Electron microscope and diffraction study on the ordered structures of platinum-rich copper-platinum alloys

The ordered structures of Cu-Pt alloys containing 65-78 at. % Pt have been investigated by a combined technique of selected-area electron diffraction and dark-field electron microscopy using thin foils prepared from bulk specimens, and three different types of ordered structure have been observed. Below the order-disorder transformation temperatures, alloys containing less than about 70 at. % Pt have the rhombohedral superlattice of the Cu₃Pt₅ type and those with more than 70 at. % Pt, the CuPt₃-type superlattice with cubic symmetry. At lower temperatures, however, the orthorhombic superlattice is stable in the composition range from about 68 to 75 at. % Pt. On the basis of the present observations, a partial phase diagram around the composition CuPt₃ has been determined.