Hot Deformation Behavior Of Ti-6Mo-5V-3Al-2Fe Alloy

Metastableβtitanium alloy has high specific strength,low elastic modulus,good corrosion resistance.The good strength-plastic-toughness match can be obtained by heat treatment.Thus,it is widely used in the aviation and aerospace industry.However,grain coarsening during hot working restricts the further improvement of mechanical properties of the alloy.In this paper,a new metastableβtitanium alloy,Ti-6Mo-5V-3Al-2Fe,was taken as the research object.The thermal deformation behavior and the evolution ofβgrains during thermal deformation under different process parameters were studied by thermal compression experiment,electron back scattering diffraction（EBSD）and transmission electron microscope（TEM）.The results can provide reference for the determination of hot deformation process window and grain size optimization of the alloy.By analyzing the true stress-true strain curve of the thermal deformation process,it can be seen that the flow stress is significantly affected by the deformation temperature and strain rate.The flow stress decreases significantly with the increase of deformation temperature and the decrease of strain rate.The thermal deformation constitutive equation is constructed inα+βphase andβphase,respectively.The error analysis results show that the constitutive equation has high accuracy in predicting the flow stress.According to the dynamic material model and Prasad instability criterion,the hot working diagram of the alloy was established,and the rheological instability region and the region suitable for processing of the alloy were determined.During the hot deformation of theα+βphase region,dislocation movement is hindered byαphase,and thenαphase affects the dynamic recrystallization behavior ofβgrains.With the increase of deformation temperature,the amount ofαphase decreases,and the effect ofαphase on the dynamic recrystallization ofβgrains becomes loss.When the deformation temperature is 700℃,dynamic recrystallization takes place at the grain boundaries and dynamic recrystallization grains with high misorientation are formed.At deformation temperature of 750℃,the softening mechanism of the alloy changes from dynamic recovery to dynamic recrystallization with the increase in strain rate.The adequately dynamic recrystallization actives when the deformation temperature is 800℃,and the recrystallization grains are formed near theβgrain boundary as well as inside the grain at the strain rate of 0.0005 s^-1.The mixture mechanism of continuous dynamic recrystallization and discontinuous dynamic recrystallization occurs in the alloy,and the refinement of the originalβgrain is the most obvious.During the hot deformation ofβphase region,dynamic recrystallization occurs when the deformation temperature is 850℃and 900℃.When the strain rate is 0.005 s^-1,the dynamic recrystallization degree of the alloy is relatively high,but when the deformation temperature is higher than 900℃,the recrystallized grains grow up obviously.When the deformation temperature is 850℃and strain rate is 0.005 s^-1,the refinement ofβgrain is better.