Microstructure And Texture Evolution Of Pure Tantalum During Asymmetrical Rolling And Subsequent Annealing Treatment

In traditional symmetrical rolling,the non-uniformity of microstructure and texture along thickness is a difficult problem to solve in the processing of BCC metal with high stacking fault energy.In severe plastic deformation mode,asymmetrical rolling(ASR)can save energy consumption during processing and impose large shear strain on the sample,thus helping to reduce the non-uniformity of the rolling structure of the sample.Because ASR is seldom used in tantalum plate,a new asymmetrical cross rolling technology is introduced to alleviate the inhomogeneity in this paper,which is represented by high purity tantalum.Two different rolling speed ratios,i.e.the rotating speed ratio of upper and lower rolls is set at 1.1 and 1.2,are adopted in ASR.In addition,in order to compare the effect of rolling deformation on the sample,another 60% and 70% of the deformation is rolled in the ASR sample.Microstructure distribution of the original sample before rolling was observed by optical microscope.Macrostructure of the rolled sample and annealed sample was measured by X-ray diffraction.Microstructure and structure of the whole thickness direction of the sample during rolling and subsequent annealing were observed by electron backscatter diffraction technology.Mechanical properties of the sample were measured by Vickers hardness value measured by hardness tester.The results show that:(1)ASR cross-rolling makes grain splitting more intense,and even makes grain splitting more serious in the central layer of the sample and its adjacent area than that in synchronous cross-rolling,so that more sliding systems are activated,which can weaken the orientation dependence of grain splitting during plastic deformation,and make the final rolled tantalum plate present {111} oriented deformed crystals.Alternate distribution of deformed grains with {100} orientation.(2)In rolling,besides the uneven orientation of deformed grains,there are usually uneven microstructures and different grain splitting behaviors in different orientations,especially in {100} oriented grains and {111} oriented grains.ASR cross rolling makes the orientation of deformed grains random,which improves the isotropy of deformed grains and reduces the difference of storage energy between grains with different orientations.(3)As for the influence of the difference of the rotational speed limit ratio of the roll in ASR on the processing effect of the sample,the grain deformation is the most serious and the difference of storage energy between grains is the smallest when the rolling speed ratio is 1.2.The homogenization effect of the orientation distribution of deformed grains is more obvious than that when the rotational speed ratio is 1.1,so it can be seen that the effect of ASR is more obvious.The homogenization effect is enhanced with the increase of linear velocity difference between rolls.(4)When the rolling speed ratio of ASR rolls is 1.2,when the rolling reduction is increased to 80%,the effect of grain breakage and splitting is more serious than that of 60% or 70% reduction.In the whole thickness direction,the difference of macro-deformation texture distribution between each thickness layer is smaller,so ASR cross-rolling makes grain splitting and texture distribution uniformity between thickness layers.With the increase of the reduction amount,it is obvious.(5)The grain orientation of {100} in synchronous cross rolling has a weak tendency of nucleation and recrystallization.During annealing,"texture inheritance" phenomenon occurs.The deformed grain shows in the annealed sample in the form of residual deformation band.The grain storage of {111} orientation can occur recrystallization faster and higher,which results in the serious inhomogeneity of grain size and orientation distribution in the final recrystallization structure.In ASR cross rolling,the grain splitting is more thorough and the slip system is more active,and the deformation structure with random grain orientation distribution is obtained.Therefore,the fine recrystallization structure with uniform grain orientation and random grain orientation can be obtained after annealing.