Order-Disorder Transformation Processes with Composition Variations Studied by IKL-ALCHEMI-EDX Method

利用分析电子显微镜，用IKL-ALCHEMI-EDX方法(相交Kikuchi线法；IKL，沟道增强原子定位；ALCHEMI和能谱；EDX)定量研究了Ti-Al-Mo合金的L10型有序态。还考察了IKL-ALCHEMI-EDX方法在B2型有序态中的应用。主要结论如下：1.面心立方相基质中Ti：Al的浓度比决定了Mo在L10_0有序的Ti-Al-Mo相中的择优位置。对于富钛基质，Mo原子倾向于占据Al亚晶格位置，而对于富Al基质，Mo原子倾向于占据Ti亚晶格位置。这表明在L1_0有序态下，Mo与Ti或Al没有表现出明显的有序化趋势。2.用Lennard-Jones原子势的团簇变分方法(CVM)计算了Ti-Al-Mo系统的L1_0有序态[Abe等人]。(…)更多的1993)]表明了Mo-Ti的优先有序化趋势。Cvm的计算结果与目前的实验结果相矛盾，认为在分析Ti-Al-Mo系统时，Cvm计算中使用的原子势应该得到更严格的估计。3.由于Ti-Al-Mo系统中L1_0或B2超晶格反射的结构因子值很小，以至于菊池谱线的分离值几乎不随有序度的变化而变化。另外一些方法，例如利用暗场电子显微镜图像中的厚度条纹，应该与IKL方法一起使用，以获得更可靠的有序度数值。4.在X射线衍射中，很难观察到超晶格反射，但在电子衍射中，它们往往是清晰可见的。多束动力学绕射计算合理地解释了B_2超晶格反射在电子衍射谱中的出现是由于电子的强烈动力学绕射效应。这表明，用IKL-ALCHEMI-EDX方法可以定量分析Ti-Al-Mo系统中的B2有序态。较少

The L1_0-type ordered state in Ti-Al-Mo alloys was investigated quantitatively by means of the IKL-ALCHEMI-EDX method (intersecting Kikuchi-line method ; IKL,atom location by channelling-enhanced microanalysis ; ALCHEMI,and energy dispersive X-ray spectroscopy ; EDX) using an analycal transmission electron microscope (TEM). The application of the IKL-ALCHEMI-EDX method to the B2-type ordered state was also examined. The main conclusions are drawn as follows.1.The site-preference of Mo in an L1_0 ordered Ti-Al-Mo phase is fixed by concentration ratio of Ti : Al in the fcc matrix. Mo atoms tend to occupy Al sublattice sites for a Ti-enriched matrix, while the site-preference of Mo turns to Ti sublattice sites for an Al-enriched matrix. This indicates that Mo exhibits no significant ordering tendency with Ti or Al for the L1_0 ordered state.2.The L1_0 ordered state in the Ti-Al-Mo system calculated by the cluster variation method (CVM) with the Lennard-Jones atomic potential [Abe et al. ( … More 1993)] indicates a preferential ordering tendency in Mo-Ti. The results of the CVM contradicts the present experimental results.It is considered that the atomic potential used in the CVM calculation should be estimated more strictly for analyzing the Ti-Al-Mo system.3.The values of structure factors of L1_0 or B2 superlattice reflections in the Ti-Al-Mo system are so small that values of separation of Kikuchi-lines hardly change with degree of order. Some other methods using, for example, thickness fringes in dark field TEM images should be utilized with the IKL method to obtain more reliable values of degree of order.4.The superlattice reflections for the B2-type ordered state in Ti-Al-Mo alloys are hardly observed for X-ray diffraction, but they often appear clearly for electron diffraction. Many-beam dynamical diffraction calculations rationalized that the appearance of B2 superlattice reflections in the electron diffraction patterns is due to strong dynamical diffraction effects of electrons. This suggests that the B2 ordered state in the Ti-Al-Mo system can be analyzed quantitatively by the IKL-ALCHEMI-EDX method. Less