Superplastic Mechanical Behavior And Deformation Mechanisms For High Nb Containing TiAl Alloys

In comparison with traditional TiAl alloys,high Nb containing TiAl alloys have higher strength,superior creep resistance and oxidation resistance,whereby the service temperature range of TiAl alloys can be increased by 50¹00°C.However,high Nb containing TiAl alloys encounter problems in forming processes due to the natural brittleness which leads to high costs and impede their industrial application.Superplastic forming may be the most promising approach for hot-forming of TiAl alloys,mainly because of the enhanced deformability and flowability,reduced forming-pressure,and the negligible springback of the superplastic material,and hence components with complex shape can be fabricated by one-time processing.In the past twenty years,considerable investigations have focused on the superplastic behavior of TiAl alloys to analyze the superplastic conditions?grain size,strain rate and temperature ranges?,explore the elongation capacity,reveal the deformation kinetics,and clarify the deformation mechanisms.Great progresses have been achieved,but there still exist huge controversies in the deformation kinetics and mechanisms.Especially when it contains appreciable amount of metastable?/B2phase,the superplastic properties the alloys can be significantly improved.However,it remains an important issue to rationalize the role of?/B2 phase during superplastic deformation.In addition,there are only a few studies concerning the superplasticity of high Nb containing TiAl alloys.Correspondingly,this study is performed for the scientific issue:the effect of metastable?/B2 phase on fine structure superplasticity of high Nb containing TiAl alloys.For this purpose,two TiAl alloys with similar composition but different microstructures,namely,??₂+??alloy and??/B2+??alloy,was prepared by specifically designed thermo-mechanical processing routes.Tensile testing was performed to study the superplastic properties and deformation mechanisms.The main contents and results are as follows.In order to prepare fine and uniform microstructures suitable for superplastic deformation,uniaxial compression and stress-relaxation tests were performed to study the kinetics of dynamic recrystallization?DRX?and meta-dynamic recrystallization?MDRX?.The results showed that the microstructure of the alloys was very sensitive to intragranular deformation so that the DRX was readily onset but the kinetics was fairly slow,which was beneficial for grain refinement.Meanwhile,the slow MDRX kinetics permitted the possibility to eliminate the dead-metal zone in the forged-pancakes.In addition,it was found that when the alloys were hot-isostatic-pressed?HIPed?or forged at??+??phase field,the?/B2 phase amount was dramatically increased,namely,the?/B2 transition was effectively impeded by thermo-mechanical effect.Based on these observations the ingot breakdown procedures for high Nb containing TiAl alloys have been designed.Two pancakes with similar composition but different constituents?i.e.,??₂+??alloy and??/B2+??alloy?have been prepared for the subsequent superplastic testing.High temperature tensile tests have been performed at temperature range of850¹050°C under a strain rate of 10^-4 s^-1.It was found that both alloys exhibited optimum superplastic elongation?⁴00%?at 1000°C.However,the elongation of??/B2+??alloy exceeded 100%even at 850°C,which indicated that the?/B2 can notably decrease the superplastic temperatures.The deformation kinetics has been analyzed by using the tensile flow curves.For both alloys,the strain rate sensitivity exponents were estimated to be?0.5.The apparent deformation activation energy of??₂+??alloy was about 390 kJ/mol which was similar with that of Al self-diffusion in TiAl lattice,whereas it was about 295 kJ/mol for??/B2+??alloy.Further discussion revealed that for all TiAl alloys with??₂+??microstructures,their superplastic deformation kinetics was similar with those of ordinary disordered alloys,which implyed identical rate-controlling mechanisms.However,the deformation kinetics of?/B2-rich TiAl alloys was significantly affected by the phase proportion.SEM and EBSD were applied to study the microstructure evolution after deformation at 850°C and 1000°C.The results showed that severe DRX occurred in both alloys at 850°C,and the amount of low-angle grain boundaries were dramatically increased.Moreover,evident<100>+<111>fiber texture was formed,indicating intragranular slip and twinning were responsible for the deformation.When deformed at 1000°C,the microstructures of??₂+??alloy were fairly stable in terms of grain size,constituent and grain boundary characteristics,etc.Meanwhile,the initial forging texture was almost completely vanishing accompanying with the emergence of a very weak<100>+<111>fiber texture.These observations indicated the superplastic deformation mechanism of??₂+??alloy was intragranular deformation accommodated grain boundary sliding.For??/B2+??alloy,however,evident DRX and deformation-enhanced???₂ transition occurred at 1000°C,and apparent cube texture was formed.The superplastic deformation mechanism of??/B2+??alloy was thought to be intragranular slip-discontinous dynamic recrystallization accommodated by phase transformation.Cavities caused by superplastic deformation at 1000°C have been quantitatively evaluated by using SEM and X-ray tomography.It was found that the cavities preferentially nucleated in the triple-junctions of interfaces in??₂+??alloy,whereas most of the small cavities in??/B2+??alloy were located in the interface ledges.Quantitative analysis revealed that the alloys had comparable cavity number after deformation,but the??/B2+??alloy was characterized by larger cavity sizes,higher cavitation volume fraction and coalescence parameter in comparison with that of??₂+??alloy.The distinct cavitation behavior of the two alloys was rationalized by taken the deformation mechanisms into account.To further identify the high temperature deformation mechanisms and reveal the superplastic regime for TiAl alloys,this study has summarized and analyzed the mechanical property data of TiAl alloys from the literature published in the past twenty years.The effects of microstructure and compositions on deformation kinetics such as dislocation creep,grain boundary sliding and diffusion creep have been quantitatively evaluated.A series of unified rate-equations have been developed,as well as the deformation mechanism maps for various TiAl alloys.The proposed deformation mechanism maps were thought to be powerful tools in understanding deformation kinetics and predicting superplastic conditions for TiAl alloys.