Structural variations and transformation behavior of the Al 68 Cu 11 Co 21 decagonal phase
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The Al 68 Cu n Co 2 i decagonal phase was studied after annealing at 1000 °C for 24-760 h by transmission and scanning electron microscopy. The strong superstructure odd-rc reflections in the [1,-2,1,0,0] electron diffractograms were stable under annealing up to about 40 h. As a possible origin of the increased intensities of the odd-n reflections the formation of vacancy-ordered structures is discussed. The structure was modified by prolonged annealing. In several annealed samples a dense net of extra reflections overlapping the quasiperiodic reflections was observed. This observation was explained as an indication of the formation of metastable states during cooling. Differently ordered decagonal structures exhibited different transformation kinetics during cooling from high temperatures.
I. INTRODUCTION In the A l - C u - C o alloy system a decagonal quasicrystalline phase occurs which has been the subject of numerous investigations. It is generally accepted that this phase is thermodynamically stable at high temperatures. However, a survey of the literature results yields a picture that is by far not clear since the experimental results reported vary considerably and are often contradictory. The thermodynamic stability of the decagonal phase at low temperatures was doubted by many authors who reported a transition of this phase during cooling to periodic approximant structures. 1 ^ Although there is agreement that these transitions do occur, the question is still open whether this is the case for decagonal phases of all possible compositions and whether decomposition inside a multiphase field plays a role. It is not obvious that the diffraction properties attributed to the presently known perfect decagonal phase are those which this phase has in its equilibrium. Indeed, a significant part of the work where a perfect decagonal symmetry was observed has been carried out on as-cast alloys often produced under poorly specified conditions. On the other hand, there is clear evidence that the assolidified materials are not in equilibrium and subsequent annealing can result in changes of the diffraction patterns of the decagonal phase.5 The high-temperature stability of the decagonal phase in A l - C u - C o was studied only in a few experiments. Tsai and co-workers6 observed the decagonal structure in Al65Cu15Co2o after annealing for 36 h at
a) Present
address: Laboratorium fur Electronmikrospie, Universitat Karlsruhe, Engesserstr. 7, D-76128 Karlsruhe, Germany. J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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927 °C, which is close to the melting temperature of this phase. Annealing times varying from 24 h at 1000 °C to 3500 h at 550 °C were applied by Grushko7 in an investigation of a series of A l - C u - C o alloys in a wide range of compositions. Powder x-ray diffraction showed the existence of a phase with a typical decagonal pattern. However, some fine effects could not be clarified in these diffractograms due to a lack of resolution. Therefore, several of thes
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