Quasicrystal Grains Research Articles

Spontaneously formed quasicrystal grains in a pure metal

Pure metals so far have been obtained in quasicrystal (QC) forms only by templation – epitaxial growth on a QC substrate. Here, we report spontaneous formation of dodecagonal QC (DDQC) grains in pure tantalum (Ta), an early transition metal normally in a body-centered-cubic crystal structure. The DDQC grains comprise icosahedral clusters assembled in accordance with the Stampfli triangle–square tiling scheme and are formed directly from the supercooled liquid and the β-Ta phases during thermal devitrification of a Ta metallic glass in molecular dynamics simulations using a realistic quantum mechanically based interatomic potential. They co-exist with β-Ta and are retainable to and stable at room temperature, with a slightly lower configurational energy than β-Ta.

Preparation and electro-catalytic activity of nanoporous palladium by dealloying rapidly-quenched Al70Pd17Fe13 quasicrystalline alloy

Abstract The formation of nanoporous Pd was studied by electro-chemical dealloying a rapidly-quenched Al70Pd17Fe13 quasicrystal alloy in dilute NaCl aqueous solution, and the electro-catalytic activity of the nanoporous Pd towards methanol electro-oxidation was evaluated by cyclic voltammetry in 1 mol/L KOH solution. XRD and TEM analyses revealed that nano-decomposition of quasicrystal grains occurred in the initial stage of dealloying, and the fully dealloyed sample was composed of FCC-Pd phase. Scanning electron microscopy observation indicated that a maze-like nanoporous pattern was formed in the dealloyed sample, consisting of percolated pores of 5−20 nm in diameter in a skeleton of randomly-orientated Pd nano-ligaments with a uniform thickness of ~5 nm. A retention of ~12 at.% Al in the Pd nano-ligments was determined by energy dispersive X-ray spectroscopy (EDS). The nanoporous Pd demonstrated obvious electro-catalytic activity towards methanol electro-oxidation in alkaline environment.

Multilayered sandwich-like architecture containing large-scale faceted Al–Cu–Fe quasicrystal grains

Faceted quasicrystals are structurally special compared with traditional crystals. Although the application of faceted quasicrystals has been expected, wide-scale application has not occurred owing to the limited exposure of the facets. Using a facile method of heat treatment, we synthesize a multilayered sandwich-like structure with each layer composed of large-scale pentagonal-dodecahedra of Al–Cu–Fe quasicrystals. Moreover, there are channels between the adjacent Al–Cu–Fe layers that serve to increase the exposure of the facets of quasicrystals. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction are used to characterize the multilayered architecture, and the generation mechanisms of this special structure are also discussed.

Phase Evolution of Al/Cu/Co Thin Films into Decagonal Quasicrystalline Phases

Quasicrystals have drawn increased scientific attention during the past decade not only for the purpose of fundamental research, but also due to their possible applications as bulk materials or thin films [1]. In particular, decagonal (d) quasicrystals could be very attractive because of their anisotropic structure being quasiperiodic in two dimensions and periodic in the third. Recently it has been shown that icosahedral quasicrystalline Al-Cu-Fe and approximant Al-Si-Cu-Fe thin films can be prepared by annealing a multilayer thin film on a sapphire or Si substrate, respectively [2]. In this work, multilayered Al/Cu/Co thin films have been deposited by magnetron sputtering onto Al2O3 (0001) and Si (001) substrates. The multilayers were produced with a multilayer period of 100 nm, repeated 3 times to a total thickness of 300 nm. The Al:Cu:Co layer thickness ratios were adjusted to obtain films with global compositions around the ideal decagonal quasicrystalline phase d-Al65Cu17.5Co17.5. The phase evolution during annealing, and the concurrent changes in film microstructure and crystal quality was investigated. The decagonal d-Al-Cu-Co and d-Al-Cu-Co-Si phases were both found by X-ray diffraction, electron diffraction, and high-resolution (scanning) electron microscopy to form at 5000C on Al2O3 and Si, respectively, and at 6000C these were the only phases present. Figure 1 shows the HRTEM micrograph of the Al-Cu-Co-Si phase after annealing to 7000C. At increasing temperatures, the quasicrystal grains grew larger in size, up to 500 nm, and the Al-Cu-Co obtained a preferred orientation with the 10-fold periodic axis aligned with the Al2O3 substrate normal. The d-Al-Cu-Co phase persisted to more than 8500C, with a complete 00001-texturing, while the d-Al-Cu-Co-Si phase was replaced by other crystalline phases at 8000C. The d-Al-Cu-Co-Si phase was also observed to grow into the Si substrate by a solid-state diffusion reaction.

Evidence for the extraterrestrial origin of a natural quasicrystal

We present evidence that a rock sample found in the Koryak Mountains in Russia and containing icosahedrite, an icosahedral quasicrystalline phase with composition Al(63)Cu(24)Fe(13), is part of a meteorite, likely formed in the early solar system about 4.5 Gya. The quasicrystal grains are intergrown with diopside, forsterite, stishovite, and additional metallic phases [khatyrkite (CuAl(2)), cupalite (CuAl), and β-phase (AlCuFe)]. This assemblage, in turn, is enclosed in a white rind consisting of diopside, hedenbergite, spinel (MgAl(2)O(4)), nepheline, and forsterite. Particularly notable is a grain of stishovite (from the interior), a tetragonal polymorph of silica that only occurs at ultrahigh pressures (≥ 10 Gpa), that contains an inclusion of quasicrystal. An extraterrestrial origin is inferred from secondary ion mass spectrometry (18)O/(16)O and (17)O/(16)O measurements of the pyroxene and olivine intergrown with the metal that show them to have isotopic compositions unlike any terrestrial minerals and instead overlap those of anhydrous phases in carbonaceous chondrite meteorites. The spinel from the white rind has an isotopic composition suggesting that it was part of a calcium-aluminum-rich inclusion similar to those found in CV3 chondrites. The mechanism that produced this exotic assemblage is not yet understood. The assemblage (metallic copper-aluminum alloy) is extremely reduced, and the close association of aluminum (high temperature refractory lithophile) with copper (low temperature chalcophile) is unexpected. Nevertheless, our evidence indicates that quasicrystals can form naturally under astrophysical conditions and remain stable over cosmic timescales, giving unique insights on their existence in nature and stability.

Icosahedral Quasicrystal Grains Growth and Evolution in Mg-Zn-Y

Unlike the growth of crystal, the growth of icosahedral quasicrystal seems to have its own characteristics. Petal-like icosahedral quasicrystal grains (i-phase grains) morphology usually has been observed in as-solidified quasicrystalline alloys. The distinguished morphology is belonged to a given i-phase grains growth evolution mechanism. We first introduced a novel model named “coherent” model to illustrate i-phase grains growth evolution.

Comment on “Extrinsic origin of the insulating behavior of polygrain icosahedral Al-Pd-Re quasicrystals”

In a recent paper, Dolin\ifmmode \check{s}\else \v{s}\fi{}ek et al. [Phys. Rev. B 74, 134201 (2006)] suggested that the insulating behavior of icosahedral $(i)$ Al-Pd-Re is of extrinsic origin. Large resistivities and insulatinglike properties of high resistivity samples were conjectured to result from the porous structure and the observed enhanced oxygen concentration in between the quasicrystal grains in some $i\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Re}$ samples. We point out that these hypotheses are incompatible with published work on the structure and properties of $i\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Re}$.

In situ study of quasicrystal growth by synchrotron X-ray imaging

One of the main challenges of quasicrystal science is to elucidate how the quasiperiodic order can extend so far, i.e. up to several centimetres according to the size of the single grains of various alloys routinely grown nowadays [M. Boudard, E. Bourgeat-Lami, M. de Boissieu, et al., Phil. Mag. Lett. 71 11 (1995); Y. Yokoyama, R. Note, A. Kimura, et al., Mater. Trans. JIM 38 943 (1997); A. Langsdorf, and W. Assmus, J. Cryst. Growth 192 152 (1998)]. Noticing that most of the present knowledge on the growth of quasicrystal grains has been collected after their cooling at room temperature, we have carried out the first in situ and real time observation of this peculiar process, which has clearly disclosed both the shape of the growing interface and its defective state. Therefore we have studied the solidification of an AlPdMn alloy giving quasicrystal grains by synchrotron X-ray imaging, combining thereby radiography and X-ray topography techniques. Radiography allowed us to clearly evidence a facetted growth proceeding by lateral motion of ledges at the solid–melt interface and controlled by the interface kinetics, rather than by local heat flow as was widely thought. Thus, a realistic estimate of the kinetic coefficient was deduced from the solid–melt interface undercooling, which indicates that quasicrystal growth is more comparable with the growth of both semiconductors and oxides than with that of pure metals. X-ray topographs, recorded simultaneously with radiographs, revealed that a lot of strain and defects are generated in the quasicrystal grains during their growth, which could be related to the growth process itself and very informative on the origin of the stability of the quasicrystal lattice.

Synchrotron X-ray diffractometry and imaging of strains and defects in icosahedral quasicrystal grains

In order to better understand the long-range propagation of quasicrystalline order, as well as quasicrystal stability, it is important to know if defects are generated in the quasicrystal grains during their growth. Previously, we studied the degree of perfection of about ten icosahedral quasicrystal grains of various alloys (Al–Pd–Mn, Al–Cu–Fe, Zn–Mg–Y), as grown or annealed, and we disclosed that some of them were much more perfect than the others. In this work we have concentrated on another slice of such a grain of the Al–Pd–Mn alloy. Similarly, we have performed an extensive synchrotron X-ray topographic investigation of strain and defects in this grain, combined with phase-contrast radiography and high resolution X-ray diffraction examinations. Very few two-lobe contrasts associated with pores and no loop-shaped contrasts were observed on the X-ray topographs, but straight line segments and band contrasts have been identified. Line segments could be considered as the result of the climbing of polygonal dislocation loops, as observed by TEM by Caillard and coworkers. This would indicate that most strains and defects observed in quasicrystal grains, at room temperature, are the result of stresses (external and internal) acting after growth.

Extended investigation of porosity in quasicrystals by synchrotron X-ray phase contrast radiography—I: In icosahedral AlPdMn grains

The porosity was studied using coherent synchrotron X-ray radiography, firstly in several bulky icosahedral Al–Pd–Mn quasicrystals, grown in different laboratories by various techniques (Bridgman, Czochralski, Flux) and then in quasicrystal grains of many other alloys and structures (Part II). In this part I, we report a quantitative analysis which has been carried out in order to find some correlation of this porosity with some growth or after-growth cooling parameters (growth parameters). The size distribution of pores was observed to change drastically among samples grown with different growth parameters as well as in those extracted from the same ingot and no mode values were evidenced when we plotted the distribution of pores in all investigated as-grown samples. These changes point out the influence of the solidification process on the occurrence of porosity, especially the influence of the vacancy migration. These results are used to discuss the two main current hypotheses on the origin of porosity.

Anomalous transmission of x-rays in quasicrystals

The perfection of one of the quasicrystal grains with the highestdegrees of perfection was checked by means of x-ray diffractometry and x-raytopography. Full widths at half maximum of de-convoluted reflectivity curveswere close to the theoretical values and a partial Borrmann effect was clearlyvisible in the measured reflection and transmission curves, whereas thetopographs still showed rather strong intricate contrast, which means thatstrong deformations (defects) still exist in the grain. Consequently,dynamical effects like anomalous transmission were confirmed to exist inselected quasicrystalline grains, but even then the structural quality isstill far from being `highly perfect'.

Effect of annealing on the structural perfection of Al–Pd–Mn icosahedral quasicrystal grains

The effect of annealing (up to 750°C) on the structural perfection of stable quasicrystals has been studied, for the first time, in large grains of the icosahedral Al–Pd–Mn phase. This study was performed by combined synchrotron X-ray diffractometry and imaging techniques. It has revealed that the quasicrystalline quality improves as long as no precipitates are formed. The improvement was mainly ascribed to the relaxation of the strain field around voids, which are often observed in quasicrystals after growth.

Investigation of structural defects and inhomogeneities in Al-Pd-Mn icosahedral quasicrystals by combined synchrotron X-ray topography and phase radiography

Abstract Phase-sensitive radiography, made possible by the lateral coherence of the beams from third-generation synchrotron-radiation sources, has been combined with monochromatic X-ray topography in an investigation of structural defects and inhomogeneities in the bulk of Al-Pd-Mn icosahedral single quasicrystal grains. Loop-shaped defects previously observed by X-ray topography are thus related with holes and second phase precipitates revealed by phase radiography. The evolution of these defects after annealing provides clues on their nature and formation.

Complementary observations of defects in quasicrystals by X-ray topography and electron microscopy

Further information on defects previously [J. Gastaldi et al. Phil. Mag. Lett. 72 (1995) 311.] observed by X-ray topography in quasicrystalline grains, and that show a loop-shaped contrast, was obtained by combining this technique with electron microscopic observations and chemical analyses. The experiments, carried out either at room temperature or during in situ and real-time high temperature annealing, allowed to get clues on the relationships between these loop-shaped defects, the growth process of the quasicrystal grains and the defect behaviour under the annealing treatment.

Growing perfect quasicrystals.

We present an approach for aggregating Penrose tiles to form a defect-free, 2D pentagonal quasicrystal tiling. Contrary to conventional wisdom, defect-free quasiperiodic tilings can be constructed by use of local rules alone. The results provide new insights as to how materials with only short-range atomic interactions can grow large, nearly perfect quasicrystal grains.