Formation and time-evolution of three-dimensional long-range ordered domains from short-range ordered states

该项目旨在分析纳米尺度合金中有序晶域的三维 (3D) 形貌。为此，我们利用透射电子显微镜（TEM）的计算机断层扫描程序开发了一种新的实验技术，即电子断层扫描。迄今为止，电子断层扫描的应用主要限于生物或非晶体材料，因为图像强度应满足作为成像对象的质量密度和厚度的单调函数给出的简单投影关系。目前，扫描TEM中的高角散射非相干成像有时用于晶体材料，以抑制违反成像投影要求的动态衍射效应。然而，后一种技术也不能用于晶体纳米结构的成像，例如通过衍射对比度图像识别的晶粒和有序域的形态。 … 更多 在这项研究中，我们研究了 3D 断层扫描暗场成像的可行性，并证明它是揭示合金中有序域 3D 非结构的实用方法。在所提出的程序中，将样品放置在样品架上，以便包含超晶格反射的系统行变得平行于样品架的倾斜轴。一系列倾斜的暗场 TEM 图像是通过始终处于激发状态的超晶格反射拍摄的。我们应用此程序来可视化 Ni-Mo 中 D1a 型有序域以及 Ni-Al-Ti 中 L12 型有序域的 3D 形态。所得结果概括如下：1.有序域的倾斜系列观察不仅通过精确设置样品的倾斜轴，而且利用电子束的倾斜功能微调衍射条件来实现。2.在亚化学计量的Ni4Mo合金中，D1a的有序析出物在无序面心立方相的基体中形成，呈橄榄球状特征，其主轴线平行于[001]方向。3.D1a的有序域有倾向Ni4Mo..4 的单相状态中习惯面生长。Ni-Al-Ti 中的 L12 有序析出物内形成面向 {100} 面的无序相片状结构。较少的

This project aimed to analyze three-dimensional (3D) morphologies of crystalline ordered domains in alloys at nanometer scale. To this end, we developed a new experimental technique taking advantage of the computed tomography procedure for transmission electron microscopy (TEM), namely electron tomography. So far, the application of electron tomography has been mostly limited to biological or non-crystalline materials, since the image intensity should satisfy a simple projection relation given as a monotonous function of mass density and thickness of imaged objects. Currently, incoherent imaging by high angle scattering in scanning TEM is sometimes employed for crystalline materials to suppress the dynamical diffraction effects which violate the projection requirement on the imaging. However, the latter technique also cannot be adopted for imaging crystalline nanostructures, such as morphologies of crystal grains and ordered domains, which are identified by diffraction contrast images. … More In this study, we examined the feasibility of dark-field imaging for 3D tomography and proved it to be a practical method to reveal 3D nonostructures of ordered domains in alloys. In the proposed procedure, a specimen is placed on the specimen holder so that a systematic row containing superlattice reflections becomes parallel to the tilt axis of the holder. A tilt series of dark-field TEM images are taken with a superlattice reflection which is excited all the while. We applied this procedure to visualize 3D morophologies of ordered domains of D1a-type in Ni-Mo as well as of L12-type in Ni-Al-Ti. The results obtained are summarized as follows.1.Tilt series observations of ordered domains are achieved by not only precise setting of the tilt axis of a specimen but fine adjustment of diffraction conditions with the tilt function of electron beam.2.Ordered precipitates of D1a are formed in the matrix of disordered fcc phase in a characteristic rugby ball like shape, the main axis of which is parallel to [001] direction in a hypo stoichiometeric Ni4Mo alloy.3.Ordered domains of D1a have a tendency to grow with habit planes in a single phase state of Ni4Mo..4.Platelets of disordered phase is formed facing to a {100} plane within L12 ordered precipitates in Ni-Al-Ti. Less