Microstructure Evolution And Mechanical Properties Of Cold Rolled TB8 Sheet

TB8 titanium alloy can be used as structural components directly for the aerospace or matrix of composite materials as a kind of high strength titanium alloy due to its excellent cold formability,corrosion resistance and heat treatment processing performance.A good combination of strength,ductility and toughness of TB8 alloys can be obtained by various treatments.In the present study,effects of thermal mechanical and heat treatments on the microstructure,including texture and mechanical properties of the TB8 titanium alloys were studied systematically.The results show that grain size decreases with the increase in thickness reduction of cold rolling.Meanwhile,the textures change from initial {110}<001>and {113}<332>to a-fiber components({112}<110>,{113}<110>)and {001}<110>orientations after cold rolling to a thickness reduction of 50%.Increasing the thickness reduction to 67%,?-fiber textures are enhanced along with the appearance of ?-fiber textures.Increasing the thickness reduction further to 83%,?-fiber textures become dominated.The combined effects of fine microstructure and texture,specifically ?-fiber and ?-fiber textures,contribute to the enhancement in the tensile strength and hardness.The effects of heat treatments on microstructures and texture transformation of code-rolled TB8 code-rolled sheet with a thickness reduction of 50%were investigated.The results show that annealing for 1 hr at 750? does not induce the recrystallization transition,remaining the main texture components of {001}<110>,{113}<110>and {112}<110>.However,the precipitation of ? and ? phase occurred and the large number of dislocation substructure disappeared.These provide a synergistic effect on the alloy tensile strength and plasticity:the tensile strength firstly decreases,then increases and finally decreases again with the increasing of annealing temperature while,the ductility changes in an opposite way.The recrystallization of TB8 code-rolled sheet with a thickness reduction of 50%were analyzed after annealing at 780? for various time.Grain nucleation during recrystallization of cold rolled TB8 sheet can be depicted as subgrain merge.The recrystallization process can be described through JMAK equation,the Avirami index is 2.296.The recrystallization activation energy can be evaluated as 211.08KJ�mol-1.The recrystallization textures are affected by both oriented nucleation and oriented growth.The formation of {113}<110>texture in cold rolled sheet show obvious genetic characteristics.The recrystallization textures have a tendency to extend along {113} lattice plane.The growth of the recrystallized grains follows the Sellars model.The average value of n is 3.252 during annealing at 800?950? for 10?90 min.And the average activation energy of grain growth is evaluated as 186.56 KJ�mol-1.After the growth of recrystallized grains,the texture further extended along the {113} crystal plan to form {113}<uvw>texture.When the annealing temperature exceeds 900?,the properties of sheets are obvious decreased.The microstructures and mechanical properties of TB8 cold-rolled sheet after different solution and aging treatments were studied.Results indicated that the properties of TB8 alloy are sensitive to its microstructure.The formation of transgranular a phase is promoted by the defects induced by cold rolling at the phase transformation temperature during solution treatment.And the formed a phase provides obvious effects on the microstructure transformation during solution treatment.It is showed that both increasing the amount of a phase and refining the grains contribute the strength and plasticity.Dual aging treatments in virtue of the nucleate effect of ? phase were proposed.It can be seen that this aging treatment can remarkably refine and increase the volume fraction of the formed second phase a,which consequently increases the comprehensive mechanical properties.With the optimum processing of solution at 845? duplex aged at 400? plus 550?,the tensile strength and ductility of TB8 sheet could reach 1510MPa and 16.5%,respectively.