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

本项目旨在分析纳米尺度下合金晶体有序畴的三维形貌。为此，我们开发了一种新的实验技术，利用透射电子显微镜（TEM）的计算机断层扫描程序，即电子断层扫描。到目前为止，电子断层扫描的应用主要局限于生物或非晶体材料，因为图像强度必须满足一个简单的投影关系，即成像物体的质量密度和厚度的单调函数。目前，晶体材料在扫描TEM中有时采用高角散射非相干成像，以抑制动态衍射效应对成像的投影要求。然而，后一种技术也不能用于成像晶体纳米结构，如晶体颗粒和有序畴的形态，这些都是通过衍射对比图像来识别的。在这项研究中，我们检验了暗场成像用于三维断层扫描的可行性，并证明了它是一种显示合金有序畴三维非结构的实用方法。在建议的程序中，将样品放置在样品支架上，以便包含超晶格反射的系统行与支架的倾斜轴平行。利用一直被激发的超晶格反射，拍摄了一组倾斜的暗场TEM图像。我们应用该方法可视化了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貌。所得结果总结如下：1。有序畴的倾斜序列观测不仅通过精确设置试样的倾斜轴，而且通过电子束的倾斜函数对衍射条件进行精细调整。在次化学计量Ni4Mo合金中，D1a有序析出相在无序fcc相基体中形成典型的橄榄球状，其主轴平行于[001]方向。在Ni4Mo的单相状态下，D1a的有序畴有沿习惯面生长的趋势。在Ni-Al-Ti的L12有序析出物中形成面向{100}平面的无序相血小板。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