NEW QUASI-PERIODIC PHASE IN AL85CR15

A new, hexagonal, two-dimensionally quasiperiodic phase with the point group 6/mmm and the composition Al4Cr is found in Al85Cr15 after moderately rapid solidification. The phase grows on the interface between the Al matrix and the orthorhombic phase epsilon'-Al4Cr. From an analysis of the pseudo-symmetry of the electron diffraction patterns it is concluded that the new phase is made up of the same icosahedral clusters or cluster fragments as epsilon' and the icosahedral phase which forms in the same alloy at higher quenching rates. The clusters have three different orientations, related by 120-degrees rotations about a common two-fold axis, parallel to the hexagonal axis. The proposed structure accounts for the fact that there is a fixed orientation relationship between the new phase and epsilon'. The phase is highly faulted, as indicated by diffuse electron scattering and confirmed by high-resolution imaging.