Study On Preparation Of Powder Metallurgy Ti43Al9V0.3Y Alloy Sheet And Its Properties And Microstructure

Ti Al alloy as a high temperature structural material with great application prospects,owns the advantages of light weight,high specific strength and specific rigidity,and strong oxidation resistance at service temperature,etc.They have great application potentials in aerospace engine turbine blades,hypersonic aircraft thermal protection systems,automobiles and tanks compressor.However,the room temperature plasticity of Ti Al alloys is low(it is difficult for Ti Al alloys to exceed 2%at room temperature after deformation),and Ti Al alloys have high deformation resistance,severe oxidation,uneven deformation and very narrow hot working windows at the deformation temperature.These all bring great difficulties to the subsequent deformation,machining,assembly and daily use,and hinders the actual application process of Ti Al alloy.The Ti Al alloy prepared by powder metallurgy has a fine and uniform microstructure,not only has good mechanical properties,but also is very suitable for subsequent hot deformation.However,powder metallurgy also faces the problem of more impurity elements.In this paper,Ti43Al9V0.3Y pre-alloy powder was used,under the super-fast cooling rate,the Y element is over-dissolved into the Ti Al alloy powder matrix.The O element in the matrix is removed during the powder preparation and HIP.The microstructure evolution,phase composition,deformation mechanism and precipitation formation mechanism during hot isostatic sintering,hot compression and pack-rolling were systematically researched,and the relationship between microstructure and properties was studied.These studies provide theoretical and engineering guidance for the rolling of powder metallurgy Ti Al alloy sheets.After HIP at 1200?/140MPa/5h,finally,a bulk Ti Al alloy material with compact microstructure,no holes,fine and uniform grains,and excellent mechanical properties is obtained,and much of dispersed spherical nano Y₂O₃ particles are formed in the microstructure.The microstructure of the alloy is composed of(?+B2)equiaxed grains,and the average grain diameter is only 7?m.A solid phase transformation occurs during the sintering process:?₂ phase+trace B2 phase??phase+B2 phase.Moreover,the over-dissolved Y element absorbs the solid-dissolved O element in the microstructure,and finally much of nano-Y₂O₃ particles are uniformly and dispersed in the matrix.The room temperature tensile strength of the as-HIPed Ti Al alloy is 793MPa,and the elongation has reached 1.1%.The excellent mechanical properties are attributed to the precipitation enhancement of the nano-Y₂O₃ particles,the purification of the Y element and the fine and uniform microstructure.At 700?,the yield strength and tensile strength of Ti Al alloy are 589MPa and 664MPa,respectively.And at800?,the tensile strength still keeps a very high strength level,reaching 448MPa.In order to study the deformation behavior and deformation mechanism of Ti Al alloy,an isothermal compression experiment was carried out on as-HIPed Ti Al alloy,the phase transformation and microstructure evolution were analyzed,and the corresponding hot processing maps were made,these researches provide guidance for the hot pack rolling of Ti Al alloy.Due to the fine and uniform grain,the as-HIPed Ti Al alloy has a low thermal activation energy of 295.86k J/mol,indicating a good deformability.When deformed at 1200°C,The grains are obviously elongated,and the microstructure is mainly composed of?₂ phase and?phase.The formation of much of?₂ phases weakens the deformation ability.The main solid phase transformation process is:???+???₂.When deformed at 1100?,the B2 phase content increased a lot,the main solid phase transformation process is:?????.Below 1200?,the deformation mainly comes from soft?-phase grain deformation,grain boundary slip and dynamic recrystallization.At 1200?,the deformation of?-phase and?₂-phase grains,the grain boundary slip and dynamic recrystallization all contribute to the deformation.The hot processing maps with the strain of 50%and 80%were established,and the suitable hot working parameters under the 50%strain is:1150?1200?/?1s^-1 and 1000?1200?/?0.05 s^-1;the suitable hot working parameters under 80%strain is:1100?1200?/?1s^-1.The microstructure evolution and properties of Ti Al alloys at different rolling temperatures were studied,phase transformation law and deformation mechanism were found,a high plasticity Ti Al alloy sheet was obtained and the reason of high plasticity was analyzed.The microstructure of Ti Al alloy sheets is composed of?,?₂ and B2phases.During the 1100?1200?,the temperature has no obvious effect on the rolling forming ability of the alloy.As the rolling temperature increases,the B2 phase content gradually decreases,the?₂ phase content gradually increases,while the?phase content increases first and then decreases.The tensile strength at room temperature is 650MPa,the elongation reaches 3.0%.The reason for the large increase in room temperature plasticity could be attributed to:the degree of DRX is the highest,and the substructure in the microstructure is the least,thereby reducing the dislocation plugging and local stress concentration;the grain has the highest sphericity,and the spherical grain could minimize the stress concentration;a large amount of twins are formed during the rolling deformation,in the process of dislocation slip,twins can adjust the crystal orientation to facilitate the continued dislocation slip.In this way,twins and dislocations greatly increase the plasticity.Obvious texture was formed during rolling,and the texture of?phase(001)crystal plane has the greatest influence on the anisotropy of Ti Al alloy sheet.The microstructure evolution and properties of Ti Al alloy sheets under different deformation conditions are studied,and the failure mechanism of the sheet when tensile plastic deformation at 800?was analyzed.Rolling speed and pass deformation significantly affect the forming ability of Ti Al alloy sheet,too fast rolling speed and too high pass deformation will cause cracks on the surface of Ti Al alloy sheet.The rolling speed has no obvious effect on the amount of substructures and the degree of DRX,and higher passes deformation leads to a reduction of substructures and an increase in the degree of DRX.Increasing the rolling speed obviously increased the?₂ phase content.However,the deformability of?₂phase is not strong,thus it will deteriorate the forming ability.The twins formed in the?-phase and?-phase grains can promote the discontinuous dynamic recrystallization nucleation.Different rolling speed and pass deformation have little effect on the?phase(001)crystal plane texture of Ti Al alloy.However,the distribution intensity of the texture in the RD direction is very high.When tensile tested at 800?,all the Ti Al alloy sheets cracked from the grain boundary,and the cracks propagated along the grain boundary and eventually caused the sheet to fail.There is no obvious elongated deformation of the grain during the tensile deformation.It shows that the tensile process is mainly carried out through the slip of the grain boundary,and the grain boundary crack is the key factor of the material failure.