Structure Analysis of a Long Period Stacking Ordered Phase in Mg-Al-Gd Alloys
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Structure Analysis of a Long Period Stacking Ordered Phase in Mg-Al-Gd Alloys Hideyuki Yokobayashi1, Kyosuke Kishida1, Haruyuki Inui1, Michiaki Yamasaki2 and Yoshihito Kawamura2 1 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501 JAPAN 2 Department of Materials Science, Kumamoto University 2-39-1, Kurokami, Kumamoto 860-8555, Japan ABSTRACT Crystal structure of a long period stacking ordered (LPSO) phase newly found in Mg-AlGd ternary alloys was investigated by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). The Mg-Al-Gd LPSO phase was confirmed to be constructed with 6-layer structural blocks, which is similar to the case of the 18R-type LPSO phases in the other Mg-TM-RE alloys. Atomic resolution high-angle annular dark-field (HAADF) STEM imaging revealed that Gd atoms are enriched in four layers instead of two layers and are ordered in a long range within the 6-layer structural block. INTRODUCTION Ternary Mg-TM(transition metal)-RE(rare earth) phases with long period stacking ordered (LPSO) structures have received a considerable amount of attentions as new types of strengthening phases in high-strength Mg-alloys [1-4]. Although some beneficial effects of the LPSO phases on mechanical properties have been reported so far, inherent characteristics of the LPSO phases including crystal structure, formation process and deformation mechanism have not been fully clarified yet. Extensive studies of various Mg-TM-RE LPSO phases have revealed that the LPSO structures are characterized by periodic arrangements of stacking faults within the HCP stacking of parent Mg and also by enrichment of TM and RE atoms in two layers adjacent to the stacking fault [5-10]. Figure 1 shows a typical example of an 18R-type LPSO phase observed in Mg-Zn-Y alloys. An atomic resolution image taken by high-angle annular dark-field scanning electron microscopy (HAADF-STEM) (figure 1a) indicate that the LPSO phase has 18 layers in the hexagonal unit cell and stacking faults exist every six layers in a HCP stacking. In addition, the SAED patterns exhibit the presence of diffuse scattering (marked by arrows in figures 1b and 1c), which implies the occurrence of a short-range ordering of Zn and Y in the enriched layers. Although the atomic resolution HAADF-STEM imaging has great advantages in determining the ordered arrangements with the strong Z-dependence of the contrast [11,12], it is still difficult to observe the short range ordering in the Mg-Zn-Y LPSO phase. Thus, the details of the in-plane arrangement in the enriched layers have not been clarified yet. In the present study, we prepare a new Mg-Al-Gd LPSO phase containing Gd with relatively high atomic number (#64) and examine the ordered arrangement of Gd atoms in the enriched layers by the atomic resolution HAADF-STEM imaging and transmission electron microscopy (TEM).
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Figure 1. (a) HAADF-STEM image and (b,c) SAED patterns taken from a 18R-type Mg-YZn LPSO phase in (a),(b) [2¯1¯10]Mg and (c) [1¯100
