The most stable crystal structure and the formation processes of an order-disorder (OD) intermetallic phase in the Mg–Al–Gd ternary system

Abstract

The most stable crystal structure for an 18R-type order-disorder (OD) intermetallic phase in the Mg-Al-Gd ternary system and its formation processes by annealing at 525 °C have been investigated by means of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The most energetically favourable polytype at 525 °C is found to be the structurally simplest one, a maximum degree of order (MDO) polytype (monoclinic, 1M, space group: C2/m) that is described with a single stacking vector in stacking sixlayer structural blocks. The formation of this simplest polytype occurs in the sequence of (i) enrichment of Gd and Al in four consecutive close-packed planes while keeping the hcp stacking of the AB-type, (ii) formation of Al 6 Gd 8 clusters in the four consecutive atomic planes, introducing a stacking fault in the middle of the four consecutive atomic planes, (iii) thickening by the formation of Gd and Al-enriched four consecutive planes at a distance of two or three close-packed Mg atomic planes from the pre-existing Gd and Al-enriched four consecutive atomic planes so as to form 6-layer and sometimes 7-layer structural blocks, (iv) in-plane ordering of Al 6 Gd 8 clusters in the four consecutive atomic planes and the stacking of structural blocks in the preferential stacking positions to form the OD structure, and (v) elimination of different structural blocks (other than 6-layer ones) and the long-range ordering in the stacking of structural blocks.

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

“…For the OD groupoid family of the C 3 -type, six MDO polytypes are derived [15,18]. Similarly to the case of the 18R-type Mg-Al-Gd LPSO/OD phase [13,18], the crystal structure of the 18R-type Mg-Zn-Y LPSO/OD phase exhibits a tendency to transform from the one with the one-dimensional stacking disorder into a fully-ordered one after prolonged heat-treatment [17]. The stacking sequences of the structural blocks observed in the 18R-type Mg-Zn-Y LPSO/OD phase heat-treated at 500°C for 300 h (Fig.…”

“…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.…”

“…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.…”

A formation criterion for Order-Disorder (OD) phases of the Long-Period Stacking Order (LPSO)-type in Mg-Al-RE (Rare Earth) Ternary Systems

“…Our recent results on the crystal structure analysis for the 18 R -type LPSO phase in the Mg-Al-Gd system, however, revealed that the LPSO phase in the Mg-Al-Gd system should not be described as a ‘LPSO’ phase any longer in a strict sense in crystallography because of the existence of the in-plane ordering of Gd and Al atoms in the four consecutive atomic layers enriched with them in the 6-layer structural block 15 . Instead, the crystal structure of the ‘LPSO-type’ phases in the Mg-Al-Gd system is best described with the concept of the order-disorder (OD) structure 15–23 , in which a crystal structure is described with the symmetry of an OD layer (corresponding to a 6-layer structural block) and the relative relation between adjacent two OD layers 15,17,18 . Then, there is a possibility that a series of LPSO-type phases with an OD structure (i.e., OD phases) is formed in Mg-Al-RE ternary systems, although the OD phases have recently been found to develop also in the Mg-Zn-Y system when the Zn/Y concentrations are high 24–26 .…”

“…S1(a) of Supplementary) 15 . In addition, the formation of L1 2 -type atomic clusters is common to both the OD and LPSO phases in the Mg-Al-Gd and Mg-Zn-RE ternary systems 15,16,25,26 and has been considered to play a key role in the formation of the OD and LPSO phases 17,27–30 . Furthermore, recent first-principles calculations have predicted that the OD phases in the Mg-Al-RE and Mg-TM-RE ternary systems are stabilized by the inclusion of an additional Mg atom in the centre of each L1 2 -type atomic cluster 31–36 .…”