In this article, we discuss the stability of soft quasicrystalline phases in a coupled-mode Swift-Hohenberg model for three-component systems, where the characteristic length scales are governed by the positive-definite gradient terms. Classic two-mode approximation method and direct numerical minimization are applied to the model. In the latter approach, we apply the projection method to deal with the potentially quasiperiodic ground states. A variable cell method of optimizing the shape and size of higher-dimensional periodic cell is developed to minimize the free energy with respect to the order parameters. Based on the developed numerical methods, we rediscover decagonal and dodecagonal quasicrystalline phases, and find diverse periodic phases and complex modulated phases. Furthermore, phase diagrams are obtained in various phase spaces by comparing the free energies of different candidate structures. It does show not only the important roles of system parameters, but also the effect of optimizing computational domain. In particular, the optimization of computational cell allows us to capture the ground states and phase behavior with higher fidelity. We also make some discussions on our results and show the potential of applying our numerical methods to a larger class of mean-field free energy functionals.

The influence of cooling rates on the solidification and microstructure of rapidly solidified quasicrystal alloys with a nominal compositions of Mg57Zn37Y6 (at.%) prepared by melt spinning method was investigated. The microstructure, phase constitution, phase transition, and phase structure of the alloys were examined by means of scanning electron microscopy, x-ray diffraction, energy dispersive spectrometer, differential scanning calorimetry, and transmission electron microscopy. The results show that rapid solidification refines and homogenizes the microstructure of Mg57Zn37Y6 alloys, compared to the conventionally-cast master alloy. With the increasing cooling rate of rapid solidification, the thickness of the ribbon decreases greatly and a larger amount of I-phase can be formed. α-Mg, MgZn, and icosahedral phases are found in the as-cast alloy, but the MgZn phase is absent from rapidly solidified alloys. The I-phase in both as-cast and rapidly solidified alloys can precipitate directly from the melt during the solidification process. A higher cooling rate can lead to a large degree of supercooling, resulting in a decreased phase transition temperature and a large number of icosahedral short-range orders (ISROs). ISROs can act as templates in liquid and promote the nucleation of I-phase.

Quasicrystalline precipitates in ZrAlNiCuNb alloy were systematically studied by transmission electron microscopy. It was found that precipitates always contain various linear phason strains. By electron diffraction analysis, two types of linear phason strain with two different directions perpendicular to the incident beam described by strain matrices with only one nonzero element were identified. After measuring the deviations of diffraction spots and quantitatively fitting against their perpendicular components of the reciprocal lattice vectors, the phason strain matrices were obtained. Domain structures formed as a result of linear phason strain variants along directions with equal probability. Electron diffraction and high-resolution electron imaging provide supportive evidence of this result.

Large quasicrystals up to ∼10 μm in size with a volume fraction of ∼30% have been identified in a nitrogen gas-atomized marginal glass-forming alloy Al85Ni5Y6Co2Fe2 by detailed transmission electron microscopy. The formation of the large quasicrystal (Q) phase is discussed through the configuration of the valence electrons of its constituent elements, and the thermodynamic and kinetic factors associated with the solidification of this marginal glass-forming alloy during gas atomization. The finding leads to an important inference that marginal glass-forming alloys could be ideal systems for the formation of bulk quasicrystals under appropriate kinetic conditions. The Q phase is stable up to ∼500 °C and decomposes thereafter. The activation energy for the decomposition of the Q phase is similar to the self-diffusion of Al. Two new intermetallic phases associated with the formation and decomposition of the Q phase have also been identified and characterized.

Design rules are described here for high-temperature straining stages for transmission electron microscopy. Temperatures above 1000°C can be attained by electron bombardment of the specimen grips. Thermal equilibrium can be reached in a short time by carrying off the heat by water cooling. Some applications of this stage are described. Ferroelastic deformation was observed at 1150°C in t′ and partially stabilized zirconia, which changes the microstructure for successive dislocation plasticity. In the oxide-dispersion-strengthened alloy INCOLOY MA 956, dislocations are impeded by oxide particles and move smoothly between the particles. At high temperatures, both the resting and traveling times control the average dislocation velocity. In MoSi2 single crystals of a soft orientation, dislocations with 1/2〈111〉 Burgers vectors are created in localized sources and move on {110} planes in a viscous manner. The dislocations in Al-Pd-Mn single quasicrystals are oriented in preferred crystallographic directions and move in a viscous way as well. On the basis of in situ observations, conclusions are drawn for interpreting macroscopic deformation behavior at high temperatures.