Phase transitions from and to the quasicrystalline state show a typical signature due to some structural peculiarities. Both quasicrystals and most of their translationally periodic transformation products, the approximants, consist of the same basic structural units ('clusters'). This is the cause of low-energy interfaces between the newly forming phase and the parent phase. It is also the origin of the rather high stability of the frequently resulting orientationally twinned nanodomain structures. Owing to topological incompatibilities between quasiperiodic and periodic structures, purely displacive phase transitions are impossible. Diffusion of a significant fraction of atoms, at least on the scale of the cluster diameters, always has to take place. Locally similar icosahedral structural ordering between parent phase and nucleating phase is also responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. The different types of experimentally observed phase transitions are discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline and quasicrystalline to crystalline as a function of temperature, pressure, irradiation and high-energy ball milling, respectively.

Structural phase transitions from and to the quasicrystalline state.

The evolution was studied of structural order/ disorder phenomena across the wide stability field of the decagonal phase in the system Al-Co-Ni. Full single-crystal X-ray diffraction data sets were collected of 31 samples annealed at 900 °C. Based on reciprocal space reconstructions of the imaging-plate-scanner data, the variation in Bragg as well as in diffuse scattering is discussed in detail. The samples were also characterized metallographically, by powder X-ray diffraction, differential thermal analysis and energy-dispersive X-ray spectroscopy. The (quasi)lattice parameters vary smoothly over the stability range of decagonal Al 71.5 Co 28.5-x Ni x (8 ≤ x ≤ 20). A smooth variation is also observed for the diffuse scattering related to the 8 A superstructure. These observations indicate a continuous ordering process at 900 °C from the basic Co-rich phase via the type II and type I superstructures to the basic Ni-rich phase.

X-ray diffraction study of decagonal Al—Co—Ni as a function of composition

The structure of decagonal Al–Cu–Rh has been studied as a function of temperature by in-situ single-crystal X-ray diffraction in order to contribute to the discussion on energy or entropy stabilization of quasicrystals. The experiments were performed at 293, 1223, 1153, 1083 and 1013 K. A common subset of 1460 unique reflections was used for the comparative structure refinements at each temperature. A comparison of the high-temperature datasets suggests that the best quasiperiodic ordering should exist between 1083 and 1153 K. However, neither the refined structures nor the phasonic displacement parameter vary significantly with temperature. This indicates that the phasonic contribution to entropy does not seem to play a major role in the stability of this decagonal phase in contrast to other kinds of structural disorder, which suggests that, in this respect, this decagonal phase would be similar to other complex intermetallic high-temperature phases.

High-temperature structural study of decagonal Al-Cu-Rh.

Al−Co−Ni (Aluminum−Cobalt−Nickel)

Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and many useful properties, they are typically very brittle at temperatures below \ensuremath{\sim}75% of their melting points, rendering them difficult to process and often unsuitable for practical implementations. Micro-compression offers an opportunity to unveil the fundamental mechanisms of quasicrystal plasticity. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using microthermomechanical techniques over a temperature in the range 25--500 \ifmmode^\circ\else\textdegree\fi{}C. We observe a few interesting phenomena, including micropillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture at room temperature to serrated plastic flows and then to homogeneous flows at elevated temperatures). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding. Our study bridges the gap between room-temperature and high-temperature plasticity in quasicrystals, demonstrating an opportunity to study complex intermetallic phases in broad size and temperature regimes.

Probing the small-scale plasticity and phase stability of an icosahedral quasicrystal i-Al-Pd-Mn at elevated temperatures

This experimental work is aimed at confirming and broadening findings of former investigations into the type I ↔ S1 phase transition in Al-Co-Ni quasicrystals. In situ high-temperature synchrotron radiation measurements were carried out on Al 71.2 Co 12.8 Ni 16 powder samples. The cell parameters were precisely determined at various temperatures in the range 290-1110 K. The data provided a new extended insight into the thermal expansion of Al 71.2 Co 12.8 Ni 16 and into fundamental properties of the phase transition.

In situ high‐temperature powder diffraction study of reversible phase transitions in decagonal Al71.2Co12.8Ni16

Highly faceted microholes (voids) in icosahedral Al-Mn-Pd quasicrystals form during annealing at temperatures between 750 and 830°C via classical Ostwald ripening. The specimens were single-phase icosahedral with a composition of Al 71 Mn 9 Pd 20 . If such a specimen containing voids is cooled to room temperature with a constant cooling rate of typically 0.6 to 5 K/min, the facets of the voids are frequently found to be decorated by a different material. The decorations have typically a morphology similar to fractals. Specimens rapidly quenched after isothermal heat treatments around 830°C never showed similar decorations. By X-ray microanalysis in a scanning electron microscope it has been found that this decoration material is considerably poorer in Al-content that the icosahedral phase, namely: Al 60 Mn 10 Pd 30 . Accordingly, these decorations are precipitations at the surfaces of voids inside a single-phase icosahedral matrix. This implies that these decorations are formed at the void surfaces bya diffusion induced local equilibrium. The local equilibrium can be understood on the basis of the equilbrium results in a single icosahedral phase.

Phase Equilibria at Internal Interfaces of Icosahedral Al-Mn-Pd

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In situ high‐temperature powder diffraction study of reversible phase transitions in decagonal Al71.2Co12.8Ni16

Phase Equilibria at Internal Interfaces of Icosahedral Al-Mn-Pd