Study On Plastic Deformation Mechanisms In High-temperature Deformation Of Ti-42.9Al-4.6Nb-2Cr With Duplex Structure

Thrust to weight ratio of aero engines can be increased by replacing Ni-based superalloy by lightweight and high-strengthγ-Ti Al alloys for structural weight saving.Unfortunately,narrow hot working window and high sensitivity of processing quality to temperature forγ-Ti Al alloys seriously restrict plastic manufacturing thin-walledγ-Ti Al alloys components with large size.The key for the development of plastic deformation manufacturing technology is to study high-temperature deformation mechanisms ofγ-Ti Al alloys in depth.In this paper,Ti-42.9Al-4.6Nb-2Cr with duplex structure was selected.The main results and conclusions are listed in the following.Based on flow stress-strain curves obtained by isothermal compression tests,the effect of deformation parameters on the flow behavior of Ti-42.9Al-4.6Nb-2Cr with duplex structure was investigated.Results showed that the flow stress and declining trend of flow stress decreased with the increasing of deformation temperature or decreasing of strain rate.The flow stress firstly increased to the peak value and then decreased with the increasing of strain when the deformation temperature was lower thanγ-solvus temperature T_γ,solv.As the strain increased,the apparent activation energy for hot deformation in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure decreased from 613.8±3.5 k J·mol^-1 to 511.5±14.2 k J·mol^-1.The strain rate sensitivity index in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure decreased with the increasing of strain rate,and increased with the increasing of deformation temperature.The strain hardening index range in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure increased with the increasing of strain rate.Based on the kinetic analysis,constitutive models coupling strain effect in the isothermal compression were established.X-ray diffraction（XRD）,optical microscope（OM）,and scanning electron microscope（SEM）technologies were utilized to investigate the microstructure and analyze the effect of deformation parameters on microstructure morphology and characteristics when Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at elevated temperatures and then air-cooled to ambient temperature.Results showed that as the deformation temperature increased,diffraction peak intensities of atomic planes forγphase changed little,and the microstructure morphology of isothermally compressed Ti-42.9Al-4.6Nb-2Cr with duplex structure changed from nearly gamma to lamellar structure.With the decreasing of strain rate,the microstructure distribution uniformity of Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at 1200℃increased which was ascribed to enhanced dynamic recrystallization（DRX）behaviors,howbeit the isothermally compressed microstructure distribution uniformity at 1300℃degraded due to uncontrolledαgrain growth.Additionally,α₂/γlamellar colonies,massive structureγ_m,Widmannst（?）tten coloniesγ_w,feathery structureγ_f,andγgrains coexisted in Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at the deformation temperature of 1300℃.With the increasing of height reduction,the volume fraction of remnantα₂/γlamellar colonies decreased,while the angle between lamellae interface and compression direction increased,and the volume fraction ofγgrains increased from 64.39%to 78.47%when Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at the deformation temperature of 1200℃and strain rate of0.01 s^-1.Phase transformation mechanisms forγ,αandβphases during isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure were investigated via transmission electron microscope（TEM）and high-resolution TEM（HRTEM）technologies.Results showed that theγ→αphase transformation occurred when Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at the deformation temperature of 1250℃.Theγ→αphase transformation was achieved by the consumption of adjacentγvariants and controlled by atoms diffusion.Theβ→γandα→γphase transformations occurred when Ti-42.9Al-4.6Nb-2Cr with duplex structure isothermally compressed at the deformation temperature of 1200℃.Theβ→γphase transformation was accomplished by the rearrangement of（110）_βatomic plane.The main diffusion flow for theα→γphase transformation was Ti,Al,and Nb atoms diffusion.Theα→γphase transformation was analogous to step-growth mechanism assisted by screw dislocations or accomplished by the immigration ofα/γphase interface.There was anotherα→γphase transformation mode that stacking faults pairs ofγphase precipitated inα/α₂ grains during air cooling.Electron backscatter diffraction（EBSD）technology was utilized to investigate distribution and evolution for Schmid factor,grain orientation,grain and subgrain boundaries in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure.Besides,dislocation configurations,fine structure of stacking faults pairs and twins were also investigated via TEM and HRTEM.According to that,twinning,decomposition and dynamic recrystallization mechanisms in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure were elucidated.Results showed that stacking faults pairs inγgrains were produced by the interaction of super dislocations and piled-up dislocations,and were evolved into twins by absorption of each other.At the early stage of deformation,intensive stacking faults pairs in two directions intersected to form network structures.At the later stage,these stacking fault pairs in the same direction evolved into twins,resulting in the movement of intersection points,and three intersection phenomena（like deflection,displacement and rigid-body shifting）.Remnantα₂/γlamellar colonies were kinked by faults pairs inγlamellae and fracture ofα₂ lamellae,and decomposed at the kinked area by DRX ofγphase and growth of adjacent lamellae.DRX nucleation mechanisms inγphase were discontinuous and continuous dynamic recrystallization mechanisms,and the latter only happened at the interior of largeγgrains.The dominant growth mechanism for recrystallizedγgrains changed from the strain-induced grain boundary bulging to interface immigration ofγ/αphase interface with the increasing of deformation temperature.DRX inαandβphases was continuous DRX in the isothermal compression of Ti-42.9Al-4.6Nb-2Cr with duplex structure.As the strain rate decreased,αtexture changed from deformation to recrystallization fiber texture.The volume fraction of recrystallizedβgrains fluctuated with the increasing of height reduction.