| Titanium alloys are widely used in aerospace,biomedical,ocean exploration and other fields for their good corrosion resistance,high strength to weight ratio,high temperature performance,and good biocompatibility.During the hot deformation of titanium alloy in?+?phase region,a dynamic transition(DT)from?phase to?phase occurs,resulting in a decrease in the volume fraction of?phase and flow softening.The phase transformations that occur during the holding process between passes in the isothermal and non-isothermal multi-pass hot deformation has an important impact on the final microstructures and properties of the titanium alloys as well.However,there is still limited research on DT of titanium alloys at home and abroad,which lags behind the researches of steel in this field.In this paper,typical near-?titanium alloys Ti-5Al-2.5Sn-0.5Fe,?+?titanium alloys Ti-6Al-4V and Ti-5Al-3V are investigated by single-pass,isothermal and non-isothermal multi-pass hot deformation tests,combining advanced SEM,EBSD and TEM to reveal the microstructures and phase orientations relationships of primary?phase,martensite??phase,Widmanst?tten?phase,FCC-Ti,etc.to reveal the microstructural evolution and its mechanism of the alloy during hot deformation and holding stage,and to investigate the effect of strain,temperature,primary?phase grain size,etc on phase transformation.In addition,the phase reconstruction method based on the Burgers relationship,expansion and stress relaxation curves,and the thermodynamic analysis are used to study the microstructural evolution mechanism of Ti alloys.The flow stress constitutive model of the alloys in the?single-phase region and?+?phase region respectively are established as well.The main results are as follows:(1)The DT takes place during compression testing at 940 oC?970 oC?1000 oC and the reverse transformation(RT)during isthermal holding after single pass hot deformation.On an isothermal multi-pass hot deformation,the flow stress decreases with increasing deformation passes,indicating an obvious flow softening.After holding process,the peak stress of the latter pass is higher than the stress of the end of the previous pass,which shows a static hardening occurring during this process.Microstructural evolution shows DT and?-phase dynamic coarsening occurres during straining.The flow hardening during cooling process is due to the increase in flow stress when temperature decresing,the increase in the?phase volume fraction and grain size.The time-temperature-transformation(TTT)curves shows that the RT percentage increment changes little as increasing temperature when holding time is short,while prolonging the holding time,increasing the temperature significantly enhences the RT process.As the holding time increases,the RT process gradually slows down,which is attributed to the storage energy accumulated by hot deformation and the driving force of the RT gradually decreases with the increase of holding time.The RT driving force is the difference in chemical Gibbs free energy between metastable?phase and its equilibrium state,which is estimated to be 36.9 J/mol?43.5 J/mol?9.0 J/mol at 940 oC,970 oC,and 1000 oC,respectively.(2)The effect of primary?grain size on the flow softening and DT of Ti-6Al-4V alloy was investigated.The alloys were heat treated at 960 oC for 12 h to coarsen primary?grains to about 10?m,which have more obvious flow softening during hot deformation than fine-grained alloys(about 3?m)attributed to an accelerated DT.Full?microstructure was observed at strain of 1.2 in coarse grain(CG)samples while retained?phase was observed in fine grain(FG)ones.The accelerated DT in CG samples was attributed to more deformation accommodation and higher DT driving forces.Phase reconstruction showed that DT was more likely to occur where primary?phase and neighboring?phase owns a close Burgers orientation relationship.Widmanst?tten?phase was observed in deformed alloys in a low strain which was reversely transformed from metastable?phase during short holding prior water quenching.By establishing the strain-temperature-transformation(STT)figure,it reveals that the percentage of DT increases rapidly under high temperature and high strain level.(3)It reveals the flow stress and microstructural evolution of Ti-5Al-3V alloy in the?single-phase(960 oC)and?+?phase region(900 oC,930 oC)during the two-pass hot deformation process at strain rate of 0.01 s-1.When deformed in the?+?phase region,the peak stress of the second pass is higher than the stress at the end of the first pass,while deformed above transus,second-pass stress is almost the same as that in first-pass,and flow stress kept constant after reloading.According to the dilatometer curve,the stress relaxation curve and the quantitative study by SEM,a post-dynamic?to?phase transformation was first indicated during the early stage of isothermal holding.With an increase in isothermally holding(above?50 s),the RT of meta-stable?phase to?phase was observed.Observed under TEM,the evolution of primary?phase,martensite?'phase and Widmanst?tten?phase in the alloy during hot deformation and cooling is studyed.In the Widmanst?tten?colony,lath?and lath?'shares a coherent interface.Thermodynamic analysis shows that the post-dynamic transformation from?to?phase is deviating form equilibrium,driven by the stored energy of in the first pass and stress activation,where stress activation plays a more important role.(4)Compared with the?+?titanium alloy,the DT from?phase to?phase in a narrower temperature range can observed in the Ti-5Al-2.5Sn-0.5Fe near-?titanium alloy.During the isothermal multi-pass hot deformation in?+?phase region,the flow stress of the alloys gradually decreases when straining,and obvious flow softening occurs,while in the?phase region the flow stress changes little.The constitutive models of hot deformation in?single-phase region(1020?1050 oC)and two phase region(900?990 oC)with strain rate of 0.001?1 s-1 during isothermal and non-isothermal multi-pass deformation are established respectively.The activation energies of deformation in the two-phase region of isothermal and non-isothermal multi-pass deformation are 963.0 k J/mol and 873.6 k J/mol,respectively.The non-isothermal multi-pass deformation starts from the?single-phase region,when cooling to two phase region,coarse?+?Widmanst?tten?colonies are formed.Compared with the equiaxed grain microstructures after isothermal multi-pass deformation,there are more?/?phase boundary in Widmanst?tten?colonies,which promotes grain boundary sliding and is more conducive to deformation.In addition,from KAM and GOS figures,less substructrures and recrystallization grains are observed in the samples after non-isothermal multi-passes deformation.An obvious DT occurs during isothermal hot deformation at 990 oC,resulting in a flow softening.After the strain level of higher than 0.6,the flow stress is close to that in the?single-phase region.TEM observation reveals that the DT mechanisms of the alloy at this temperature are displasive nucleus and diffusion controlled growth.The phase transformation from HCP-Ti to FCC-Ti was observed by TEM during deformation with strain level of 1.2on 960 oC and strain rate of 1 s-1,with(0 0 0 1)HCP//{1 1 1}FCC orientation relationship. |
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