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Crystal Structures of Long-Period Stacking-Ordered Phases in the Mg-TM-RE Ternary Systems Kyosuke Kishida, Hideyuki Yokobayashi, Atsushi Inoue and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, JAPAN ABSTRACT Crystal structures of long-period stacking-ordered (LPSO) phases in the Mg-TM (transition-metal)-RE(rare-earth) systems were investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The 18R-type LPSO phase is constructed by stacking 6-layer structural blocks, each of which contains four consecutive close-packed planes enriched with TM and RE atoms. Formation of the TM6RE8 clusters with the L12 type atomic arrangement is commonly observed in both Mg-Al-Gd and MgZn-Y LPSO phases. The difference between the crystal structures of Mg-Al-Gd and Mg-Zn-Y LPSO phases can be interpreted as the difference in the in-plane ordering of the TM6RE8 clusters in the structural block. The Mg-Al-Gd LPSO phase exhibits a long-range in-plane ordering of Gd and Al, which can be described by the periodic arrangement of the Al6Gd8 clusters with the L12 type atomic arrangement on lattice points of a two-dimensional 2 3aMg u 2 3aMg primitive hexagonal lattice, although the LPSO phase in the Zn/Y-poor Mg-Zn-Y alloys exhibits a shortrange in-plane ordering of the Zn6Y8 clusters. INTRODUCTION Mg alloys containing ternary Mg-TM(Transition-metal)-RE(Rare-earth) phases with long-period stacking-ordered (LPSO) structures have received a considerable amount of attention in recent years [1-3]. Although reasons why these alloys can simultaneously exhibit high strength and high ductility have been remained largely unsolved, ternary LPSO phases have been believed to play important roles in endowing them with excellent mechanical properties. Mg-ZnRE LPSO phases are reported to consist of structural blocks with five to eight close-packed atomic planes, forming various polytypes with different numbers of the close-packed atomic planes in the structural blocks and with different stackings of the structural blocks [4]. In the absence of the in-plane long-range ordering of the constituent atoms (as usually assumed in most studies in Mg-TM-RE LPSO phases), polytypes expressed as 10H, 14H, 18R and 24R polytypes are reported to form, among which 14H and 18R polytypes are the most dominantly observed ones [1,2]. The Mg-TM-RE LPSO structures have been generally characterized by periodic occurrence of stacking faults within the HCP stacking of parent Mg and also by enrichment of TM and RE atoms in two atomic layers adjacent to the stacking fault. Figure 1 shows a typical example of an 18R-type LPSO phase observed in Zn/Y-poor Mg-Zn-Y alloys [5]. An atomic resolution image taken by high-angle annular dark-field scanning electron microscopy (HAADFSTEM) (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 selected area electron

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