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

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 (K_p and 2k_F), 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 K_p⁄2k_F\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 K_p⁄2k_F=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.