Real-Time Observations of Quasicrystal Formation

Abstract

Since the discovery of quasicrystals, there have been continued efforts to explain their growth mechanisms given that their unique structures violate the rules of conventional crystallography. Despite decades of research on the topic, the growth mechanisms of quasicrystals remain one of the fundamental puzzles in the field of crystal growth. To elucidate the growth of quasicrystals, many theories have been proposed. However, there have been very few experimental investigations with which to test the various theories, and quasicrystal growth often accompanies complicated interactions and unexpected growth pathways beyond the scope of these theories. Therefore, to corroborate these theories, it is essential to utilize the benefits of advanced in situ characterization techniques, such as X-ray tomography (XRT) and dynamic transmission electron microscopy (DTEM). The results obtained through these advanced techniques provide direct evidence to support the theories, with high spatial and temporal resolutions. Especially, such in situ approaches allow extracting the information regarding the growth kinetics, growth shapes, and growth interactions which cannot be retrieved from ex situ characterization techniques. In the first and primary part of this thesis, the growth of a single quasicrystal will be discussed. We demonstrate how growth and dissolution pathways of a decagonal quasicrystal are different from each other with respect to the underlying mechanism (interfacial attachment in the former case and bulk transport in the latter). In addition, we compare the growth kinetics of a decagonal quasicrystal with its crystalline approximant, which shares a similar structural motif. Furthermore, we investigate the kinetic and equilibrium shapes of icosahedral quasicrystals. These observations are only possible when we incorporate 4D (i.e., 3D space + time) approaches. The second part of this thesis concerns the growth interactions between multiple quasicrystals. We examine the interfacial phenomena when quasicrystals impinge on each other using 4D XRT and describe the preconditions required for forming a single quasicrystal from multiple quasicrystalline nuclei or ‘seeds’ with the aid of molecular dynamics (MD) simulations. From the XRT results, we can directly observe the formation of a single quasicrystal based on the gradual disappearance of grain boundary grooves. In typical solidification experiments, it is often unavoidable to produce polycrystalline materials, which often deteriorates materials’ properties. Therefore, our joint experiment-computational discovery paves the way toward fabrication of single, large-scale quasicrystals to solve engineering problems. The last part of this thesis covers the solid-state phase transformation from approximant to quasicrystalline phases induced by a short-pulsed laser irradiation. To the best of our knowledge, the real-time investigation of quasicrystal growth far-from-equilibrium has not been reported in a time-resolved manner. Additionally, the solid-state dendritic growth is extremely rare in Nature and several preconditions have to be satisfied for this growth form to manifest. Interestingly, this study demonstrates how quasicrystals grow dendritically showing a huge deviation from their well-known polyhedral growth shapes and what contributes to this unique precipitation pathway. Through ab initio MD simulations, we identify common structural motifs that facilitate the phase transformation between the approximant and quasicrystalline phases. Overall, the findings in this dissertation work are at the forefront of solidification science and have expanded our knowledge on the growth mechanisms of quasicrystals and their approximants using advanced characterization techniques and corresponding simulations.