Content uploaded by D. Gratias

Author content

All content in this area was uploaded by D. Gratias on Apr 15, 2014

Content may be subject to copyright.

A metallic solid (Al-14-at. pct.-Mn) with long-range orientational order, but with icosahedral point group symmetry, which is inconsistent with lattice translations, has been observed. Its diffraction spots are as sharp as those of crystals but cannot be indexed to any Bravais lattice. The solid is metastable and forms from the melt by a first-order transition.

Content uploaded by D. Gratias

Author content

All content in this area was uploaded by D. Gratias on Apr 15, 2014

Content may be subject to copyright.

... The surfenergy γ s appearing in Equation (1) as the solid-vapour interfacial tension γ sv can be calculated from the contact-angle Θ measurement as graphically illustrated in Figure 1a. In contrast, the liquid-vapour interfacial tension γ lv and the solid-liquid contribution γ sl need to be determined from Equation (1). Therefore, it is necessary to perform the measurement of the contact angle with two different liquids. ...

... The surfenergy appearing in Equation (1) as the solid-vapour interfacial tension can be calculated from the contact-angle measurement as graphically illustrated in Figure 1a. In contrast, the liquid-vapour interfacial tension and the solid-liquid contribution need to be determined from Equation (1). Therefore, it is necessary to perform the measurement of the contact angle with two different liquids. ...

Super-hydrophobic surfaces and coatings have stimulated a great deal of research, with the aim being to achieve better wetting properties. Factors such as surface chemistry and roughness play an important role in changing the surface energy, which in turn leads to changes in the wettability. Here, we have analysed the time dependence of the oxide layer and possible surface adsorbates on the surface topography of an Al59Cu25Fe13B3 quasicrystalline material in relation to changes in the wettability. The quasicrystalline matrix phase was 94% of the sample volume, and it was covered by a very smooth, amorphous oxide layer. The AlB12 and AlFe2B2 boron-rich phases were embedded in the quasicrystalline material as a result of the 3 at.% boron addition, which made atomisation of the material a simpler process. Under ambient conditions, the sample was naturally covered by an oxide layer; therefore, it is referred to as “surfenergy” to distinguish it from the conventional surface energy of a bare quasicrystal surface. The growth of the oxide layer with atmospheric ageing and annealing at 500 °C in air for various times was investigated for both cases. The phase most prone to oxidation was the boron-rich AlFe2B2, which influenced the topography of the surface and accordingly the wetting behaviour of the specimen. We demonstrated that the surfenergy depends on the polar component, which is the most sensitive to the operating conditions. A correlation between the surfenergy components and the surface roughness was found. In addition, theoretical models to determine the wettability were included.

... The connection of aperiodic protosets and nonperiodic order in tilings to quasicrystals (discovered in 1982 by Shechtman [35]) stimulated much research in the 1980s onward. An excellent overview of order in aperiodic tiling spaces is given in [3]. ...

Eshelby tensors serve as the basis of micromechanics which should be explored first to study the effective mechanical behavior of heterogeneous materials. In this paper, Eshelby tensors are extended from isotropic materials to quasicrystals. By utilizing Green’s functions and Cauchy’s residue theorem, simple and unified expressions of Eshelby tensors for one-dimensional (1D) orthorhombic quasicrystal are derived. Specifically, the closed-form Eshelby tensors are given when the shapes of the inclusions are spheroid, elliptic cylinder, rod-shaped, penny-shaped, and ribbon-like, respectively. Furthermore, the effective stiffnesses of 1D orthorhombic quasicrystal are obtained in view of the received Eshelby tensors and the Mori–Tanaka mean theory. Finally, parameter studies are carried out, and the effect of material properties and volume fraction on the effective overall material properties are investigated.

Cold spray (CS) technology has proven an enormous potential in the production of composite coatings, enabling a production of materials with superior qualities such as enhanced tribological behavior. This study aims to investigate the tribological properties of CS Al-based composite coatings reinforced by quasicrystalline (QC) particles. Two different Al alloys were used as the matrix, AA 6061 and AA 2024, and mixed with Al-based QC particles (Al-Cr-Fe-Cu) at different Al/QC ratios. A room-temperature ball-on-disc test was then used to evaluate the wear resistance of the CS composite coatings in air and compared to those of the CS non-reinforced Al alloy coatings as well as cast counterparts (AA 6061-T6). We have demonstrated that CS could be employed to produce dense and thick Al-QC composites. Further, the addition of the QC particles into the structure increased the wear resistance of the matrix resistance up to 8 times.

The atomic structure of the recently discovered antiferromagnetic (AFM) Ga50Pd35.5Tb14.5 2/1 approximant to quasicrystal with the space group of Pa3¯ (No. 205), a=23.1449(0)Å was determined by means of single-crystal x-ray diffraction. The refined structure model revealed two main building units, namely, a Tsai-type rhombic triacontahedron (RTH) cluster with three concentric inner shells and an acute rhombohedron filling the gaps in between the RTH clusters. One of the interesting findings was a very low number of chemically mixed sites in the structure, which amount to only 7.40% of the all the atomic sites within an RTH cluster. A disorder-free environment was noticed within a nearest neighbor of an isolated Tb3+ ion, which is presumably one of the main contributors in enhancing AFM order in the present compound. The second significant finding was the observance of an orientationally ordered trigonal pyramidlike unit with a height of 4.2441(7)Å at the center of the RTH cluster. Such a unit is noticed to bring structural distortion to outer shells, particularly to the surrounding dodecahedron cage being another possible contributor to the AFM order establishment in the present compound. The results, therefore, are suggestive of a possible link between chemical/positional order and the AFM order establishment.

By their very nature, discoveries are often unexpected and thus unpredicted. To a considerable extent, discoveries in the geological realm are disconnected from those in the laboratory sciences. In unusual situations, spectacular advances in cognate sciences result in geological or mineralogical discoveries. Such is the case with fullerenes and quasicrystals, whose histories will be briefly explored in the following pages.KeywordsFullerenesQuasicrystalsDiscoveryIcosahedriteSolar system

Bond-orientational order in molecular-dynamics simulations of supercooled liquids and in models of metallic glasses is studied. Quadratic and third-order invariants formed from bond spherical harmonics allow quantitative measures of cluster symmetries in these systems. A state with short-range translational order, but extended correlations in the orientations of particle clusters, starts to develop about 10% below the equilibrium melting temperature in a supercooled Lennard-Jones liquid. The order is predominantly icosahedral, although there is also a cubic component which we attribute to the periodic boundary conditions. Results are obtained for liquids cooled in an icosahedral pair potential as well. Only a modest amount of orientational order appears in a relaxed Finney dense-random-packing model. In contrast, we find essentially perfect icosahedral bond correlations in alternative "amorphon" cluster models of glass structure.

Complex alloy structures, particularly those of transition metals, are ; considered as determined by the geometricnl requirements for sphere packing. A ; characteristic of the class of structures discussed is that tetrahedral groupings ; of atoms occur everywhere in the structure--alternatively stated, coordination ; polyhedra have only triangular faces. The topological and geometrical properties ; of such polyhedra are examined and rules and theorems regarding them are deduced. ; Justification is given for the prominence of four such polyhedra (for ; coordination numbers of 12, 14, 15, and 16) in actual structures. General ; principles regarding the combination of these polyhedra into full structures are ; deduced and necessary definitions are given for terms that facilitate the ; detailed discussion of this class of structures. (auth);

Three-dimensional bond orientational order is studied via computer simulations of 864 particles interacting through a Lennard-Jones pair potential. Long-range orientational fluctuations appear upon supercooling about ten percent below the equilibrium melting temperature. The fluctuations suggest a broken icosahedral symmetry with extended correlations in the orientations of local icosahedral packing units.

An analysis of orientational order in dense, three-dimensional liquids is presented. The alignment of neighboring regions of local orientational order with both icosahedral and cubic symmetry is studied with the use of mean-field theory. The theory predicts a first-order transition to a phase which possesses long-range orientational order but no translational order. This is the phase which appears to have been seen in recent computer simulations of a supercooled liquid.

A defect description of liquids and metallic glasses is developed. In two dimensions, surfaces of constant negative curvature contain an irreducible density of point disclinations in a hexatic order parameter. Analogous defect lines in an icosahedral order parameter appear in three-dimensional flat space. Frustration in tetrahedral particle packings forces disclination lines into the medium in a way reminiscent of Abrikosov flux lines in a type-II superconductor and of uniformly frustrated spin-glasses. The defect density is determined by an isotropic curvature mismatch, and the resulting singular lines run in all directions. The Frank-Kasper phases of transition-metal alloys are ordered networks of these lines, which, when disordered, provide an appealing model for structure in metallic glasses.

The stability of the shape of a moving planar liquid‐solid interface during the unidirectional freezing of a dilute binary alloy is theoretically investigated by calculating the time dependence of the amplitude of a sinusoidal perturbation of infinitesimal amplitude introduced into the planar shape. The calculation is accomplished by using gradients of the steady‐state thermal and diffusion fields satisfying the perturbed boundary conditions (capillarity included) to determine the velocity of each element of interface, a procedure justified in some detail. Instability occurs if any Fourier component of an arbitrary perturbation grows; stability occurs if all components decay. A stability criterion expressed in terms of growth parameters and system characteristics is thereby deduced and is compared with the currently used stability criterion of constitutional supercooling; some very marked differences are discussed.

Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies