Microstructure Evolution And Mechanical Properties Of Hot-rolled Ti₂AlNb Prepared By Powder Metallurgy

Ti₂AlNb alloys have been increasing used in aerospace field because of its low density,high specific strength and good creep resistance.Ti₂AlNb alloy sheets prepared by traditional process often have defects such as composition segregation and uneven microstructure.Also,the preparation cycle is long and the efficiency is low.In this work,we prepared the compacts by spark plasma sintering the pre-alloyed powders,and then hot packed rolled the compact to fabriacte Ti₂AlNb alloy sheets in different phase zones using different thickness reductions,trying to obtain Ti₂AlNb alloy sheets with good mechanical properties.Such method has the advantages of short process and low cost.The effects of rolling temperature and rolling process on the microstructure evolution and mechanical properties of the sheet were mainly studied.Ti₂AlNb pre-alloyed powders were first sintered by spark plasma sintering to fabricate raw compact.It was found that the density of the compact sintered at1060°C and 50 MPa was close to the total density of Ti₂AlNb powders.The B2 phase grains in the sintered compact had an average grain size of 25.9μm.Some O phases,precipitated during the cooling process,were observed inside the B2 phase.The phase zones of target rolling temperatures were determined by SEM and XRD.The results showed that the temperature of 850°C located at the O+B2 phase field;the temperature of 950°C located at theα₂+O+B2 phase field;the temperature of 1030°C located at the single B2 phase field.Cracking was observed at the edge of packes when rolled in single B2 phase field.The B2 grain sizes increased and they showed elonged morphology and curved boundaries with the increase of rolling passes.Both high strengths and good ductility were obtained when the sheets were rolled at single B2 phase field.The tensile strengths of the sheets were all higher than 1100 MPa.The elongation of the sheet prepared by four-pass hot rolling was 9.5%.High-temperature tensile test result showed that the tensile strength and elongation of the sheet rolled by three passes were931.4 MPa and 3.4%at 650°C.The low ductility can be attributed to noα₂ phase was precipitated at the grain boundaries,which is easy to break at high temperature,resulting in intergranular fracture of the sheet.Cracking was observed at the edge of the pack when rolled at the O+B2two-phase field.Multi-pass rolling process reduced the surface roughness of sheet.The most of the O phases in the sheet rolled using two passes were acicular.The O phases in the sheet gradually spherodized and its content gradually increased with the increase of rolling passes.The O phases in the sheet rolled by four passes are mostly spherical.The tensile experiment showed that the strengths of the sheet prepared at O+B2 phase field were all higher than 1400 MPa.The tensile strength of the sheet rolled using two passes was 1540.7±24.7 MPa.The tensile strength of the sheet decreased with the increase in rolling passes.However,the ductilities of the rolled sheet in O+B2 two-phase field were poor,and the elongations of the three sheet rolled by different schemes were less than 3%.The high temperature tensile test showed that the tensile strengths of the sheet were low,but the elongations were higher than 70%.Cracking was only observed at the out edge of the pack rolled using two passes at theα₂+O+B2 three-phase field.The B2 grain size increased with increasing the rolling passes.The sheets rolled atα₂+O+B2 three-phase field had high strength and good ductility,this was mainly becasue the coexistence of dispersed nano-size O phase and micron spherical O phase,which produced synergistic effect of strengthening and plasticizing.The tensile strengths of the sheet fabricated atα₂+O+B2 three-phase field were higher than 1050 MPa with elongations higher than 9%.The elongation of the sheet prepared by four passes was 13.9±1.6%.The high temperature tensile test showed that the tensile strength and elongation of the sheet rolled by three passes were956.6 MPa and 7.7%at 650°C.