SIMULATIONS ON THE PHASE TRANSFORMATION OF DECAGONAL AL-CO-NI TO ITS CRYSTALLINE APPROXIMANTS Matthias Honal, Torsten Haibach and Walter Steurer, Laboratorium für Kristallographie, ETH Zentrum, CH-8092 Zurich, Switzerland.

Model calculations simulating the transformation from the decagonal phase to an crystalline nanodomain structure have been performed, in direct and reciprocal space, respectively.

Starting from an idealised structure model of decagonal Al₇₀ Co₁₅ Ni₁₅ [1], a purely geometrical mechanism is applied, leading to a patchwork-like arrangement of crystalline nanodomains [2]. This is achieved by slight uncorrelated displacements of the atoms. The resulting orientation of the different nanodomains is determined by the minimum total sum of atomic shifts.

Models with dimension 8000 * 8000 * 4 ų, including about 2^.10⁶ atoms, have been used for the calculations. Resolution and coherence parameters have been considered corresponding to experimental conditions.

The resulting nanodomain-structure reproduces quite well the experimental observations [3]. The characteristics of different nanodomain models will be discussed in direct space as well as in reciprocal space.

[1] Steurer, W., Haibach, T., Zhang, B., Kek, S., Lück, R.: The Structure of Al₇₀Ni₁₅Co₁₅. Acta Crystallogr. B49 (1993) 661-675.

[2] Steurer, W., Honal, M. and Haibach, T.: Mechanism of phase transformations in decagonal phases. Proceedings of 5th International Conference on Quasicrystals ICQ95. Singapore - World Scientific (1995) in press.

[3] Kalning, M., Press, W., Kek, S.: Investigation of Decagonal Al₇₀Co₁₅Ni₁₅ Single Crystals by Means of High-resolution Synchrotron X-ray Diffraction. Phil. Mag. Lett. 71(6) (1995) 341-349.