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 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 R3C, is assigned when the simplest stacking of structural blocks is assumed