Precipitation of nanoscale icosahedral quasicrystalline phase in Hf-Cu-(Rh, Ir) amorphous alloys

Hf70Cu25Rh5 and Hf70Cu25Ir5 amorphous alloys were prepared by a single roll er melt-spinning method and the crystallization processes were studied by X -ray diffraction, differential scanning calorimetry and transmission electr on microscopy. For both of the two alloys, the crystallization proceeds thr ough the reactions of Am -->I-phase --> Hf2Cu+alpha -Hf, where Am and I-pha se represent the amorphous and icosahedral quasicrystalline phases, respect ively. Under optimum annealing conditions, the particle size of the I-phase is 10-25 and 10-20 nm for Rh- and Ir-containing alloys, respectively. Cons idering the fact that the crystallization process of Hf70Cu30 amorphous all oy does not accompany the precipitation of an I-phase, the present result d emonstrates that the addition of Rh or Ir is significant. It is suggested t hat icosahedronlike atomic clusters exist in the amorphous state of Hf70Cu2 5Rh5 and Hf70Cu25Ir5 alloys, which serve as the seeds for the precipitation of the I-phase in the initial crystallization process.