Formation of a single quasicrystal upon collision of multiple grains

Quasicrystals exhibit long-range order but lack translational symmetry. When grown as single crystals, they possess distinctive and unusual properties owing to the absence of grain boundaries. Unfortunately, conventional methods such as bulk crystal growth or thin film deposition only allow us to synthesize either polycrystalline quasicrystals or quasicrystals that are at most a few centimeters in size. Here, we reveal through real-time and 3D imaging the formation of a single decagonal quasicrystal arising from a hard collision between multiple growing quasicrystals in an Al-Co-Ni liquid. Through corresponding molecular dynamics simulations, we examine the underlying kinetics of quasicrystal coalescence and investigate the effects of initial misorientation between the growing quasicrystalline grains on the formation of grain boundaries. At small misorientation, coalescence occurs following rigid rotation that is facilitated by phasons. Our joint experimental-computational discovery paves the way toward fabrication of single, large-scale quasicrystals for novel applications.

An Alternative Method of Grain Boundary Characterization

EBSD (Electron Backscatter Diffraction) is a modern experimental technique which allows to represent the information about texture and microstructure in the form of a topological map comprised of a very large number of acquisitioned orientation points. Such a map can be easily used to analyze grain boundaries. In TSL OIM Data Analysis software it is mainly done by Line Segments Method, in which grain boundaries are represented as lines separating pairs of EBSD points for which the misorientation value is within a specified range. The aim of this work is to present a complementary method of grain boundary characterization. In this case, a GB consists of specially selected EBSD points and is thus represented as a two dimensional area. As a result, new possibilities of GB analysis emerge, such as texture of GB areas. The provided description may be also more compatible with a real microstructure, especially after deformation, in which grain boundaries (especially the one with small misorientation) are indeed areas of lattice defects accumulation.

Where are the geometrically necessary dislocations accommodating small imprints?

Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses of small indentations in copper single crystals exhibit only slight changes of the crystal orientation in the surroundings of the imprints. Far-reaching dislocations might be the reason for these small misorientation changes. Using EBSD and TEM technique, this work makes an attempt to visualize the far-propagating dislocations by introducing a twin boundary in the vicinity of small indentations. Because dislocations piled up at the twin boundary produce a misorientation gradient, the otherwise far-propagating dislocations can be detected.

Effect of thermal annealing on grain boundary chemistry of polycrystalline TiBa2Ca2Cu3Oy films

Recent studies have shown that spray-pyrolyzed films of the Tl-1223 compound (TlxBa2Ca2Cu3Oy, with 0.7 < × < 0.95) on polycrystalline yttrium stabilized zirconia substrates can be prepared which have critical current density Jc near 105 A/cm2 at 77 K, in zero field. The films are polycrystalline, have excellent c-axis alignment, and show little evidence of weak-link behavior. Transmission electron microscopy (TEM) studies have shown that most grain boundaries have small misorientation angles. It has been found that the films have a nigh degree of local texture indicative of colonies of similarly oriented grains. It is believed that inter-colony conduction is enhanced by a percolative network of small angle boundaries at colony interfaces. It has also been found that Jc is increased by a factor of 4 - 5 after the films were annealed at 600 °C in oxygen. This study is thus carried out to determine the effect on grain boundary chemistry of the heat treatment.