The Structure of the T′ Phase in the System AI-Cu-Zn

Abstract

The T'' phase in the system AI-Cu-Zn has been investigated using electron diffraction, X-ray diffraction, and electronprobe microanalysis. It is established that T'' has a rhombohedral structurewith a==8''676 ÅA and α==27.41∘°. This structure is a superlattice based on an assembly of five small CsCI-type cells, joined corner to corner along [111]. The intensities of the electron-diffraction reflections show that T' consists of planes of aluminium and of copperatoms arranged alternately perpendicular to the triad axis. Every fifth heavy-metal atom plane is vacant. Measurements of composition and density suggest that zinc replaces a small proportion of the aluminium in the Iattice in order to reduce the electron/atom site ratio. At the low zinc contents commensurate with T'' additional vacancies, both random and ordered, are necessaryto stabilize the structure. However, it is improbable that an electron/atom site ratio of much less than 1.72is achievedin the T'' structure.