Formation of Icosahedral Quasicrystal and Its 1/1 Crystal Approximant in Al-Pd-Sc System

抄録

An icosahedral quasicrystal (i-phase) and its 1/1 crystal approximant (1/1-phase) have been found to form in Al-Pd-Sc system. The formation areas of the i-phase and the 1/1-phase are limited to very small areas around the compositions of Al₅₄Pd₃₀Sc₁₆ and Al₅₆Pd₂₉Sc₁₅, respectively. The valence electrons per atom ratio (e/a ratio) is calculated to be 2.10 for the i-phase of Al₅₄Pd₃₀Sc₁₆. The 16 at% Sc concentration and e⁄a=2.10, as well as the intensity profile of the measured X-ray diffraction spectrum, indicate that this i-phase can be classified into the Tsai-type: the Tsai-type i-phases found so far commonly contain an element having relatively large atomic size with 15–16 at% and have e/a within the range 2.0–2.15. The formation of the i-phase has also been found in a series of quaternary Al-Pd-Cu-Sc alloys with different compositions keeping 16 at%Sc and e⁄a=2.10. This supports the fact that the two conditions are crucially important in the formation of this type of i-phase.