Investigation of room temperature ageing of powdered decagonal Al-Co-Ni

We report on long-term observations obtained from powder X-ray diffraction (XRD) measurements with milled Al-Co-Ni of decagonal structure at room temperature under ambient conditions. Several powder samples were prepared from different pieces of initially bulk polycrystalline Al70.7Co13.3Ni16.0 by ball milling for a total period of 14 min. The as-milled samples were investigated without any intermediate annealing. While a number of them exhibited X-ray powder patterns without sharp reflections, the specimen in the focus of this report exhibited X-ray reflections, which could be unambiguously attributed to the decagonal structure. Evaluating the time-dependent course of integral intensity and position of the four strongest reflections we found evidence that, due to milling, order along the periodic direction is disturbed more than order within the quasiperiodic planes. In a first stage of room temperature ageing, order is restored within the first ∼100 h. In a second stage, quasiperiodic order recovers within the planes, which is probably caused by the relaxation of phason strains induced by the milling procedure. Fourier analysis of the XRD patterns yielded differences in the radial distribution function (RDF) at about 6 A and less pronounced at 3 A, indicating the preferred occupation of bond lengths between transition metal (TM = Co, Ni) atoms rather than pairs involving Al