Study On Phase Transformation Behavior Of Ti-6Al-4V Alloy And Its Strengthening Mechanism Via Electropulsing Treatment

Ti-6Al-4V alloy has excellent comprehensive mechanical properties,that is the most widely used band in titanium products.Therefore,the thesis mainly study the phase transformation law during electropulsing for Ti-6Al-4V.SEM,EBSD,TKD,TEM,etc were used to reveal the microstructure evolution and relation between microstructures and mechanical properties.The optimal microstructural design was also carried out based on the discovered results to further improve its strength and plasticity.At the same time,this work also explored the unique role of electric current in order to provide the basis of application and promotion in experimental and theoretical aspects.The main research findings are as follows:(1)The mechanism of acceleration of element diffusion and phase transformation process via electropulsing has been revealed.Theβ-transus temperature reduces by～50℃comparing with the equilibrium heat treatment at a current density of 1.8×10~7A/m~2and heating rate of 300℃/s,and the alloy elements diffused evenly within 3.2 seconds.The study has found that the continuous current can introduceωand O′nanodomains inβphase.The lattice distortion caused by nanodomains reduces the V element diffusion activation energy inβphase,thus accelerates the process of phase transformation.While the effect of electromigration and electron wind force is limited.The microstructural evolution is almost the same when the heating rate increasing to 1500℃/s.While the the current duration time reduces to 680 ms,resulting in theβ-transus temperature reduces by～20℃comparing with the equilibrium heat treatment.In addition,theβphase that containing nanodomains will transform to nano-martensiteα′′during quenching,because the nanodomains can limit the growth of martensite.Nanodomains only exist within a certain temperature range and gradually disappear.The quenched microstructure is micro-martensiteα′if beyond that range.(2)The reverse massive transformation that is independent of alloy elements diffusion has achieved under heating condition.Theα→βtransformation is complete at～980℃when the heating rate exceeds a critical value(2300℃/s),while there is almost no long-range diffusion of alloy elements during the transformation process.A novelα′+βdual phase microstructure has formed after quenching.The element composition of martensiteα′is almost same as primaryα,the preexistingβhas be left.Al-enrichedα′+βdual phase microstructure exhibits good comprehensive mechanical properties.The yield strength is～960 MPa,the tensile strength is～1180 MPa and the total elongation is～20.5%.This treatment separates the phase transformation from the elements diffusion.(3)According to the reverse massive transformation and formation of nanodomains inβphase during electropulsing in Ti-6Al-4V alloy,the bimodal structure(α_p+α′)was subjected to the electropulsing treatment.After electropulsing,bimodal martensite structure with different composition and grain size was formed(Al-enriched coarseα′and V-enriched fineα′).After aging,the yield strength is～1289 MPa,tensile strength reaches to～1387 MPa,and the total elongation is～14.3%.The high strength-plasticity match is result from the combined effect of solid solution strengthening,fine grain strengthening,precipitation strengthening,and heterogeneous deformation induced strengthening.(4)High-voltage electroshock could achieve direct recrystallization of priorβwith coarse-grain martensite of Ti-6Al-4V.It was found that the temperature rise was only～600℃during 400μs at 7 k V.The coarse priorβgrain with many subgrain boundaries recrystallized and the the grain size decreased.The texture intensity also reduced.Therefore,the grain size of martensite at room temperature also decreased after quenching.Consequently,the yield strength of Ti-6Al-4V alloy increased from～1049 MPa to～1116 MPa,the tensile strength increased from～1196 MPa to～1259MPa,and the total elongation increased from～4.7%to～10.4%after electroshock.(5)A new mechanism of current treatment with low parameter on plasticity of Ti-6Al-4V dual phase microstructure was elucidated.It was found that the strength and plasticity of Ti-6Al-4V with dual-phase microstructure decrease simultaneously after electroshock(～600℃,400μs).This is because that onlyωphase generated in theβphase.ωembrittle theβphase,making it becomes the source of microcracks.The fracture dimple shows a tear characteristic.When it subjected by the electropulse heating to 800℃(540 ms),the spinodal decomposition and small amount of O′phase were also formed exceptωphase inβphase.A uniform strain field formed inβphase due to the spinodal decomposition,making its plastic deformation becomes more uniform during tensile and it can better coordinate the plastic deformation of surroundingαgrains.The fracture dimple presents a shear characteristic with good plasticity.