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 mode rately rapid solidification. The phase grows on the interface between the Al matrix and the orthorhombic phase epsilon'-Al4Cr. From an analy sis of the pseudo-symmetry of the electron diffraction patterns it is concluded that the new phase is made up of the same icosahedral cluste rs or cluster fragments as epsilon' and the icosahedral phase which fo rms in the same alloy at higher quenching rates. The clusters have thr ee different orientations, related by 120-degrees rotations about a co mmon two-fold axis, parallel to the hexagonal axis. The proposed struc ture accounts for the fact that there is a fixed orientation relations hip between the new phase and epsilon'. The phase is highly faulted, a s indicated by diffuse electron scattering and confirmed by high-resol ution imaging.