STRUCTURAL INVESTIGATIONS ON Al₇(Mn_0.7Fe_0.3)₂ AND Al₄ Pd DECAGONAL QUASI CRYSTALS BY FIM

Abstract
Two types of metastable alloys, Al₇(Mn_0.7Fe_0.3)₂ and Al₄Pd, with decagonal symmetry were prepared by the melt spinning technique. The solidification morphology of the melt spun ribbons of Al₇(Mn_0.7Fe_0.3)₂ appeared to contain 5 µm sized pentagonal facets which in fact showed to be constituted of many lobes (SEM). The decagonal symmetry was revealed by transmission electron diffraction, the tenfold axis being always normal to the ribbon plane, as is expected for splat cooling on a cold surface. Bright field imaging (TEM) of quasi-crystalline grains while continuously tilting the specimen revealed a rapid, fine structured contrast variation unlike the usual appearance of crystalline materials. Bright field transmission electron micrographs of an Al₄Pd tip revealed decoration with 5 nm sized patches. Both Al₇(Mn_0.7Fe_0.3)₂ and Al₄Pd had similar field ion micrographs (Ne, 80 K) in the sense that globally disordered images had local ordering on the 5 nm scale. This is in contrast to simulations, which show ordering throughout the image field. This result, combined with the transmission electron micrographs, leads to the conclusion that the stacked Penrose model cannot universally describe the decagonal phase in the investigated alloys. A high defect density (like phasons) could be responsible for the observed phenomena, however the model in which the material is built of a large number of crystallites should not be ruled out either.