Characterization of two different types of long-period ordered alloys by high-resolution electron microscopy

High-resolution electron microscopy can be used to differentiate between two structural models of long-period ordered alloys. These models differ in the nature of the disorder as seen in the stacking irregularities which can have planar boundaries as exemplified by Ag₃Mg or wavy boundaries as in AuCu II. In the composition range 22-27 at.% Mg, Ag₃Mg is built up with a regular arrangement of two kinds of structural layers, 1 or 2 L1₂ cells in thickness. Some stacking disorder exists, but this alloy can be locally described using a space-group notation and has the character of being an infinitely adaptative structure. High-resolution images have been used to show the perfect planarity of the boundaries in Ag₃Mg, thus demonstrating the way in which disorder is accommodated in this alloy.