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

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 the 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 Al6Gd8 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 Mg35Al3Gd4 (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 P6322 or R3¯c, is assigned when the simplest stacking of structural blocks is assumed.

► The crystal structure of the 14H-type Mg–Al–Gd LPSO phase was investigated by STEM. ► The 14H-type Mg–Al–Gd LPSO phase is formed by stacking-ordered structural blocks. ► One-dimensional disorder occurs in the stacking of the structural blocks. ► The crystal structure of the LPSO phase is described by the OD theory. ► The space group of either P  6322 or R3¯c, is assigned for the simplest polytypes.