Study On Microstructure Evolution Of TB17 Titanium Alloy During Hot Working Process

With the development of new-generation aerospace vehicles in the direction of high speed,large size,complicated structure and improved fuel economy,ultra-high strength and toughness titanium alloys will be applied to bearing structural parts in the fields of aviation and aerospace to achieve greater weight reduction.The mechanical properties of titanium alloys are determined by the microstructure.The development of ultra-high strength and toughness titanium alloys is limited by the matching between high strength and high toughness.Exploring the relationship between titanium alloy hot processing and organization,revealing the plastic forming regulation,phase transformation regulation and optimizing the processing parameters are of great significance for improving the quality and performance of forgings,promoting the development and application of ultra-high strength and toughness titanium alloy.In this paper,the thermal deformation behavior and microstructure evolution of TB17 titanium alloy at high temperature were studied systematically by isothermal hot compression experimental,and the Arrhenious constitutive relationship was establish.The dynamic recrystallization behavior and mechanism of the alloy were studied and the processing map was constructed based on dynamic material model and Murty criteria.The effect of aging temperature of TB17 titanium alloy on the type,morphology,distribution and formation of precipitated phase of the alloy was studied.The high temperature flow behavior of TB17 titanium alloy shows that the alloy exhibits flow softening phenomenon in the temperature range of 795⁹80°C.With the increase of deformation temperature and strain rate,the flow stress of the alloy reduced obviously.By considering the influence of strain on flow stress,the constitutive model between flow stress and temperature,strain rate and strain is established,and it could accurately predict the flow stress of TB17 titanium alloy.Based on the dynamic material model and Murty criterion,the thermal processing map of TB17 titanium alloy was established.The instability processing domains in the thermal processing map focused on the range of high strain rates;The safe processing domains mainly in the low strain rates.The deformation mechanism of the safe processing is mainly dynamic recovery and dynamic recrystallization,and local plastic flow occurs in the instability processing domains,and an adiabatic shear band is formed as the strain increases.The microstructure of TB17 titanium alloy after high temperature thermal deformation shows that increasing the deformation temperature and reducing the strain rate are beneficial to the dynamic recrystallization.With the decrease of deformation temperature and the increase of strain rate,the dynamic recrystallized grain size decreases.In theβsingle phase field,when the strain rate is no more than 0.01 s^-1,it could acquire a relatively uniform microstructure,otherwise an adiabatic shear band is easily formed.The dynamic recrystallization kinetics of theβphase region satisfies the Avrami equation.Both elevated temperature and reduced strain rate could increase the degree of dynamic recrystallization;In addition,the critical strain increases as the temperature decreases and the strain rate increases.The deformation mechanism of TB17 titanium alloy at high temperature is mainly dynamic recovery and dynamic recrystallization.There are two dynamic recrystallization mechanisms in theβphase region.The dynamic recrystallization at the low strain rate is mainly nucleation inside the grain,and the transformation mechanism is discontinuous dynamic recrystallization of the sub-grains combined with rotation.At high strain rate,it is mainly nucleated near the originalβgrain boundary.There are two kinds of transformation mechanisms,one is discontinuous dynamic recrystallization of the grain boundary bow nucleation,and the other is continuous dynamic recrystallization;In the（α+β）phase field,dynamic recovery occurs in the originalβcrystal grains to form sub-grains,and dynamic recrystallization occurs at the grain boundaries to form highly mis-orientation dynamic recrystallized grains.The results of isothermal aging of TB17 titanium alloy results show thatβ→β+ωphase transformation mainly happens at 350℃aging,and the precipitated phase exhibits as fine granular shape.β→β+ω→β+αphase transformation mainly happens at 450℃aging,when the aging time is less than 1h at 450°C,theωphase is mainly precipitated at first,and theωphase is converted into theαphase as the aging time increases,and the formedαphase is a fine sheet-like shape.β→β+αphase transition occurs when aging at 550℃and 650℃,there are different types variants of theαphase directly precipitated from theβphase,and the morphology is mainly like a needle shape.The aging temperature and aging time have a significant effect on the hardening effect of TB17 titanium alloy.Short-time aging can significantly increase the hardness of the alloy,but after the aging time increases to a certain time,the hardness tends to be stable;as the aging temperature increases,the hardness of the alloy increases first and then decreases,the hardness is at the largest level when at 450℃.