Research On Pulse Current-assisted Hot Deformation And Microstructure Evolution Of Ti₂AlNb Alloy

Due to light-weight,high-strength and excellent high-temperature properties,Ti₂AlNb alloy has gradually become one of the most potential new light-weight high temperature structural materials which can be used at 600-800?.However,Ti₂AlNb alloy needs to be processed at higher temperatures(T>950?)due to its inherent brittleness.Moreover,the narrow processing temperature range and high microstructure sensitivity make it difficult to deform.It is important to improve the plastic deformation ability and the mechanical properties of Ti₂AlNb alloy.This study introduced the pulse current heating method during the deforming process,which could reduce the flow stress of the material and improve its plasticity.It is necessary to investigate high temperature deformation mechanism and microstructure evolution of Ti₂AlNb alloy.The compression deformation temperatures were located in four phase regions,and the strain rate ranged from 0.05 to 1.0 s^-1.The true stress-strain curves of current-assisted hot compression and furnace compression were analyzed.It can be found that the peak stress of current-assisted hot compression could be greatly reduced.And the relationship between current and temperature was obtained base on the curves.The flow stresses near the yield point(?=0.05)were selected,and the high-temperature constitutive equations of current-assisted hot compression and furnace compression Ti₂AlNb alloy were established on the basis of Arrhenius formula.Meanwhile,the Joule heating effect and the non-Joule heating effect of electric current were used to study the influence of electric current on the flow stress.In this work,the defomaed microstructure were investigated by optical microscope(OM)and scanning electron microscopy(SEM).The results showed that the stress softening behavior is related to dynamic spheroidization and dynamic recrystallization during hot compression deformation.The main softening mechanism was dynamic spheroidization when the deformation temperature of current-assisted hot compression was 930?.However,recrystallization began to occur as the deformation temperatures rised to 980?.Moreover,the size and quantity of the recrystallized grains were significantly affected by the compression temperature and strain rate.It can be concluded that dynamic spheroidization was dominant softening mechanism when deformation temperature was below 980°C during furnace compression,but the dynamic recrystallization was dominant as the temperature increased to 1030°C.Comparing the microstructure of current-assisted hot compression and furnace compression,the results indicated that the more dynamic spheroidizated and dynamic recrystallized grains produced during current-assisted hot compression.The transmission electron microscope(TEM)was used to further investigate the microstructure evolution and the current mechanism.The results showed that the current could promote the movement of some unmovable dislocations,and those entangled dislocations were gradually activated by pulse-current.The existence of current was conducive to dislocation glide.On the one hand,most of the dislocations were hindered at the boundary of the second phase,which could lead to the generation of twins.On the other hand,the current promoted the dislocations to pass through the second phase and form a dislocation wall,which caused the O phase to accomplish the shear separation and spheroidization.