Study On Microstructure,Properties And Hot Deformation Behavior Of Tialbased Alloy Fabricated By Vacuum Hotpressing Sintering

With the further demanding conditions for“lighter and stronger”materials in the field of automotive and aircraft engines,TiAl-based alloys can partial replace high density components,such as Nickel-based superalloys,due to low density,high specific strength,high specific stiffness and excellent elevated temperature performance.However,low room temperature plasticity and poor hot workability limit the wide engineering applications of TiAl-based alloys.In the paper,powder metallurgy and hot deformation processing were made to control and refine the microstructure of TiAl-based alloys,in order to improve mechanical properties.TiAl pre-alloy powders with a nominal composition of Ti-43Al-4Nb-1Mo-0.1B?at.%?were used by vacuum hot-pressing sintering to fabricate alloy billets at different temperatures.The effect of temperatures on microstructures and properties was investigated.Sintering temperatures were at 1200,1275,1300 and 1325 ^?,near-gamma and duplex microstructures with average grain sizes of 3 and 5?m,and near lamellar and full lamellar microstructures with average colony sizes of 70 and 120?m were obtained,respectively.During the sintering process,the metastable?₂ phase in the original pre-alloy powder was transformed to the equilibrium?phase.As the temperature rose to?single-phase region,several?phase was transformed to?.In subsequent cooling,?phase was precipitated in?phase to form?₂/?lamellar due to eutectoid reaction.For above four kinds of microstructures at different temepratures,the engineering strains were higher than 30%in room temperature compressive test,but the elongations were only 0.2⁰.34%in room temperature tensile test.While the ductility of near lamellar microstructure was increased to 5%,and the ultimate tensile strength was increased to 500 MPa at 800 ^?.The hot deformation behavior of TiAl-based alloy sintered at 1300 ^? was studied by means of thermal simulation tests at the temperature of 1050 to 1250 ^?,the strain rate of 0.01 to 0.5 s^-1,and the true stress of 0.7.The results showed that the flow stress increased with decreasing of deformation temperature and increasing of strain rate.The activation energy of sintered TiAl-based alloy was 664.75 k J/mol.It was found that lamellar colonies were bent and twisted,some of lamellar colonies were broken.The orientation of numerals lamellar structures was perpendicular to the compression direction.With the deformation temperature rising,the content of B2???and?phase decreased.When deformed at 1240 ^?,a number of dynamic recrystallization?grains appeared and lamellar spacing was refined to less than 20 nm with the strain rate increasing.The work hardening was caused by the increase of dislocation density.The softening mechanism was dynamic recrystallization,bent and twisted lamellar,and phase transformation.Combining the results of thermal simulation tests,the hot extrusion temperature of TiAl-based alloys was determined to be 1250 ^?.The microstructure evolution and mechanical properties of extruded alloy were researched.The microstructure was composed of fine lamellar colony and?phase with linear distribution of flow.The?₂phase formed<10-10>?₂?ED and<2-1-10>?₂?ED related with strongly plastic deformation.The?phase exhibited<110>??ED texture due to the orientation relationship with?₂ in the lamellar structure and<211>??ED recrystallization texture.Comparing the microstructure and mechanical properties of extruded TiAl-based alloys after heat treatment,it was indicated that the equiaxed?and?₂ grains resulting from static recrystallization.With the temperature increased to1200 ^?,lamellar structure appeared.The microhardness of alloy was reduced due to the release of internal stress caused by air cooling after heat treatment.At 1100 and 1200^?,the engineering strains were improved to 35%.After 900 ^? heat treatment,the deformation texture still remained in the microstructure.Since the slip system direction of the?₂ phase was parallel to the tensile stress,the dislocations were easy to move and the room temperature elongation increased to 1.37%.