In-plane positional correlations among dopants in 10H type long period stacking ordered Mg75Zn10Y15 alloy studied by X-ray fluorescence holography
Graphical abstract
Introduction
Recently, a series of Mg alloys, containing transition and rare earth metals, has attracted much attention owing to their excellent mechanical properties [1], [2], [3] and their curious microscopic structure, the so-called synchronized long-period stacking ordered (LPSO) structure [4], [5], [6], [7]. The LPSO structure is a long period stacking derivative of the hcp Mg structure, where the hcp structure is modified by a periodical insertion of stacking faults. Moreover, it was reported that the impurity elements are enriched around the stacking faults [7], that is, the concentration of the impurity elements are synchronized with the stacking faults. The excellent mechanical properties of these Mg alloys are believed to originate from the synchronized LPSO structure, but the strengthening mechanism has not been fully understood so far.
To obtain profound insight into the relation between mechanical and structural properties, it is important to clarify the arrangement of impurity elements around the stacking faults. According to an electron microscope observation on Mg–Zn–Y LPSO alloy [8], Zn and Y form L12-type clusters around the stacking faults as shown in Fig. 1. It has been inferred that the superior mechanical properties, such as a high elastic modulus [9], are caused by the formation of these clusters, and are highly dependent on their stability. Also, according to a recent study on the plastic deformation behavior of Mg–Zn–Y LPSO alloy [10], it was suggested that the ductility is strongly dependent on the in-plane ordering in the Zn/Y concentrated layers, i.e., the incompleteness of the in-plane ordering is essential for obtaining a good ductility. Thus, to understand the origin of the excellent mechanical properties, it is important to investigate the disordering of the arrangement of the L12 clusters as well as their stability. However, such a disordering makes it difficult to apply transmission electron microscope (TEM) techniques for obtaining clear atomic images. Although scanning tunneling microscopy (STM) is a candidate for visualizing in-plane atomic arrangement including disordered structure, the spatial resolution is not enough for observing inner structure of the L12 clusters at present [11].
X-ray fluorescence holography (XFH) [12] is a powerful technique for visualizing the three-dimensional local atomic configurations around a selected element, and has been applied to various materials [13], [14], [15], [16], [17], [18]. Thus, XFH is useful for clarifying the detailed structure of the L12 clusters. Furthermore, because the intensity and the shape of the atomic image obtained from XFH are very sensitive to the positional fluctuation of atoms, this technique can give information on the disordering of the arrangement of the L12 clusters.
Previously, we performed XFH measurements on a single crystal Mg85Zn6Y9 18R type LPSO alloy [13]. The atomic images exhibited broad features, which were found to be attributed to a rotational fluctuation of the L12 clusters. However, it was difficult to obtain detailed structural information of the L12 clusters from these broad atomic images. Thus, XFH measurements on more highly ordered LPSO alloys, such as a 10H-type LPSO alloy [19], are required for the exact understanding of their structural properties. In this paper, we report the in-plane atomic image around Zn obtained from the Zn Kα holograms of the Mg75Zn10Y15 single crystal alloy, having a 10H type LPSO structure [19]. We discuss the inner structure of the L12 clusters and evaluate the inter-cluster positional correlations.
Section snippets
Material and methods
The master ingot of Mg75Zn10Y15 was prepared by induction melting in a carbon crucible. The master ingot was directionally solidified (DS) in a furnace using the Bridgman method at a solidification rate of 10 mm per hour under an Ar atmosphere [20]. The DS sample was annealed for 24 h at a temperature of 773 K. Fig. 2(a) shows a picture of the sample. The single crystal region is indicated by the solid lines. The size of the region is about 0.3 × 1.0 mm2. Fig. 2(b) shows XRD pattern of the
Results and discussion
Fig. 3(a) shows the experimentally obtained and simulated in-plane atomic images around Zn. The Barton algorithm [22] was used for the reconstruction of the images. The simulated atomic image is based on the cluster model proposed in Ref. [8]. The upper part corresponds to the three different atomic configurations around Zn indicated in Fig. 3(b). In the model [8], Zn atoms form a regular triangle along the ab plane. The atomic configurations around the three Zn atoms at the vertices of this
Conclusions
In summary, we performed XFH measurements on a single crystal Mg75Zn10Y15 alloy, having a 10H type LPSO structure, and reconstructed the in-plane atomic image around Zn. The clear atomic images corresponding to the surrounding Zn atoms within the L12 cluster were observed, which evidences the formation of the robust L12 clusters. On the other hand, the atomic images corresponding to the Zn atoms outside the L12 cluster are hardly observed, which indicates the weak inter-cluster positional
Acknowledgment
This study is supported by Grant-in-Aid for Scientific Research on Innovative Areas (“3D active Site Science” and “MFS Materials Science”) from the Japan Society for the Promotion of Science (Nos. 2605006 and 18H05479, respectively). The X-ray fluorescence holography experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal nos. 2016A0128, 2015A0116, 2014B1296, 2014B1289, and 2014A1172).
Declaration of interest
We declare that we have no known
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Recovery features of kink boundaries upon post-annealing of a hot-extruded Mg-Zn-Y alloy
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Present address: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan