Research On Rheological Softening Behavior And Mechanism Of CP-Ti At High Temperature

With the advantages of excellent corrosion resistance and good biocompatibility,CP-Ti has been widely used in aerospace,chemical industry,shipping and medicine.Because most CP-Ti parts need to be produced by hot working,it is very important to study the thermal deformation behavior and dynamic structure evolution of CP-Ti.In this paper,the special softening behavior and dynamic microstructure evolution of CP-Ti during high temperature deformation were studied,and its formation,mechanism and influencing factors were discussed.The results show that the thermal flowing softening are caused by dynamic recovery,dynamic recrystallization and ? ? ? dynamic transformation(DT).The DT is closely related to the dislocation activities,and presents as follows:(1)the dislocations are generated at the grain boundaries and expand from the grain boundary to the interior of the ? phase grains,and the part swept by the dislocations transform into the high-temperature ? phase,and the ? phase grains are split into(? + ?)lath structure;(2)a large number of dislocations gather at the ?phase boundaries make the DT occur at the grain boundaries.With the increase of the strain,the DT region extends and finally a whole ? phase grain becomes a ? phase grain;(3)during thermal deformation,dislocations are produced inside the ? phase and part of the ? phase becomes ? phase.When the CP-Ti is hot deformed at high strain rate,the DT takes place before the dynamic recrystallization;when the CP-Ti is deformed at low strain rate,the dynamic transformation and dynamic recrystallization take place almost at the same time.The initial crystallographic orientation has a significant effect on the DT behavior.The lath structures caused by the DT in the specimen with pre-deformation is obviously more than that in the specimen without pre-deformation,and the peak stress of thermal flowing curve is also reduced.This shows that the orientation of the initial ? phase in CP-Ti is adjusted by pre-deformation,which is more conducive to the DT of ? phase.Further study shows that the initial ? phases in CP-Ti are mainly T-texture orientation,while the ? phase in CP-Ti is mainly R-texture orientation after pre-deformation.At the same time,it is confirmed by Schmidtfactor analysis that the orientation of R-texture is more conducive to the occurrence of the DT than that of T-texture,and the dislocations are more likely to slide on the pyramid surface after the beginning of thermal deformation,leading to the occurrence of the DT.