Superordering in Ni-rich and Ni-poor decagonal Al-Co-Ni phases

Abstract

Single-crystal X-ray diffraction was carried out on a decagonal Ni-rich phase in the Al-Co-Ni system with a nominal composition of Al72.5 Co11Ni16.5. Apart from sharp Bragg reflections, satellite scattering was observed in Al72.5 Co11Ni16.5 which is different from the satellite scattering observed in a Ni-poor phase with nominal composition Al72.5 Co16.5Ni11. The two types of superorder scattering are interpreted in an unified way by periodic order superimposed on the quasiperiodic averaged substructure. This periodic regimes have a limited correlation length (short-range ordering). While the modulation period stays almost constant over a wide range of compositions in these decagonal alloys, the modulation vector is rotated by π/10 if passing from Ni-rich (S satellites) to Ni-poor (C satellites) compositions. Arguments are given in favour of this interpretation which is also supported by computer simulations.