PHASE-TRANSFORMATIONS AND STRUCTURAL CHARACTERISTICS OF AL-CU-CO(-SI)DECAGONAL PHASE

The phase transformations and structural characteristics of the Al-Cu- Co-Si alloy have been studied by neutron diffraction and high-resoluti on electron microscopy. The Al65Cu17.5Co17.5 decagonal phase is stable in the temperature range between 973 K and 1350 K. At the low-tempera ture end, it relaxes to a microcrystalline approximant structure. At t he high-temperature end, it melts directly into liquid. Two distinct o rthorhombic phases are identified in the Al63Cu17.5Co17.5Si2 microcrys talline structure. They are composed of several structure units that c an also construct the Penrose tiling. Because of lack of units, a sing le orthorhombic phase cannot undergo the transformation towards the hi gh-temperature decagonal phase. An analysis of the orientation relatio nships between the CsCl and orthorhombic phases leads to the definitio n of Penrose tiling-like subnetworks inside the orthorhombic unit cell s so that these orthorhombic phases can be regarded as the periodic pa tchworks of quasiperiodic subnetworks.