COMPOSITION AND HEAT-TREATMENT EFFECTS ON ARCS OF DIFFUSE-SCATTERING IN TI-MN-TRANSITION-METAL ICOSAHEDRAL PHASES

Arcs of localized diffuse scattering have been observed in the icosahedral phase of many alloys. Generally, however, they are too weak to be quantitatively compared with theoretical predictions. Since the arcs are very strong in titanium-based quasicrystals, they are ideal systems for which to study the origin and the stability of this diffuse scattering. The positions, shapes and relative intensities of the diffuse scattering in alloys of Ti-Mn-Si TM (where TM=V, Cr, Mn, Fe, Ni, Cu and Ge) are shown to be identical. No changes are observed with annealing, even though the samples annealed to the highest temperatures have partially transformed to several crystalline approximant phases. Similar arcs have also been observed in a b.c.c. crystal approximant. The transition-state theory based on intermediate chemical ordering is inadequate to explain these experimental observations. Further, if the arcs arise from structural ordering, the ordering temperature must be very high in these alloys.