【作者】 邓超；

【导师】 刘庆； 刘施峰；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2014， 博士

【摘要】 钽作为一种高熔点难熔金属，具有独特的物理与化学性质，在多种工业领域获得了广泛应用，被称为“金属王国”里的后起之秀，但是关于钽的基础科学研究相对落后。本文结合X射线衍射（XRD）、背散射电子衍射（EBSD）、差示扫描量热法（DSC）以及透射电子显微技术（TEM），对高纯多晶钽板在两种轧制方式下（单向轧制与周向轧制）的组织演变规律与再结晶织构形成机理做了系统研究，得到了以下主要结论：①锻造退火态钽板展现出极大的组织不均匀性。这种不均匀性主要体现在钽板表面层晶粒尺寸远大于中心层的晶粒尺寸，个别晶粒呈现出异常长大形态，尺寸达到了数百微米甚至几个毫米。另外，钽板不同厚度层具有很大的织构差异，表面层含有强烈的{100}织构，而中间层具有强烈的{111}织构。以上组织的不均匀性对材料的使用性能具有不利影响。②两种轧制方式下钽板具有不一样的织构演变规律。经过70%变形后，单向轧制钽板中含有较强θ纤维织构与γ纤维织构，另外还形成了中等强度的不完整α纤维织构。随着轧制变形量的增加，γ纤维织构的强度逐渐增大，而θ与α纤维织构的强度保持相对稳定。钽板在周向轧制过程中，仅含有θ与γ两种纤维织构。这两种织构的强度在轧制过程中均不稳定，随着轧制道次的改变成周期性变化。由于轧制方向的连续改变，钽板中不再含有α纤维织构。两种终轧道次钽板（87%变形量）均呈现出一定的织构梯度，这种织构梯度的存在主要归因于钽板初始组织存在极大的不均匀性。对于周向轧制钽板，这种织构梯度主要表现在，表面层具有强烈的θ织构与很弱的γ织构，其中γ织构的强度随着离样品表面距离的增加而逐渐增强，而θ织构的强度随着位置的改变没有明显变化。③两种轧制方式下晶粒分裂行为均显示出取向相关性，其中{111}晶粒的分裂程度要高于{100}晶粒的分裂程度。{111}晶粒在单向轧制中容易呈现出平行的变形带组织，内部具有大的平均取向差，并含有较高的储存能。而{100}晶粒在变形过程中显得非常稳定，内部具有非常小的平均取向差，并含有较低的储存能。周向轧制对这两种晶粒的分裂行为产生了较大的影响，一方面部分地消除了{111}晶粒中的平行变形带组织，另一方面使部分{100}晶粒产生了较大的分裂。因此周向轧制明显缩小了这两种晶粒之间的储存能差异，提高了变形组织的均匀性。对位错结构的透射电镜观察表明，周向轧制钽板大多数晶粒具有位错胞结构，而单向轧制钽板大多数晶粒显示出平行的层状位错界面结构。④采用三束离子束抛光技术，并结合高分辨场发射电镜上背散射电子衍射系统，可以对变形钽板内部的真实显微结构进行表征，所获得的花样质量成像图接近于透射电镜照片。不仅可以对显微结构参数，诸如位错界面间距、界面取向差等做定量的统计分析，还可以获得晶体取向与储存能之间的相互关系。⑤周向轧制钽板的再结晶行为受轧制道次与退火条件的影响较大。轧制道次的增加会增加轧制织构中的{100}组分，这种组分的存在可以抑制优先形核{111}晶粒的长大，有助于细化钽板的退火晶粒结构。钽板的退火处理不适宜在低温情况下进行，低温退火很难激活{100}变形晶粒中的再结晶行为，不利于消除退火组织中的变形带。退火处理也不适宜在高温长时间条件下进行，这容易引起晶粒快速长大并粗化，不过高温短时退火可以同时激活{111}与{100}晶粒的再结晶行为。另外，在退火过程中进行回复处理对能够减小优先形核晶粒的长大速率，对细化晶粒是有利的。与单向轧制相比，周向轧制有助于消除退火样品中的变形带组织。⑥周向轧制钽板的再结晶织构类型与轧制织构类型相同，为{111}与{100}混合织构，不过{111}织构要强，并且高温退火有助于加强这种织构。由于不同晶界显微结构的巨大差异，导致晶界类型对形核行为具有重要影响，其中γ-θ晶界位置为择优形核点，这个位置的形核源于亚晶长大机制，主要产生{111}与{100}两种晶粒取向，而θ-θ晶界位置为第二形核点，这个位置仅产生{100}取向晶核。强烈{111}织构的形成源于{111}再结晶晶粒的生长优势，在再结晶过程中，{111}晶粒的平均尺寸始终要大于{100}晶粒的平均尺寸。这两种晶粒的尺寸差异由晶粒生长过程中的取向钉扎产生。在生长过程中，由于轧制织构中具有更强的{100}取向，因此{100}晶粒与{100}变形基体形成小角度界面的几率更大，受到取向钉扎的影响更为明显。更多还原

【Abstract】 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”.更多还原