Icosahedral, Decagonal and Amorphous Phases in Al–Cu–M (M=Transition Metal) Systems

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Abstract
Rapidly solidified phases in Al–Cu–M ternary alloys ranging from 15 to 20 at%Cu and 10 to 20%M were found to change, with the group number of transition metal M, in the sequence of an amorphous phase for Y, La, Ti, Hf or V, followed by an icosahedral (I) phase for Cr, Mn, Fe, Ru or Os, a decagonal (D) phase for Co or Rh and a crystalline phase for Ni, Pd or Pt. Furthermore, the I-phase in Al65Cu20(Fe, Ru or Os)15 and the D-phase in Al65Cu15(Co or Rh)20 were thermodynamically stable and the amorphous Al75Cu15V10 phase transformed to an I-phase by annealing. The formation of the stable I- and D-phases was examined in terms of the electronic structure parameters (Kp and 2kF), the atomic size factor (λ) and the outer electron per atom ratio (e⁄a). Their stable quasicrystalline phases were found to exist in narrow composition ranges where the criteria of e⁄a\simeq1.75, λ\simeq0.09 and Kp⁄2kF\simeq1.0 are satisfied, though the metastable I- and D-phases are formed in relatively wide ranges of e⁄a=1.4 to 1.8, λ=0.06 to 0.11 and Kp⁄2kF=1.0 to 1.3. It is therefore interpreted that the formation tendency and stability of the I- and D-phases in the Al–Cu–M alloys are enhanced when the energy gap lies near the Fermi surface.