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

本项目的目的是在纳米尺度上分析合金中晶体有序微区的三维形态。为此，我们开发了一种新的实验技术，利用透射式电子显微镜的计算机断层扫描程序，即电子断层扫描。到目前为止，电子层析成像的应用大多局限于生物或非晶体材料，因为成像强度应该满足作为成像对象的质量密度和厚度的单调函数的简单投影关系。目前，对于晶体材料，有时会采用扫描透射电子显微镜中的大角度散射非相干成像来抑制动态衍射效应，这种效应违背了成像的投影要求。然而，后一种技术也不能用于晶体纳米结构的成像，如晶体颗粒的形貌和有序结构域，这些结构是通过衍射对比图像来识别的。…在这项研究中，我们进一步研究了暗场成像用于三维层析成像的可行性，并证明了它是一种揭示合金有序磁区三维非结构的实用方法。在所提出的方法中，将样品放置在样品支架上，以使包含超晶格反射的系统行平行于样品支架的倾斜轴。用一直处于激发状态的超晶格反射拍摄了一系列倾斜的暗场电子显微镜图像。我们应用这一程序可视化了Ni-Mo中D1a型有序结构域和Ni-Al-Ti中L12型有序结构域的三维形态。得到的结果概括如下：1.不仅通过精确设置样品的倾斜轴，而且通过利用电子束的倾斜函数来精细调整衍射条件，实现了有序磁畴的倾斜系列观察。2.在无序fcc相的基质中，D1a以典型的橄榄球形状形成有序析出物，在低化学计量比的Ni4Mo合金中，其主轴平行于[001]方向。3.在Ni4Mo的单相态中，D1a的有序域有随惯性面生长的趋势。4.在Ni-Al-Ti中的L12有序析出物中，形成了面向{100}面的无序相片层。较少

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