Local icosahedral symmetries in Al-Mo alloys prepared by ball milling

Annealing products of mechanically treated Al₇₅Mo₂₅ powders have been studied by X-ray powder diffraction with the Rietveld method. The differential scanning calorimetry traces of as-milled powders showed transformation features whose temperature decreased as a function of mechanical treatment time. Different phases were identified across the calorimetric events, which were also reported in the high-temperature phase diagram of the Al-Mo system, such as Al₁₂Mo, Al₅Mo, Al₄Mo, Al₃Mo and Al₂₂Mo₅. Nevertheless, the solid state transformation processes activated in the calorimeter at ambient pressure are of different nature with respect to those observed by means of neutron diffraction during in situ annealing under vacuum. In the specimen mechanically alloyed for 32 hours the Al₁₂Mo phase was observed after annealing with both processes. In the crystal geometry of Al₁₂Mo, which is regarded as a first order approximation of the quasi-crystalline phase, the molybdenum atoms are arranged in a BCC fashion and are surrounded icosahedrally by twelve aluminum atoms.