The Microstructure,Mechanical Properties And Hot Deformation Behavior Of As-extruded TiAl Alloys

With the rapid development of science and technology,the aerospace field has put forward higher requirements on the mechanical properties of structural materials.Owing to the high density of traditional Nickel-based alloys,it is urgent to find new substitute materials to replace them.Ti Al alloys are the most competitive candidate material due to their low density and high temperature strength.However,the poor plasticity of Ti Al alloys at room temperature have severely limited their production and application.To improve the poor mechanical properties,extrusion process improves the comprehensive mechanical properties not only by improving the density of the alloy,but also refining the grain size.In this work,the as-extruded alloy bars with different components of Ti-44Al-(Nb,Mo,V,Y)and Ti-46Al-(V,Cr,Ni)were studied.The microstructure,mechanical properties and the high temperature deformation behavior of both alloys were observed and determined.The optimized processing deformation parameters were obtained in order to guide the practical industrial production.The main constituent phases of both alloys were determined by XRD analysis.It was preliminarily determined that both alloys contain two phases: γ-Ti Al,α2-Ti3 Al and a small amount of β0 phase.The results of SEM microstructure analysis showed the as-extruded Ti-44Al-(Nb,Mo,V,Y)alloy possessed typical duplex microstructure which consisted of equiaxial γ grains and similar-sized α2/γ lamellae.The grain size measured about 10μm.β0 grains distributed along the grain boundaries.Besides,trace amount of Y2O3 particles were observed in matrix.The as-extruded Ti-46Al-(V,Cr,Ni)alloy possessed near lamellar microstructure,the size of lamellar colony measured between100-200μm.Equiaxial γ grains distributed between lamellar colonies,β0 grains were seen between γ grains and lamellar colonies.EBSD analysis was carried out on both alloys,the results showed that both alloys had the highest proportion of γ-Ti Al phase,which was subsequent to the content of α2-Ti3 Al phase,and the least amount of β0 phase.A large number of high angular grain boundaries(HAGB)were observed in both alloys,which proved the occurrence of dynamic recrystallization during extrusion.The results of tensile testing illustrated that both alloys possessed the highest ultimate tensile strength(UTS)at room temperature.The UTS of both alloys were736.2MPa and 756.7MPa,respectively.With the increase of temperature,the UTS ofboth alloys decreased and on the contrary,the plasticity increased.The elongation of both alloys at 850℃ were 30.8% and 9.2%,respectively.The results of fracture morphology observation demonstrated that both alloys showed the characteristics of brittle quasi cleavage fracture at room temperature.More plastic deformation was observed with the temperature increasing,dimples were observed in specimens fractograph tested at 850℃.It was inferred that the ductile-brittle transition temperatures of both alloys were between 750 ℃ and 800 ℃.The Vickers hardness testing of both alloys were carried out,the axial mean hardness of as-extruded Ti-44Al-(Nb,Mo,V,Y)and as-extruded Ti-46Al-(V,Cr,Ni)alloy were 375.71 Hv and312.49 Hv,respectively.The radial mean hardness were 357.56 Hv and 320.01 Hv,respectively.The compressive testing at room temperature was carried out,in terms of the results,the axial compressive strength of both alloys were 2679.3MPa and2016.2MPa,respectively,higher than the corresponding radial compressive strength.The hot deformation behavior of as-extruded Ti-46Al-(V,Cr,Ni)alloy was studied by thermal compression testing with the temperature range of 1020℃-1260℃,the strain rate regime of 0.001s-1-0.1s-1.The degree of deformation was 60%.The apparent activation energy of the alloy was calculated as 586.2k J/mol according to Arrhenius equation.The hot working window and processing map of the alloy at deformation degrees of 60% was obtained.With the increase of deformation temperature and the decrease of strain rate,the volume fraction and grain size of DRXed γ grains gradually increased.Taking the hot working window,processing map,effect of strain rate and deformation temperature on the microstructure into consideration,the optimized deformation condition of as-extruded Ti-46Al-(V,Cr,Ni)alloy was determined to be1180℃-1220℃/0.005s-1-0.01s-1.