Microstructure Design And High Temperature Mechanical Properties Of TiAl Alloy Based On β→α Transformation

With the continuous development of the aerospace industry,the requirements of alloy materials are increasing daily,such as light weight,high temperature strength,and oxidation resistance.γ-TiAl alloys have high specific strength,good oxidation resistance and excellent high temperature creep resistance.It has attracted more and more attention from researchers and become a promising high-temperature lightweight structural material in the aerospace materials.The traditionalγ-TiAl alloy has a good casting performance.The solidification path is peritectic solidification,and the microstructure of traditionalγ-TiAl alloy is coarse lamellar colonies.Theβ-γTiAl alloys passes through theβsingle phase zone during solidification,and the size of the lamellar colonies is relatively small.In the process of smelting,elements of B,C,or Y are generally added to bring the effect of heterogeneous nucleation.The transformation ofβ→αis the characteristic phase transformation of theβ-γTiAl alloys.During the transformation ofβ→α,the primaryβphase is separated by a large number ofαphases,which can refine the lamellar colonies.The transformations ofβ→αinβ-γTiAl alloys include massive transformation,Widmannst（?）tten transformation and martensite transformation.Due to the transformation of martensite is difficult,there are currently few studies on the phase transformation mechanism and decomposition mechanism.The hot workability of theβ-γTiAl alloys is excellent because of the B2 phase.Besides,the addition ofβstabilizing elements increases the range of hot working temperature.Many researchers have studied the deformation kinetics and deformation mechanisms of TiAl alloys,but the high temperature plastic deformation mechanism of the（B2+γ）structure with ultra-large B2matrix was still unclear.Based on the above research background,this thesis studied the effects of different cooling rates on the microstructures of Ti-42.1Al-8.3V alloy（TAV alloy）.And in order to compare the transformation ofβ→α,the TiAl alloys with and without heterogeneous nucleating elements（Ti-38.6Al-8.1V-0.2（B,C）,TAVBC alloy）were studied.The precipitation and decomposition mechanisms of martensite in TAV alloy were explored,and the high temperature plastic deformation mechanism of TAV alloy was stuied.The main research contents and results were as follows:The effects of different cooling rates were studied on the microstructures of TAV alloy and TAVBC alloy.Under the condition of furnace cooling,the results shown that the model ofβ→αof TAV alloy was the Widmannst（?）tten transformation,and a large number of coarse Widmannst（?）tten were precipitated in parallel.The martensite transformation ofβ→α₂’occurred,and fine needle-shaped martensites precipitated on the B2 phase.A large number of equiaxedαgrains with various orientations were precipitated in the TAVBC alloy.Under the condition of the slow cooling（30℃/h,100℃/h）,the Widmannst（?）tten laths were spheroidized and distributed in parallel in the TAV alloy.The spheroidization mechanism was the terminal material migration mechanism,and a large amount of martensites were precipitated on the B2 phase.Equiaxedαgrains with various orientations were syill precipitated in TAVBC alloy,and there was no martensite transformation occurred on the B2phase.A martensite transformation ofβ→α₂′on the B2 phase during oil quenching was occurred in TAV alloy,and a large amount of martensites with different sizes,lenticular shaped and various orientations were precipitated.On the midrrbis,there were high dislocation densities,and there were also a large number of dislocations inside the martensite.There were threeβ→αtransformation modes ofβ→α₂′martensite transformation,Widmannst（?）tten transformation andβ→αdiffusion phase transformation in TAV alloy during air cooling.Theβ→αdiffusion phase transformation andβ→α₂′martensitic transformation were occurred during air cooling in TAVBC alloy.During oil quenching,TAVBC alloy also occurred a non-diffusion type ofβ→α_m massive transformation.After oil quenching,the TAV alloy was aged at different temperatures and times.The results shown that a large number of fineγlamellae precipitated in martensites at low temperatures from 750°C to 800°C.A large number of fine and equiaxedγgrains precipitated on the B2 matrix,which was a transformation of B2→γ.Martensites were not decomposed,which indicated that TAV alloy exhibited an excellent thermal stability.The microstructure after low temperature aging was the（α₂+γ）lamellar colonies with a small amount of fine（B2+γ）phase.A large number of lamellae precipitated on the martensites at high temperatures from 900℃to 1000℃.Fineγand B2 phase precipitated around and inside the martensites,which is a transformation ofα₂+γ→B2+γ.A transformation of B2→γoccurred on the B2 matrix.When the treatment was 1000℃/100 min,the martensites were basically decomposed.The microstructure is a fine structure of（B2+γ）phase.The high temperature tensile was performed at temperatures from850℃to 1000°C with strain rates from 5×10^-4 s^-1 to 5×10^-3 s^-1of TAV alloy after 1000℃/100 min aging treatment.The results shown that the B2 phase andγphase were followed the K-S orientation relationship.The rheological curves were the work hardening phase curves.TAV alloy exhibited superplasticity at 1000℃,and the highest elongation was 177.5%.The value of activation energy（Q）and stress index（n）of TAV alloy was 464 KJ/mol and 3.1,respectively.The deformation of TAV alloy obeyed the nearly superplastic deformation mechanism,and the strain rate was determined by grain boundary/interface sliding and dislocation creep.By observing the high temperature tensile patterns of TAV alloy,it was found that the ultra-large B2 phase matrix was elongated and the patterns were deformed unevenly.B2 phase occruured intragranular deformation.As the straiin increased,dynamic recrystallization appeared on the B2 phase.The orientation of the B2 phase was rotated and finally transformed to the{111}<112>texture,which was a part ofγ-fiber texture.The ultra-fineγphase moved with the movement of the B2 phase,and the<110>fiber texture was appearred.Theγphase was slightly deformed.Theγphase and the B2 phase were still maintained a K-S orientation relationship.The K-S orientation relationship was relatively weak,because of the strong sliding of the grain boundary/interface.