Atomic structure and translational order in the stable decagonal Al_(70)Mn_(18)Pd_(12) quasicrystal

摘要

The structure of the stable decagonal Al-Mn-Pd quasicrystal was studied on an atomic level using a Hitachi H-1500 HVTEM, operated at 800 kV with a resolution of 1.4 A at Scherzer defocus. Image simulations based on the structure determination of decagonal Al-Mn (W. Steurer, J. Phys.: Condens. Matter 3, 3397 (1991)) showed a reasonable good agreement with the HRTEM images. High-resolution images normal to the 10-fold axis revealed a triangular pattern near the centers of all decagonal clusters, indicating a reconstruction at the cluster centers in the thinnest part of the quasicrystal fragments. A probable model for the reconstruction was obtained by considering slight shifts of atoms in the puckered layers. The decagonal phase is usually obtained by a solid state phase transformation. Depending on the structure of the starting material, different disorder is present in the decagonal phase. This has been verified in two AlyrjMnisPdis starting alloys which were conventionally solidified and rapidly quenched, respectively. Subsequently, both alloys were annealed at approximately 800 °C, In the rapidly quenched alloy an almost perfect decagonal phase was observed. By contrast, in the slowly cooled alloy the decagonal phase consisted of a non-periodic texture including nanometer-size crystalline domains and linear phason strain. The corresponding SAD patterns show variations from an almost perfect decagonal pattern to periodic pattern of a pseudo-decagonal approximant with a large unit cell (a= 6.25 nm, b= 8.65 nm, c= 1.25 nm).