Microstructure,Property And Hot Deforming Behavior Of TiAl-based Alloy Prepared By Spark Plasma Sintering

TiAl-based alloys with low density,high specific strength,good high temperature oxidation resistance and excellent corrosion resistance are considered as one of the most concerned high temperature structural materials.However,their application are limited due to their low plasticity at room temperature and poor formability in processing.In order to improve the comprehensive properties and processing formability of TiAl-based alloys,Ti-46.5Al-2.15Cr-1.90Nb-?B,Y,Mo?powders were prepared by gas atomization with refined grain size and homogeneous microstructure.Then,the Ti-46.5Al-2.15Cr-1.90Nb-?B,Y,Mo?alloys with considerable properties were prepared through optimizing temperature-related technological parameters of spark plasma sintering..By means of isothermal double-pass hot deformation experiments,the influence of different processing parameters on hot deformation behaviors was explored,and the softening mechanism during deformation was analyzed to provide experimental basis for improving the formability of TiAl-based alloys.The effects of sintering temperature,temperature control mode and heating steps on the microstructures,densification and properties of spark plasma sintering were studied.It was found that,the metastable transition of ?₂?? occured with the increase of sintering temperature for the atomized powders.The near gamma microstructure formed at sintering temperature of 1100?.When the sintering temperature rised to 1250?,TiAl-based alloys had ?₂/? lamellae,which exhibited typical duplex microstructure.Moreover,the temperature of manual control mode was relatively stable,and the temperature fluctuated ±25? for the automatic control mode.The microstructures of TiAl-based alloys were affected slightly by the different heating steps,but the grain size of TiAl-based alloys increased for three heating steps sintering due to the increase of sintering time.The temperature had a great influence on the densification of TiAl-based alloys.There were obvious holes in the alloys and poor densification was achieved,when the temperature was low.Nearly full densification took place,when the sintering temperature exceeded 1100?and the sintering time was more than 12 min.The properties of TiAl-based alloys were affected by densification and microstructure.The comprehensive properties of TiAl-based alloys were poor,which was determined by the low density at relatively lower sintering temperature.The yield strength of TiAl-based alloys decreaseed obviously when the temperature rised to 1250?since the microstructure of TiAl-based alloys changed from near-gamma structure to duplex structure.The TiAl-based alloys had optimal mcehcnical properties when the sintering temperature was 1150?,i.e.the yield strength,fracture strength and plastic strain were1084 MPa,2477.6 MPa and 39.45%,respectively.Isothermal double-pass hot deformation experiments of TiAl-based alloys showed that the flow stress decreased with the increase of temperature and the decrease of strain rate.The second peak stress decreased with the increase of holding time due to the metadynamic recrystallization softening when the deformation temperature was 1150?and 1200?.When the deformation temperature was 1250?,the flow stress was affected by both softening and phase transformation.With the increase of inter-pass duration,the second peak stress increased gradually.Furthermore,the grain size of the alloy was influenced by recrystallization nucleation and growth.With sufficient recrystallization,the quantity of nucleation increased and thus the grain size reduced.When the recrystallization of TiAl alloy was sufficient,the grain size increased with further increasing the temperature and deformation.The metadynamic recrystallization of interval pass could reduce the small sized grain after deformation and make the grain distribution more uniform.The dynamic recrystallization mechanism of near grammar TiAl-based alloys was mainly continuous dynamic recrystallization.The low-angle grain boundaries formed by increasing the orientation difference through the accumulation of dislocations,and then sub-grain boundaries formed by the further accumulation of orientation difference.Finally,the recrystallized grains appeared by the rotation of sub-grains.In addition,the alloys with near-gamma microstructures also included local discontinuous dynamic recrystallization and twin nucleation recrystallization as minor mechanisms during hot defroming.The dynamic recrystallization mechanism of lamellae for TiAl-based alloy was discontinuous dynamic recrystallization.Through the accumulation of dislocations at grain boundaries,the nucleation of grain boundary protrusion occured and the recrystallized grains were formed.The metadynamic recrystallization took place for TiAl-based alloys during the interval of passes.By growth of recrystallized grains,the internal distortion energy was consumed and therefore the alloys were softened.