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

Abstract

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 Al 54 Pd 30 Sc 16 and Al 56 Pd 29 Sc 15 , respectively. The valence electrons per atom ratio (e/a ratio) is calculated to be 2.10 for the i-phase of Al 54 Pd 30 Sc 16 . The 16 at% Sc concentration and e=a ¼ 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 ¼ 2:10. This supports the fact that the two conditions are crucially important in the formation of this type of i-phase.

Formation of Icosahedral Quasicrystals and 1/1 Crystal Approximants in Al-Pd-RE (RE: Rare Earth Metals) Systems

“…Moreover, electron diffraction experiments showed that the positions of the diffraction spots slightly deviate from the ideal positions. 18) These results are attributable to the introduction of a phason strain. The i-phase remained unchanged by the annealing at 873 K for 100 h whereas the i-phase transformed to crystalline phase(s) by the annealing at temperatures higher than 973 K. The diffraction intensities and peak widths of the alloy annealed at 873 K were essentially the same as those of the as-solidified alloy.…”

“…The formation area of the i-phase is limited to the very narrow composition area around the Al 54 Pd 30 Sc 16 , whereas the formation area of the 1/1-phases is extended along 15 at%Sc line. As discussed previously, 18) 16 at%Sc and e=a ¼ 2:10 are crucially important in the formation of the Tsai-type i-phases. We note that e=a ¼ 2:10 is also important in the formation of the 1/1-phase, although the condition for the 1/1-phase is not much strict in comparison with that for the i-phase.…”

“…1(a) are indexed as an P-type i-phase with the quasilattice constant a q 19) of 0.507 nm. As discussed previously, 18) the i-phase can be classified into the Tsai-type. Figure 2 shows the formation areas of the Al-Pd-Sc i-, 1/1-and the Ga-Pd-Sc 1/1-phases.…”

“…The single i-phase has been found at the nominal composition of Al 53:6 Pd 30:0 Sc 16:4 which possesses slightly larger amount of Sc than that of Al 54 Pd 30 Sc 16 previously reported. 18) In our previous study, precise tuning of the alloy compositions of produced samples (0.5 at%) was not enough to determine the single i-phase region. In Fig.…”

ScPd 2 Al 3 – New polymorphic phase in Al-Pd-Sc system

“…The change of crystal structure of rare-earth intermetallic compounds is sometimes seen at the boundary of lighter rare-earth and heavier rare-earth compounds 29) . Phase diagram of the Al-Pd-Sc system is frequently studied because Al54Pd31Sc15 is a wellknown 1/1-crystal approximant to the icosahedral quasicrystal Tsai-type i-phase of the composition Al54Pd30Sc16 30,31) . Detailed study has shown that the approximant phase and i-phase exist in the extremely narrow region of the Al-Pd-Sc phase diagram.…”