Icosahedral quasicrystal formation in Ti-Zr-based alloys and a new classification technique

Until recently, icosahedral phase (i-phase) formation was studied primarily in Al-transition metal alloys. The Al-based i-phases generally fall into one of two classes; those believed to be based on the Pauling triacontahedron, fundamental to the Bergman l/l phase, and those based on the double-shell Mackay icosahedra found in the 1/1 α-(Al-Mn-Si) phase. Notable Bergman-type quasicrystals include i-(Al-Li-Cu) and i-(Al-Mg-Zn); i-(Al-Mn-Si) forms the best known Mackay-type i-phase. The large number of Ti-based i-phases now known, and the differences in their diffraction features raise the question of their fundamental structural units. To address this partially, results of X-ray and electron microscopy studies of Ti-Zr-Ni alloys, where Ni is replaced by Fe and Co, are reported. The character of the i-phases varies smoothly from the Ti-Zr- Ni quasicrystals, which probably are Bergman-type i-phases, to the Ti-Zr-Fe quasicrystals, which probably are Mackay types. A new classification method for icosahedral quasicrystals based on the ratio of the quasilattice constant a_qto the average atomic separation (a_s), computed from the measured density, is introduced and applied to both Al- and Ti-based quasicrystals. On the basis of this scheme, most Ti-based i-phases, including the Ti-3d transition metal-Si-O phases and Ti-Zr-Fe, form a third group, different from the Al-based Mackay and Bergman groups. Ti-Zr-Ni and Ti-Zr-Co quasicrystals fall into the same class as the Bergman-type Al-based i-phases.