Study On Rolling Deformation And Annealing Behavior Of High Purity Polycrystalline Tantalum

Tantalum (Ta) is a refractory metal with bcc structure. Due to unique properties, ithas been widely used in many fields, such as electronics industry, cutting-tool industry,chemistry industry, medical and military fields. Unfortunately, its fundamental studiesdrop behind. In this thesis, the deformation under two rolling ways (unidirectionalrolling and clock rolling) and subsequent annealing behavior of high puritypolycrystalline Ta were studied by multiple characterization methods, such as X-raydiffraction (XRD), electron backscattered diffraction (EBSD), transmission electronmicroscope (TEM), and differential scanning calorimetry (DSC). According to thisstudy, the following results and conclusions can be drawn:①The forged and annealed Ta plate displayed extreme heterogeneity inmicrostructure. The heterogeneity was in that the grain size in the surface layer wasmuch bigger than that of the center layer. Some grains owned millimeter size andexhibited abnormal growth. In addition, the texture in the surface layer was dominatedby strong {100} component compared to the intense {111} component in the centerlayer. The above heterogeneity would have deleterious effect on material performance.②Different texture evolutions were under different rolling ways. After70%deformation, strong θ-and γ-fiber as well as moderate α-fiber would be produced inunidirectional rolling. With increased rolling deformation, the intensity of the γ-fiberwas enhanced, while kept stable in θ-and α-fiber. Duing clock rolling, only θ-andγ-fiber could be found and the intensity of the two textures varied with rolling passes.The disappearance of the α-fiber was attributed to the continuous change in rollingdirection. Both rolling methods led to a through-thickness texture gradient in Ta platesand this was caused by heterogenous microstructure and texture in the staring materials.As for clock rolling with87%thickness reduction, the intensity of γ-fiber increasedgradually from the surface layer to the center position.③Orientation dependence during rolling was common in deformationmicrostructure. Compared to unidirectional rolling, the extent of orientation dependencecould be eased by clock rolling.{111} grains underwent severe deformation duringrolling and would accommodate high density dislocation. This type of orientation oftendisplayed parallel deformation bands or GNBs after heavy unidirectional rolling.{100}grains were stable during deformation, and this type of orientation did not subdivide since internal misorientations were very small. Clock rolling, on one hand, cleared upparallel deformation bands or GNBs in {111} grains; on the other hand, subdivision of{100} grains was enhanced.④Triple focused ion beam polishing, in combination with EBSD system in highresolution field emission scanning electron microscopy, would be very useful inrevealing real deformation microstructure of Ta.⑤Rolling pass and annealing temperature had great effect on recrystallizationbehavior of clock-rolled Ta. Increasing rolling pass would enhance {100} rolling texture,which is beneficial to inhibit the growth of nuclei. Heat treatment of clock-rolled Tashould not be done under very low (950℃)or high (1300℃) temperatures, as annealingduring low temperature was hard to eliminate deformed bands, whereas hightemperature would lead to grain coarsening. However, high temperature annealing couldactivate nucleation in {100} and {111} deformed bands simultaneously. Recoveryreleased the most stored energy of deformed Ta and would slowd down the grain growthduring subsequent recrystallization. Compared to unidirectional rolling, clock rollingwas relatively helpful for eliminating deformed bands.⑥The recrystallization texture of clock-rolled Ta was the same as the rollingtexture, consisting of {111} and {100} components. The {111} component would bestrengthened under high annealing temperature. Nucleation preferred to occur in γ-θgrain boundries. This type of boundary owned subgrain substructure and the nucleationwas caused by subgrain growth. The nucleation in θ-θ grain boundaries could beactivated under relative high temperatures or needed to undergo a long embryo timebefore recrystallization. The intense {111} recrystallization texture was attributed to thebig size in {111} recrystallized grains, since during the whole recrystlliztion process,the mean size of {100} grains was always samller than that of {111} grains. This sizedifference could be explained by “orientation pinning”.