Research On Hot Deformation Induced Phase Transformation Behaviour Of TA15 Titanium Alloy

Titanium alloy,as a light structural material with excellent comprehensive performance,has been widely used in aerospace and other fields.Titanium alloy components are usually formed by hot forging processing（HFP）,but there are some problems during the HFP,such as narrow forging processing window,huge fluctuation of structure and property,etc.How to reasonably control the hot working process parameters to achieve accurate regulation of structure and property is always a difficult problem in the hot manufacturing of titanium alloy forgings.Deformation-inducedα→βtransformation,also named dynamic transformation（DT）,is regarded as a universally special phenomenon in titanium alloy during hot deformation in the（α+β）two-phase region,that is,the deformation can induce the harderαphase conversion into the softerβphase,resulting in the decrease or even completely disappeared ofαphase content（fαp）with the increase of deformation amount,and further softening of the tested material.Currently,the DT in titanium alloys during hot deformation mainly focused on the qualitative analysis of phase transformation.The quantitative description of the thermal/dynamic conditions and evolution rules of DT,and their effects on the mechanical property response are relatively insufficient,which has posed a new challenge to the accurate control of the structure and property of titanium alloy.In this paper,the method of theoretical analysis combined with experimental research was used to investigate TA15 titanium alloy.The features of microstructure evolution and rules of mechanical response in TA15 titanium alloy during hot deformation in the two-phase region were investigated by systematic hot compression experiments,and the micro mechanism of DT was revealed.The results show that the main mechanisms of flow softening in TA15 titanium alloy are dynamic recovery（DRV）,dynamic recrystallization（DRX）,deformation heat（DH）and DT.In particular,it is found that the DT can make the primaryαphase（α_p）completely dissolve in the matrixβphase under certain deformation conditions,which is equivalent to the transus temperature of TA15 reduced by about 35°C.The DT is most sensitive to deformation temperature,followed by strain rate and deformation amount.The DT can be preferentially triggered on theα_p with the Schmid Factor higher than 0.4,of which mechanisms are mainly manifested as the interface penetration ofαphase and interface migration ofα/βphase.The metastableβphase,the product of DT,is transformed into（quasi-）Widmanst（?）ttenαphase after cooling to room temperature.By considering the driving force of phase transformation softening,the DT thermodynamics conditions of TA15 titanium alloy and its quantitative relationship with deformation parameters were explored,and then the energy dissipation proportion of DT was analyzed.The results show that the DT belongs to a stress-driven type,of which the driving force of DT is from the mechanical activation and deformation stored energy,but the driving force provided by deformation stored energy is far lower than that provided by mechanical activation,which can be negligible.The critical driving force of DT declines with the rise of deformation temperature or the decrease of strain rate.In addition,the proportion of energy consumed by DT to the total mechanical work reduces with the increase of strain rate.According to the Johnson-Mehl-Avrami theoretical model（JMA）,the kinetic quantitative model of DT was established,and the quantitative effect of deformation parameters on the transformedα_p was clarified.The results show that the value range of the exponent n in the DT kinetic equation is 1.25～2.1,indicating that the DT process of the tested alloy is diffusion-controlled,and the kinetics curves of transformedα_p show the typical sigmoidal features.The rate of DT increases first and then decreases with the increase of deformation amount,and rises with the increase of deformation temperature or strain rate,while the saturation value of transformedα_p kinetics curve drop with the decline of deformation temperature or the increase of strain rate.The effects of DH,DRV,DRX,and DT on the dynamic softening were separated by the methods of double differentiation,the self-consistent model,etc.Quantifying the contribution of different softening mechanisms,the interactions between their mechanisms were discussed.The results show that the main control mechanism of dynamic softening is different under different deformation conditions.With the increase of strain rate,the softening degree caused by DH gradually rises,the softening degree caused by DT gradually drops,and the softening degree caused by DRX first decreases and then increases.When the strain rate is less than 0.1 s^-1,the DT plays a dominant role,and DRV/DRX play a minor role;However,when the strain rate is larger than 0.1 s^-1,the DT,DRV,DRX and DH control the dynamic softening together.Among them,the DT and DH have approximately the same softening degree.The DRV and DH are beneficial to DT,while the DT is inhibited by DRX.Conversely,the DT can restrain DRV and accelerate DRX.The feature and evolution of phase transformation in TA15 titanium alloy during multi-pass hot deformation were investigated.The results show that the DT occurs during pass deformation to form the over-saturatedβphase,and the post-dynamicα→βtransformation（PDT）and reverseβ→αphase transformation（RT）successively take place in the isothermally holding stage between pass deformation.What’s more,the DT and PDT can reduce the fαp,yet the RT can increase the fαp.The over-saturated metastableβphase formed by the DT or PDT can be dissolved back and transformed intoαphase during the isothermally holding stage with a longer time between pass deformation.On the other hand,the originalβgrains undergo the DRX during the pass deformation stage,the residual originalβgrains and DRX grains in the early stage of isothermally holding between pass deformation can grow up,and then the static recrystallization can be found in theβgrains with the prolongation of isothermally holding time between pass deformation.