Hot Working, Structure And Properties Of ??+?₂+B2? Multiphase Tial Alloy

TiAl-based alloys have lightweight,high modulus and elevated temperature performances,and have become a new generation of high-temperature structural material.At present,there are urgent demands of TiAl alloy in areas of aero engine high pressure compressor blades,low pressure turbine blades,tail nozzle expansion flap components and spacecraft thermal protection structures.For years,the research of TiAl alloy mainly concentrated in??+?₂?two-phase TiAl alloys.With the development of TiAl alloy,the??+?₂+B2?multiphase TiAl alloy has attracted a wide spread attention as the excellent hot deformation ability.However,the phase transition and microstructural evolution in the hot working process are complicated.The influence of B2 phase to mechanical properties of alloy was not clear,especially to ductility at room temperature?RT?was still controversial.In this paper,the multiphase Ti-44Al-5V-1Cr alloy and Ti-42Al-6V-1Cr alloy were studied.The essential aims of the present study were to explore the plastic deformation behavior and microstructural evolution;the relationship of microstructure and mechanical properties during hot deformation and heat treatment;the deformation mechanisms of nearly lamellar microstructure in multiphase TiAl alloy.The hot deformation behavior and microstructural evolution of as-cast and as-annealed Ti-44Al-5V-1Cr alloy were investigated by hot compression tests.It was indicated that the optimum hot working condition of as-cast TiAl alloy occurred in the temperature range of 1175–1225? and the strain rate range 0.05–0.1s-1.The large-size TiAl alloy rectangular bars with crack-free appearance were successfully prepared by hot extrusion.After annealing,the fine and uniform microstructure with excellent deformation ability was obtained,the average grain size was 13.7?m.The flow stress of as-annealed alloy was lower and the deformation was more uniform,the hot working window significantly increased.In the??+?+??phase region,the lamellar colonies bended and kinked heavily with the strain increased,laths coarsened and decomposed partially in lamellar colonies.Soft ? phase can promote lamellar colony sliding and grain rotating and prevent premature failure.The ?,?,? phases are all transformed by dynamic recovery and dynamic recrystallization.In the??+??phase region,the DRX of ? grains play a dominant role in controlling the flow softening and hot deformation,the existence of ? phase effectively inhibit the growth of ? grains.The effect of ? grain size on the flow stress is weak.With the Ti-44Al-5V-1Cr alloy rectangular bars for billets,using near isothermal forging and hot pack rolling technology,the large-size Ti-44Al-5V-1Cr alloy forging productions and sheets were successfully fabricated.Their sizes were 450mm×220mm×35mm and 300mm×200mm×2.7mm,respectively.The microstructure of this sheet was mainly composed of equiaxed ? grains,irregular B2 grains,and little massive ?₂.its average grain size was 3.5?m.At 950?/1×10-4s-1,the superplastic elongation reached 345%.In this paper,two types of TiAl superplastic forming parts with the application background were successfully prepared,which were of great significance to promote the application of TiAl alloy sheet.Effect of heat treatment temperatures on the microstructures and tensile properties of the as-forged Ti-44Al-5V-1Cr alloy were investigated systematically.The alloy was nearly lamellar microstructure when treated at 1230?¹310?.The heat treatment temperature had little effect on the composition of B2 phase.After treating at 1230?,the tensile strength of the alloy was 893 MPa,with the elongation 2.5%.The alloy maintains a tensile strength more than 965 MPa and the elongation 1.0%¹.2%,when heat treated in??+??phase region.The strengths decreased slowly when tensile temperatures lower than 700?,however,the strengths decreased markedly when tensile temperatures higher than 700?.Judging from the property results,the increase of the lamellar volume fraction increased the strength of the alloy.Increasing ? grains markedly decreased the tensile strength,however,increasing ductility.The Ti-42Al-6V-1Cr alloy was fabricated and an??+??phase region extrusion process was adopted to optimize its microstructure and mechanical properties.The as-extruded microstructure was consisted of fine lamellar colonies and mixtures of ? and B2 grains around the lamellar colony boundaries.The lamellar colony size and spacing of the as-cast alloy were significantly refined by hot extrusion.At room temperature,the tensile strength of the as-extruded alloy was 1113 MPa and the tensile ductility was 2.4%,demonstrating a good combination of high strength and ductility.Even up to 800?,the as-extruded alloy maintains a tensile strength more than 860 MPa.It was considered that the high lamellar fraction,fine lamellar colony size and fine lamellar spacing in the as-extruded alloy may be responsible for the high strength and improved ductility.Deformation mechanisms of the nearly lamellar microstructure in the present multiphase TiAl alloy that had been deformed at RT and high temperature were investigated.The results show that:At RT,the contribution of the ?/?₂ lamellar structure to the plastic deformation is small.The plastic deformation mainly comes from the deformation of ? and B2 grains.The <110> type dislocations were observed in B2 grains for the first time.The {111}<110> slip modes satisfy the von Mises criterion and it is a contributor to the ductility.The ? twin deformation induced dislocation emission in the B2 grain.This allowed the stress concentration at grain boundaries to be released.The evaluated hardness values were 5.3GPa,4.7GPa and 3.9GPa for the B2 phase,?/?₂ lamellar and ? phase,respectively.The deformation ability of ?,B2 and ?₂ in the multiphase TiAl alloy are in order: ? ?B2??₂.After deformed to 34% at 800?,the ?/?₂ lamellar structure was rotated and partial decomposition.Recrystallization was observed in ? and B2 grains.At 800?,three different types of dislocations were observed in B2 grain,containing <111>,<110> and <100>-type dislocations.The more slip modes are activated and this would lead to softening.No ? phase presented in B2 grains in the present V and Cr alloyed alloy.The deformation ability is better than that of Nb and Mo alloyed B2 phase.The B2 phase is shown to be beneficial to the room-temperature ductility of the present alloy.In this study,the two multiphase TiAl alloys demonstrate good combination of high strength and ductility,which show potential for engineering applications.