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

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.