| In this work,the metastable?-titanium alloy(Ti-5Al-5Mo-5V-3Cr-1Fe)bar was selected as the research object,and the hot compression tests were conducted by a Gleeble-3800 thermal-mechanical simulator.Multiple characterization and analysis techniques including XRD,EBSD,ECC,EDS and hardness measurement were jointly utilized to comprehensively investigate microstructural characteristics and mechanical properties of the as-received specimens,heat-treated specimens and hot-deformed specimens.The causes of microstructure evolution of hot-forged specimen,the effect of cooling rate on microstructure evolution and hardness change of hot-forged Ti-55531Fe alloy in dual-phase region and the hot deformation behavior and microstructure evolution mechanism of hot-forged Ti-55531Fe alloy in dual-phase region and?-phase region were systematically studied.Main conclusions are obtained as follows:(1)The microstructure of the Ti-55531Fe alloy after forging in the dual-phase region(a strain of 54%at 820?)and air cooling consists of bulk?grains,fine?plates and?matrix.Among them,the bulk?grains are primary?phases(?p,average grain size?2.4?m),while the fine?plates are secondary?phase(?s,average width?70 nm)precipitated from the?matrix during air cooling.And during the forging process,the?matrix experiences dynamic recovery with the formation of many subgrains(87.8%of all boundaries belonging to low angle boundaries)and significant orientation gradients.The?s phase is found to well maintain the Burgers orientation relationship with the?phase,while this relationship is lost between some?p and?phase,which should be related to thermal deformation-induced changes of their orientations.(2)Microstructural characteristics of a typically forged Ti-55531Fe metastable?Ti alloy after?-solution treatments(at 890? for 10 min)and cooled at different rates are clarified.Both the WC and the AC specimens are mainly composed of recrystallized?grains with no?phase,while the FC specimen consists of coarse equiaxed?grains,intragranular acicular?s phase(width?48 nm)and grain boundary?GB phase.The Burgers orientation relationship is strictly obeyed during???transformation in the FC specimen.When adjacent?grains share a common?110?axis,the?GB phase precipitated at their boundaries tends to maintain such orientation relationship with both of them.Hardness analyses show that compared with the as-received specimen(428±9 HV),the WC(309±5 HV)and the AC specimens(297±3HV)are softer,which can be attributed to the coarsened?grains and the disappearance of?phase and deformed structures.For the FC specimen,a large number of fine needle?phases precipitate inside?grains during slow cooling,leading to increased hardness(444±9 HV).(3)The softening mechanisms of Ti-55531Fe alloy in the hot deformation process in dual-phase region(at 820? and strain rate 0.01 s-1)mainly includes dynamic globularization of?phase and dynamic recovery of?phase.With the strain increases,a small amount of discontinuous dynamic recrystallization and geometric dynamic recrystallization will occur in the?phase in turn.While the softening mechanisms in the hot deformation process in?-phase region(at 910? and strain rate 0.01 s-1)are mainly dynamic recovery and progressive lattice rotation dynamic recrystallization of?phase.(4)The constitutive equations of hot deformation of metastable?Ti alloy Ti-55531Fe in different phase regions are constructed in the range of temperature 820?910? and strain rate 0.01?10 s-1 by the calculation and analysis of the hot compression test data.Based on the dynamic material model and the corresponding instability criteria,the thermal processing map(a true strain of 0.9)of the metastable?Ti alloy Ti-55531Fe was made in the range of deformation temperature 820? to 910? and strain rate 0.01 s-1 to 10 s-1,and the optimal processing parameters was determined as the deformation temperature 820?910? and strain rate 0.01?0.22 s-1.Through microstructure analysis,the instability area under high strain rate is mainly due to the flow localization is easy to occur and the residual strain energy is difficult to release. |
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