conclusions are as follows:(1)Amongαphase,βphase andβ_t,αphase is the most sensitive to strain.Growing rolling reduction reduces the texture intensity,and improve the strength and hardness of alloy while reducing ductility.Meanwhile,the tensile properties of alloy along rolling direction(RD)are better than that of transverse direction(TD).Ductile fracture occurs when stretched along RD,and mixed ductile-brittle fracture happens during stretching along TD.(2)Numerous defects make work hardening phenomenon of cold rolled alloy stronger than that of the hot rolled(HR)alloy.When stretched along RD,the long dislocation slip distance and wellαgrain orientation enhance the work-hardening phenomenon and ductility in RD.The dislocation slip distance along TD is short,and theαgrain orientation is not favorable for the activation of slip system,so the work-hardening phenomenon and ductility is low.(3)Annealing can coarsen theα_s phase and improve the Burgers orientation relationship for HR alloy.Besides,the rising temperature promotes the spheroidization ofαphase and the decomposition and reconstruction ofβ/β_t.For the cold rolling reduction of 50%(CR-50%)alloy,αphase gradually grows and spheroidizes with the higher annealing temperature,andα_s phase precipitates after the growth ofβphase.After annealing,the hardness and ductility of HR alloy fluctuates,and the strength gradually decreases.The hardness/strength of the annealed CR-50%alloy decreases with increasing temperature,and the ductility also fluctuates but is better than the alloy before annealing.(4)The HR alloy annealed at 700℃and the CR-50%alloy annealed at 700℃/800℃have abundant slip system involved and coordinated in deformation,and the low density of defects guarantees continuous slip of dislocations.Therefore,the alloys generate a well-matched strength/ductility.At the same time,the different microstructural morphologies in the three alloys also improve the match of strength/ductility.