Decagonal phase and pseudo-decagonal phase in the Al–Cu–Cr system

Abstract

A decagonal phase with a periodicity of 3·78 nm (= 3 × 1·26nm) along the periodic axis has been found for the first time in Al–Cu–Cr alloys in the as-cast state as well as in the annealed state. The temperature range of stability has been found to be very narrow around 1000°C by investigation of differently annealed specimens in a transmission electron microscope. The broken bulk specimens exhibited a few particles with a tenfold-facetted decagonal prismatic shape. The chemical composition of these particles was determined to correspond to Al₇₂Cu₁₂Cr₁₆ by energy-dispersive X-ray spectrometry. High-resolution electron microscopy image of the decagonal phase taken along the periodic axis (tenfold axis) has shown a ring contrast with ten bright spots situated symmetrically and located at the vertices of a random tiling with four tiles with the same edge length of about 2·0 nm. The decagonal phase has been examined by comparing with the pseudo-decagonal phase obtained at a lower annealing temperature.