The crystal structure of the LPSO phase of the 14H-type in the Mg–Al–Gd alloy system

Abstract

The crystal structure of a long-period stacking-ordered (LPSO) phase of the 14H type formed in a Mg-Al-Gd alloy as a local small part in the intergrowth structure together with that of the 18R type (the majority) has been investigated by scanning transmission and transmission electron microscopy. The LPSO phase of the 14H type in the Mg-Al-Gd system is found to form by stacking structural blocks, each of which consists of seven close-packed atomic planes. In each of structural blocks, a long-range ordering occurs for the constituent Mg, Al and Gd atoms with the enrichment of Gd atoms in the four consecutive atomic planes. The in-plane long-range ordering in the four consecutive atomic planes occurs so as to form Al 6 Gd 8 clusters in a periodic manner. This is exactly the same as what is observed in the LPSO phase of the 18R type. The crystal structure of the 14H-type LPSO phase can thus be described to form simply by adding a Mg layer to the crystal structure of the 18R-type LPSO phase so as to form triple (three consecutive) Mg layers to sandwich the Gd-enriched quadruple layers. The ideal chemical formula of the structural block is Mg 35 Al 3 Gd 4 (Mg-7.1 at.%Al-9.5 at.%Gd). The crystal structure of the LPSO phase can thus be crystallographically described as one of the order-disorder (OD) structures, and the space group of either P6 3 22 or R3c, is assigned when the simplest stacking of structural blocks is assumed.

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

“…Various types of the LPSO phases expressed as 10H, 14H, 18R and 24R-types, which are formed by stacking structural blocks composed of five, six, seven and eight close packed atomic layers, respectively, have been reported in various Mg-Zn-RE systems [8][9][10][11][12][13][14][15][16]. 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].…”

“…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.…”

First Principles Calculations of Solute Ordering in Mg–Zn–Y Alloys

“…The cluster formations are observed in the more stable LPSO structure, for instance, in the MgAl Gd system. 13) In the MgZnY system, such a clear cluster formation has not been observed as it is under discussion.…”

Effects of stacking sequence and short-range ordering of solute atoms on elastic properties of Mg–Zn–Y alloys with long-period stacking ordered structures

“…For an 18R-LPSO phase, it was suggested that the clusters formed partially ordered domain structures [23][24][25] even though they did not exhibit long-range ordering in directions parallel and perpendicular to the stacking direction [23,26]. In contrast, in a 10H-LPSO phase of Mg-Zn-Y alloys, the clusters show long-range ordering in the direction perpendicular to the stacking direction, i.e., inside the ABCA-type stacking layers [19]; the ordered arrangement of clusters is similar to the order-disorder (OD) structure formed in Mg-Al-Gd alloys [26,27].…”