Microstructures and phase transformation of quasicrystals

Abstract

Quasicrystals are relatively new materials, which are arranged in perfect long range order and exhibit non-crystallographic symmetries. They can be classified into two major groups, Aluminum-Transitional Metal (Al-TM) type, such as icosahedral Al-Cu-Fe, and Frank-Kasper (F-K) type, such as icosahedral Zn-Mg-Y. Quasicrystalline surfaces and coating exhibit many desirable characteristics such as low friction and good wear resistance. One of the commercial applications of quasicrystals is on cookware surface coating. Many researchers have investigated the thermal stability of quasicrystalline alloys as it is an important property for cookware coating. While some researchers reported that A165Cu20Fe15 quasicrystal is thermally stable, thermal instability phenomenon of Al-Cu-Fe quasicrystals was observed in other experimental investigations. More work is therefore needed to investigate the thermal stability of quasicrystals. In this project, two groups of quasicrystals, Zn60Mg30Y10 alloy and A165Cu20Fe15 alloy, were investigated to examine their morphological features and structure (5-fold, 3-fold and 2-fold rotational symmetry). Typical pentagonal dodecahedron grains were found in Al-Cu-Fe ahoy, but they were not found in Zn-Mg-Y alloy, which may be due to uneven heat transfer. With regard to the microstructural change of quasicrystals and their approximants during ageing treatment, coarsening of precipitate particles occurred in both quasicrystalline Zn-Mg-Y and Al-Cu-Fe phases under certain ageing conditions. However, the coarsening effect vanished after prolonged ageing. It proved that the quasicrystalline phases were very stable in most ageing conditions. In this project, electron back-scatter diffraction (EBSD) is shown to be an effective method in identifying the quasicrystalline phase and its coexisted crystalline approximant directly from bulk specimen on scanning electron microscopy (SEM). On the other hand, precipitates coarsened continuously during prolonged ageing in the MgZn2 phase and the A171Cu5Fe24 phase which are the approximants of the Zn-Mg-Y alloy and Al-Cu-Fe alloy, respectively. The rhomboid networks of rare earth containing coherent precipitates disappeared completely in the MgZn2 phase and the line-shaped form precipitates in the A171Cu5Fe24 phase are very weak and disappeared when over-aged. Phase transformation occurring in the coexisting crystalline phases were also detected by X-ray diffraction (XRD) technique. This showed that the thermal stability of the quasicrystalline phases is better than those of their approximants. This is the main reason why most researchers tried to obtain single quasicrystalline alloys by heat treatment methods. The outcomes of the present project are significant in better understanding the microstructural stability and phase transformation behaviour of quasicrystals and had laid down the foundations for further investigations.