Crystal Structures of Long-Period Stacking-Ordered Phases in the Mg-TM-RE Ternary Systems

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 Mg-Zn-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$\sqrt 3 $aMg × 2$\sqrt 3 $aMg 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.

Crystal structures of highly-ordered long-period stacking-ordered phases with 18R, 14H and 10H-type stacking sequences in the Mg–Zn–Y system

“…However, they did not fully describe how they deduced the atomic ordering in the structural blocks and the space groups based on their experimental results obtained from the imperfectly ordered LPSO phases [16]. Their proposed in-plane order and space groups for two crystal structure models of the 18R-type LPSO phase are well consistent with our crystal structure models for the two MDO polytypes designated as 1M and 3T of the Mg-Al-Gd LPSO/OD phases [13,16,18]. However, there is no apparent reason why the Mg-TM-RE LPSO/OD phases in the different ternary systems should possess the same crystal structures.…”

“…In the OD theory, a crystal structure is described with (i) a set of partial symmetry operations (POs) transforming a structural block (an OD layer) into itself (k-POs) and (ii) a set of POs transforming an OD layer into an adjacent one above it (r-POs) [13,15,18,21]. In the case of the Mg-TM-RE LPSO/OD phases, a set of k-POs is described with the layer group of P( 3)1m irrespective of the number of close-packed atomic layers (either 5, 6, 7 or 8), as far as an OD layer is selected so that TM 6 RE 8 atomic clusters with the L1 2 -type atomic arrangement are periodically arranged on lattice points of a 2 ffiffiffi 3 p a Mg  2 ffiffiffi 3 p a Mg two-dimensional primitive hexagonal lattice [13,15,18].…”

“…The crystal structures of the Mg-TM-RE LPSO phases have been generally characterized by the periodic stacking faults associated with the enrichment of TM and RE atoms in two consecutive atomic layers [9,12]. It should be noted that the Ramdell's notations used to describe the LPSO structures are assigned under an assumption that all close-packed atomic layers are identical, which are in fact not adequate in a strict crystallographic sense because of the RE and TM enrichment in some of the close-packed atomic layers [9,[12][13][14][15][16][17][18]. However, the above mentioned Ramsdell notations have been utilized because no atomic ordering in any closepacked atomic layers was considered to exist originally.…”

“…Published by Elsevier Ltd. All rights reserved. annular dark-field (HAADF) STEM and have revealed the following features [13,15,17,18]. First, each structural block in the Mg-Al-Gd LPSO phases have a long-range in-plane order, which is characterized by a periodic arrangement of Al 6 Gd 8 atomic clusters with the L1 2 -type atomic arrangement in the Gd and Al-enriched four consecutive atomic planes.…”

“…First, each structural block in the Mg-Al-Gd LPSO phases have a long-range in-plane order, which is characterized by a periodic arrangement of Al 6 Gd 8 atomic clusters with the L1 2 -type atomic arrangement in the Gd and Al-enriched four consecutive atomic planes. Second, although the Mg-Al-Gd LPSO phases have the perfectly ordered structure in each structural block, they generally exhibit one-dimensional disorder in the stacking of the structural blocks and consequently, their crystal structures are best described with the crystallographic concept of the order-disordered (OD) structure [13,15,[18][19][20][21][22][23]. The Mg-TM-RE LPSO phase with an OD structure is hereafter designated as the Mg-TM-RE LPSO/OD phase.…”