Enrichment of Gd and Al atoms in the quadruple close packed planes and their in-plane long-range ordering in the long period stacking-ordered phase in the Mg–Al–Gd system Original Research Article

The crystal structure of a long period stacking-ordered (LPSO) phase newly found in the Mg–Al–Gd ternary system has been investigated by scanning transmission and transmission electron microscopy. The LPSO phase in the Mg–Al–Gd system is found to form by stacking structural blocks, each of which consists of six close-packed atomic planes. In each of the structural blocks long-range ordering occurs for the constituent Mg, Al and Gd atoms, with enrichment of Gd atoms occurring in four consecutive planes of the six close-packed atomic planes. The ideal chemical composition of the structural block is determined to be Mg29Al3Gd4 (Mg–8.3 at.% Al–11.1 at.% Gd). However, the stacking of structural blocks is largely disordered. Strictly speaking, the technical term LPSO cannot be used to describe this phase because of the long-range ordering of the constituent atoms in each structural block. The crystal structure of the LPSO phase can thus be crystallographically described as one of the order–disorder structures, and either the C2/m, P3112 or P3212 space group is assigned when the simplest stacking of structural blocks is assumed.