Hot Deformation And Microstructure And Mechanical Properties Of High Nb Containing TiAl Based Alloy

High Nb containing TiAl based alloy is the typier of high temperature and highstrength TiAl alloy. Due to their attractive propertis, such as high specific strengthand modulus, good high temperature oxidation resistance and creep resistance, etc,high Nb containing TiAl alloys have a high potential application in aerospace andautomobile industry. However, the difficulty in ingot preparation and the largedeformation resistance of high Nb containing TiAl alloy limit their processing andapplication. It is proved that β solidifying can effectively reduce segregation andrefine the structure. Two kinds of β solidifying high Nb containing TiAl alloy withnominal composition of Ti-45Al-9Nb-Y and Ti-44Al-8Nb-0.2W-0.2B-Y areprepared by using ISM and VAR technologies, respectively. The preparation ofingots, hot working technology, heat treatment processing, and microstructure andmechanical properties were investigated systematically in this paper.Ti-45Al-9Nb-Y and Ti-44Al-8Nb-0.2W-0.2B-Y alloys were prepared by ISMand VAR respectively. Ti-45Al-9Nb-Y alloy shows a typical near lamellar structure,with same orientation in the adjacent lamellar colonies. And γ phase precipitateddiscontinuously in the residual β phase with the relationship of（110） β//（111） γ,forming mixed structure between β/B2and γ phase in the grain boundary. ISMtechnology can effectively reduce segregation and improve the uniformity of theorganization. Elements in Ti-44Al-8Nb-0.2W-0.2B-Y ingot after HIP distributeduniformly. TiB formed during solidifying act as heterogeneous nucleation siteswhich increase the nucleation rate of α phase and refine the lamellar strcutureresulting in the formation of non Burhers relationship α phase. The top of the ingothas the best mechanical properties; room temperature strength is about680MPa.High temperature compression test and canned forging of Ti-45Al-9Nb-Y alloywere carried out. The θ=0°lamellar colony with medium yield strength bent androtated to the orientation of θ=90°which has highest yield strength, and dynamicrecrystallized at this bent area of lamellar colony and its grain boundary. The0°＜θ＜90°lamellar colony with soft orientation also rotated to the orientation of θ=90°accompanied with numerous dynamic recrystallization, while the θ=90°lamellarcolony was elongated with dynamic recrystallization at grain boundary. Theoptimum deformation process was determined as1250℃/0.05s^-1. The as-forgedstructure is typically duplex structure, composed of γ phase, β phase and γ/α₂structure. The coarsened α₂phase in the residual lamellar colony containstweed-like pattern perpendicular to the lamellar orientation. Widmannst tten α₂ phase was obtained in equiaxed γ phase after heat treated at1330℃with30min/FC,which promoted the formation of lamellar structure. The full lamellarstructure has the highest strength and the elongation of as-forged structure is thelargest.Ti-44Al-8Nb-0.2W-0.2B-Y alloy was canned forged at1275℃withdeformation of50%and70%, and pancakes with70%deformation were furnacecooled and air cooled, repectively. And with the increase of deformation, as-forgedstructure has much higher strength and plasticity at room temperture, for example,the strength at room temperature was930MPa of alloy with70%deformation. Theair cooled deformation structure was work hardened obviously which indeedincreased the strength at lower temperature and resulted in the crack of the pancake.After tensiled of the alloy with70%deformations at900℃and above tempertautre,ω phase with white contrast formed, and its content increased with temperatureincreasing or strain rate slowing. Considerating the solid state phase transformationduring the tensile tests, it was believed that ω phase fromed in the transformationα₂â†'ω+γwhich was promoted by the dynamic recrystallization. Fine duplexmicrostrucutre with globularized γ grain and small lamellar structure formed after atwo steps heat treatment of1310℃/20min+1240℃/1h. Full lamellae structure couldbe obtained at1340℃for10min. The mechanical properties were measured forduplex structure and full lamellar structure.Microstructure evolution of α laths and γ laths during hot working and thefollowing cooling processss was discussed in detail. During deformation, ordered γphase in remnant lamellae dynamic recrystallized prior （γ_D） in the form ofsubboundary rotaion，while the disordered α lath which has high stacking fault weremerely bended. As deformation increase, dynamic recrystallization occurred in αphase and the grown of γ_Dinto α₂phase by bulging increased. The higher rate ofcooling, the lower rare of swallowing ofγ_D, the more quantity of α₂phase residual.The microstructure of as-rolled Ti-45Al-9Nb-Y alloy is typically near γstructure with dynamic recrystallized grain in RD, ND and TD directions. As thedeformation increase, the stress induced α phase transformation of γâ†'αoccurredfrom forming α₂+γ mixed structure into forming single α₂phase. Annealing twinsformed by stacking fault in the sheet with65%deformations. ω phase tranformedfrom β/B2phase was dramatically improved by rolling, the morphology of ω phasechanged, and the number of ω phase increased, as deformation increase. β/B2phasehas high contents of ω phase after rolling with65%deformations. Full lamellae heattreatment at1420℃holding20min could eliminate ω phase completely which isbrittleness and is bad to the properties.