Study On Cold/Warm Deformation And Aging Precipitation Behavior Of TB17 Titanium Alloy

Under the premise that the chemical composition of the material does not change,it is a very effective method to promote the specific properties by controlling the precipitates.This text aimed to metastable beta titanium alloy TB17,through several methods such as stress-strain curve analysis,phase detection,and microstructure observation,studied the precipitation behavior during cold deformation,warm deformation,cold/warm deformation and the subsequent aging behavior,the effects of deformation and ageing factors on phase change precipitates was researched,and got the experimental basis for the selection of deformation and aging parameters for TB17titanium alloy.Firstly,in the research of cold deformation and the subsequent aging behavior to TB17 titanium alloy,this material expressed more sensitive to strain rate from the analysis of stress-strain curve,it showed apparent rheological softening phenomenon with a lower strain rate(0.1s^-1)condition,and behaved apparent rheological hardening phenomenon in a larger strain rate(0.01s^-1).Via structure observation and XRD analysis,martensite phase was both detected in cold deformation with a strain of0.105 and in a low-temperature aging at 350℃for 0.5h with a predeformation of strain 0.22,no obviousαandωphase transition occurred during low temperature aging at 350℃for 0.5h.While under the same deformation conditions,no obvious phase transition occurred after aging treatment at 250℃for 0.5h.Deformation induced martensite and isothermal martensite which were formed in room temperature deformation and low temperature aging process respectively,both belong to lath martensite,the same point was that both were precipitated near the grain boundary,the difference was that the isothermal martensite was distributed in parallel,and the deformation induced martensite exhibited an irregular distribution.As aging condition changed to 500℃for 8h,the martensite was decomposed,fine alpha phase precipitated along the deformation band and appeared a phenomenon of“connected into a line”,a mass of needle-like alpha phase staggered distribution in microstructure.Secondly,Microstructure evolution of warm deformation during 400℃-500℃and the subsequent aging precipitation behavior based on TB17 titanium alloy was studied,result showed that dislocation glide,twining,deformation induced martensite transformation and deformation band were contained to its warm deformation mechanism.Besides,not only temperature but also strain rate had great effect to precipitation behavior.According to the trend of the stress strain curve,as strain rate kept to 0.01 s^-1,compared to 400℃,the resistance force performed larger under 500℃.The analysis experiment confirmed that alpha phase obviously precipitated under 500℃rather than 400℃.However,alpha phase and martensite both prefer precipitated in a low strain rate condition.Besides,the subsequent low temperature aging treatment showed that the martensite presented dynamic variation,deformation induced martensite resolved gradually and the isothermal martensite was precipitated,but no significant phase transition occurred after the low-temperature aging treatment of the undeformed sample at 250℃for 0.5h and 350℃for 0.5h.When aging condition changed to 500℃for 8h,martensite were decomposed completely and a large amount of alpha phase were precipitated,then,the thin strip microstructure was formed.Finally,in the study of step-by-step deformation of TB17 titanium alloy,it had been found that with the precondition of no more any other secondary phase,the isothermal martensite which formed in low-temperature aging treatment after first step cold deformation played a role in coordinated deformation during secondary cold deformation,degraded deformation resistance and reduced modulus of elasticity.During cold/warm deformation,the precipitation phase was changed with the varied of the deformation order,deformation induced martensite and alpha phase was emerged with the condition of warm deformation the former cold deformation the latter,and deformation induced alpha phase was occurred in plastic stage,while only martensite was appeared under the condition of cold deformation the former warm deformation the latter.Besides,the deformation resistance raised sharply as the low-temperature aging（350℃0.5h）added between the two steps deformation,analysis showed that the deformation induced martensite precipitated in first step deformation gradually decomposed in aging treatment,alpha phase was precipitated by the induced of secondary warm deformation,and no obvious isothermal martensite was detected.From the above,the evolution of the microstructure and phase change of TB17titanium alloy under deformation and aging conditions is known.The deformation factor has an important influence on the phase transformation,and it has a close relationship with the phase transformation in the subsequent deformation and aging process.Under the condition of combination of low temperature aging and step deformation,the phase transition precipitation behavior is complicate,which also indicates that the effect of using the aging precipitation phase to achieve coordinated deformation needs to meet certain conditions.