Study On Phase-transformation Behaviors And Microstructure Control Of The High-strength TB8 Alloy

TB8?Ti-15Mo-2.7Nb-3Al-0.2Si?is a typical metastable beta titanium alloy.The alloy has an excellent deformed performance,oxidation and corrosion resistance due to the single ? phase after solution treatment.The previous studies indicate that the solution plus aging treatment at the reasonable temperature and duration makes the alloy have the high-strength and good-plasticity properties.The microstructure evolution mechanism and effects of precipitation process on morphology control and performance improvement were rarely reported.Therefore the systematic study on precipitation phase transformation and morphology control in the TB8 alloy is beneficial for the engineering application.The experimental in this paper is conducted to the rolled TB8 bars.The microstructure evolutions in the deformation and recrystallization behaviors during the solution treatment have been studied by the SEM?XRD and EBSD technique as the fundamental points.The phase transformation of the solution-treated alloy in the heating up process has been studied by the DSC technique systematically,same as the analysis of the pre-aged alloy in the isothermal treatment process.The studies are in order to establish the phase transformation kinetics model of TB8 alloy during the continuous heating,and recognize the mechanism of the effects of the isothermal aging conditions on the precipitation morphology,distribution and orientation relationship.The ? precipitates of the pre-aging and crystal defects of the pre-deformed play an assistant role for the nucleation of ? precipitates in the final-aging process.Main works of this paper are listed as follows:?1?The analysis of microstructure/texture evolution of the ? grains and dynamic recrystallization behaviors during the hot deformation has been conducted.To study the effects of the hot rolling temperature on microstructure evolution,it is beneficial for analyzing the causes of the different textures in ? or ? phase after hot rolling.Texture evolution of ? grains and formation mechanism of the crystal defects during the cold rolling was studied intensively.Furthermore,the analysis of the microstructure evolution mechanism of ? grains in the TB8 alloy has been conducted.?2?The recrystallization behaviors of the ? grains in the TB8 alloy solution-treated at 800 ? 800 ? for 60 min have been studied,and achieve the recrystallization kinetics equation as ??28?TD)314.841.6exp(-8955.On the other hand,the alloy was solution-treated at 840 ? for 10 min 180 min,and expression of growth of the ? grains is?28?7.41 t D204.0.Through analyzing the effects of the scale of ? grains on precipitation behavior in the aging process,the relationship of the solution treatments and properties of the aged alloy has been studied.?3?The analysis of the effect of the continuous heating rates on phase transformation process in the TB8 alloy has been conducted.The relationship of the activation energy and ??? phase transformation degree in the heating up process has been studied.The mechanism of precipitation nucleation and growth in the phase transformation process during continuous heating has also been discussed.?4?The analysis of nucleation and growth of ? precipitates during the isothermal aging has been conducted.The mechanism of effects of phase conditions on morphology,distribution and orientation relationship of ? precipitation has been studied.In the TB8 alloy,the more length ?WGB?? widmanstatten precipitates that developed from the ?/? boundaries or ? phase at grain boundaries in parallel colonies?and a tiny amount of ?WM?? widmanstatten precipitates in the intragranular area?have transformed at the 600? isothermal aging;the length of the ?WGB decreases and the mounts of ?WM increases at the 550? isothermal aging;the short length ?WGB and a tremendous amount of ?WM have transformed at the 500? isothermal aging;the transformation of ???/? has been occurred with the reasonable conditions at the 450? isothermal aging,and cause of the heterogeneous microstructure.?5?Precipitation in the TB8 alloy has been growing along the certain direction rapidly during the isothermal aging.The composition deviation of ?WM is more than that of ?WGB in the alloy aging-treated at the 550 ?for 1 h,and suggests that the growth rate of ?WM should be higher than that of ?WGB.Meanwhile,the textures of ? and ? phase in the alloy isothermal aging-treated 500? and 600? have been studied.The results illustrate that the textures of the ? phase transformed during the both aging treatments are random,and the ? phase formed at 500? aging has an obvious texture with <112₀> orientations,but that formed at 600? aging is random.?6?The analysis of final-aging precipitation behaviors in the pre-aged TB8 alloy has been conducted.According to JMA dynamic calculation formula,the Avrami indexes of the final-aging process in the 320? and 400? pre-aged alloys are 2.83 and 3.04 respectively.The mechanism of ? precipitation growth during the final-aging in the pre-aged and pre-deformed alloy has been investigated.The crystal defects of pre-deformed and the ? precipitates of pre-aged have an obvious function for the assistant nucleation of precipitation.?7?The analysis of the effects of phase transformation conditions on the precipitation morphology and tensile properties in the TB8 alloy has been conducted.Through the analysis of the relationship of precipitation morphology and fracture,the mechanism of the phase transformation control and the performance strengthening of the TB8 alloy has been studied intensively.The results of the analysis of the tensile testing behaviors in the solution plus aging treated TB8 alloy,show that a good combination tensile properties in the single aged alloy are achieved with the 840? solution plus 500? aging treatment.The duplex aging treatments could raise the strength of the alloy,and remain a good plasticity.