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

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 16at% 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 1516 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 16at%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. [doi: 10.2320/matertrans.MC200827]