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

利用透射电子显微镜（TEM），采用IKL-ALCHEMI-EDX方法（交叉Kikuchi-line法；IKL，通道增强微分析原子定位法；ALCHEMI，和能量色散x射线光谱法；EDX）对Ti-Al-Mo合金中的l1_0型有序态进行了定量研究。并对IKL-ALCHEMI-EDX方法在b2型有序态中的应用进行了探讨。主要结论如下：1。Mo在L1_0有序Ti-Al-Mo相中的位置偏好由fcc基体中Ti: Al的浓度比决定。在富Ti基体中，Mo原子倾向于占据Al亚晶格位，而在富Al基体中，Mo原子倾向于占据Ti亚晶格位。这表明Mo与Ti或Al在L1_0有序态中没有明显的有序趋势。利用Lennard-Jones原子势[Abe et al.（…More 1993）]用簇变法（CVM）计算Ti-Al-Mo体系的L1_0有序态表明Mo-Ti具有优先有序的倾向。CVM的结果与目前的实验结果相矛盾。认为在分析Ti-Al-Mo体系时，对CVM计算中使用的原子势应进行更严格的估计。在Ti-Al-Mo体系中，L1_0或B2超晶格反射的结构因子值非常小，以至于基池线的分离值几乎不随有序度的变化而变化。利用暗场TEM图像中的厚度条纹等方法，可以与IKL方法相结合，得到更可靠的有序度值。Ti-Al-Mo合金中b2型有序态的超晶格反射在x射线衍射中很难观察到，但在电子衍射中却很明显。多束动态衍射计算证明了电子衍射图中B2超晶格反射的出现是由于电子的强动态衍射效应。这表明可以用IKL-ALCHEMI-EDX方法定量分析Ti-Al-Mo体系中的B2有序态。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