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

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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