NEW SUPERLATTICE ORDERING IN AL-PD-MN AND AL-PD-MN-SI ICOSAHEDRAL QUASI-CRYSTALS

Al-Pd-Mn Si alloys with nominal compositions (Al0.710Pd0.205Mn0.085)1-x Si(x) and 0 less-than-or-equal-to x less-than-or-equal-to 0.03 were annealed at various temperatures between 1075 and 779 K. The existence of the following five structures has been recognized in these alloys by electron microscopy observations: traditional F-type icosahedral phase, an F-type icosahedral phase with a new superlattice ordering, a third F-type icosahedral phase with a highly dense distribution of reflections, and two kinds of approximant structures. The main feature of the second icosahedral phase was interpreted by a doubled superlattice of the original six-dimensional cubic lattice; a6D=2.58 nm. This ordering occurred in the alloys annealed at lower temperatures: 876 and 779 K for x=0 alloys and 1025, 980 and 878 K for x=0.01 alloys. A drastic structural improvement by the addition of silicon was discovered in the third F-type icosahedral phase, which was formed in the alloys annealed below 1025 K. The occurrence of these two highly ordered phases at low temperatures suggested that the quasiperiodic model is favourable, rather than the random-tiling model, as a model structure of the icosahedral phases.