金属塑性变形过程中位错组态化和位错界面形成的起源研究

The origins of dislocation patterning, by which dislocations begin to arrange into boundaries during the very early stages of plastic deformation, remains a key unsolved question in the area of dislocation plasticity. In this study it this fundamental question will be addressed using a combination of carefully chosen experimental samples and by use of new advanced experimental techniques. The research work will focus on three aspects of dislocation boundary formation: (i) the condensation of dislocations into planar walls in orientations aligned for single slip deformation; (ii) the transition from cell to cell block microstructures during uniaxial testing of samples with loading directions near the [001] corner of the unit triangle, and (iii) the effect of grain size on the early stages of boundary formation, using samples prepared by a novel spark plasma sintering route, where grains in fully recrystallized condition with near- and sub-micrometer grain size can be produced. Experiments will be carried out primarily on single and polycrystalline samples of Al, Cu, and Ni, to take advantage of existing knowledge about the types of deformation microstructure formed in these metals and as they provide a range of both stacking fault energy and room temperature 3D-dislocation mobility. Studies of the formation of planar walls will be carried out using a novel approach combining bending followed by tensile deformation of coarse (cm-scale) grain samples. The resulting strain gradient allows systematic study of dislocation patterning as a function of initial dislocation density. In order to collect the required experimental data use will be made of advanced experimental techniques including high angular resolution (cross-correlation) EBSD to measure pattern of crystal rotations over larger areas than possible in TEM studies, and digital image correlation method to measure local plastic strains, so that quantitative relationships between plastic strain and dislocation pattering can be made in the very early stages of plastic deformation. Detailed TEM measurements to verify the EBSD data and to identify dislocation Burgers vectors and dislocation arrangements will be also be made as part of the study. The results of the study will provide data on dislocation patterning of previously unavailable accuracy, and as such will form a basis for the understanding of this important process and for the development of dislocation dynamics models of dislocation patterning.

位错组态化的起源，是位错在塑性变形初期演变成界面的必经阶段，目前仍是位错塑性领域一个未解决的核心问题。本项目将选择合适样品和使用新的先进技术来解决这一基础问题。研究将集中在位错界面形成的三个方面：（1）单滑移变形中位错演变成平面位错墙的过程；（2）沿样品近[001]取向加载的单轴变形中位错胞结构向胞块结构的转变；（3）晶粒尺寸对位错界面形成的影响。实验将采用单晶和多晶的铝、镍和铜样品来开展。关于初始位错密度对平面位错墙形成的影响，将通过对粗晶样品进行先弯曲后拉伸的方法来研究。本项目将采用高角分辨率的交互电子背散射衍射进行大区域的取向测定，使用数字图像相关方法测量局部应变，从而建立塑性变形初期应变和位错组态化间的定量关系。同时也会使用透射电镜进行位错排布和柏氏矢量的测定。本项目将会得到十分详细而又准确的关于位错组态化的数据，从而为理解位错组态化起源和发展其位错动力学模型奠定基础。

本项目研究了塑性变形过程中的位错组态的起源和晶粒分裂演化过程。主要采用了电子通道衬度成像（ECCI）和高角度分辨率电子背散射衍射（HAR-EBSD）两种手段来原位直接观测低应变量下位错界面的形成过程。为了评估HAR-EBSD的实际角分辨率和其应用于小取向差界面表征的可行性，我们通过在TEM和EBSD中对10%变形Al样品的同一区域进行了原位组织观察：首先在TEM对样品进行倾转以观察位错界面，然后使用高灵敏度固态EBSD检测器和CC-EBSD或高角度分辨率索引方法对样品进行观测。结果表明，只有取向差角> 0.1°的位错边界才能被可靠地成像，这表明这些方法具备研究位错界面起源和转变的潜力和局限性。通过精确倾转样品来实现控制衍射条件的ECCI方法同样可以被用于研究低应变量下变性组织的定性观察。为研究位错胞块界面（GNBs）的起源，选取一组取向、尺寸相近、预期会获得GNBs的晶粒，结合HAR-EBSD对其进行了研究。结果表明，较小的初始取向变化并不会对后续变形过程中位错界面的产生和晶粒分裂产生影响；晶粒内部形成的GNBs与滑移方向极不一致，这与以往的GNBs只会偏离滑移面较小角度的认识向左。.塑性变形的原位研究的另一个挑战是同时通过数字图像相关技术（DIC）测量局部塑性应变，和采集EBSD数据通常是不可能的，这是因为常用的表面处理方法会严重降低EBSD信号。为了解决这个问题，项目开发了一种基于电解抛光的新表面处理方法，该方法可用于大范围塑性应变下同时进行EBSD和DIC测量。使用该技术对Al进行的原位变形研究揭示了局部塑性晶体旋转（由EBSD确定）和塑性应变（由位移梯度张量确定）存在复杂的相关性。同时，项目还采用具有0.01°角分辨率的基于劳厄微衍射的3D XRD同步辐射方法对极低塑性应变下的晶粒分裂进行了研究。针对该方法，我们开发了一种基于互相关技术的原位实验样品位置修正方法，可实现准确度优于1 μm的位置校准。 通过该技术，观察到晶粒的变形存在显着的不均匀性，这种不均匀性对晶体取向的依赖性强于晶粒尺寸。