Icosahedral Alloys Research Articles

Substitutional site preference in Al−Cu−Ge−Mn icosahedral alloys

Evidence is presented, based on analyses of57Fe Mossbauer spectra of magnetically ordered Al−Cu(Fe)−Ge−Mn icosahedral alloys measured at 4.2 K, for the dependence of Fe substitution upon the concentration of Fe. It is found that Fe substitutes approximately randomly on transition-metal (TM) sites, i.e., it enters both magnetic and nonmagnetic class of sites, for very small concentration of Fe. However, for larger concentration, Fe enters preferentially the nonmagnetic class of sites. This is discussed in terms of recent theoretical calculations which predict the dependence of a magnetic moment of a TM atom upon the volume associated with it in an alloy.

Quasicrystalline Ga–Pd–M (M=Cr, Mn or Fe) Alloys Prepared by Rapid Solidification

New icosahedral alloys were found to form at Ga 70 Pd 20 M 10 (M=Cr, Mn or Fe) in a rapidly solidified state. The icosahedral phase has the simple P-type quasicrystalline structure and its grain size is in the range from 1 to 3 μm. The icosahedral phase (1) in these alloys is in a metastable state, and the subsequent heating causes the decomposition into crystalline phases (Cryst.) through two-stage reactions of I→I+Cryst.→Cryst., the total exothermic heat (ΔH x ) being 2.3 kJ/mol for M=Cr, 1.5 kJ/mol for M=Mn, and 0.76 kJ/mol for M=Fe

Thermal stability of Al-Cu-Fe icosahedral alloys

Thermal stability of Al-Cu-Fe icosahedral alloys

Electric Quadrupole Interactionsand Magnetic Susceptibility of Icosahedral Al70Fe20Ta10

X-ray diffraction, 57 Fe Mössbauer spectroscopy, and magnetic susceptibility measurements have been performed in the temperature range 4.2-293 K on the Al 70 Fe 20 Ta 10 icosahedral alloy. It was found that Fe atoms are distributed among a multiplicity of sites, which was interpreted as evidence of intrinsic disorder characteristic for icosahedral alloys. It was shown that the distribution of electric quadrupole interactions is better described by the Gaussian shape than by the shape predicted by the shell model. It was found that Al 70 Fe 20 Ta 10 is not magnetically ordered. We find that Al 70 Fe 20 Ta 10 is a paramagnet and that the claim made in the literature of the simultaneous occurrence of Curie-like and Pauli paramagnetism is an artifact resulting from previously unconsidered second phases.

Random cluster models for icosahedral phase alloys

We present results on computer generated random cluster models for icosahedral phase alloys. By the application of physically motivated constraints on the local atomic cluster configurations, the model achieves long range translational order comparable to, or greater than, that found in simple icosahedral alloys such asi-AlMnSi andi-AlLiCu. The parallel and perpendicular space structures are explored in some detail including a comparison with experimental powder diffraction patterns and an examination of the phason fluctuations. The latter are shown to decrease markedly with increased constraint on the local environment, but the remaining phason strain would seem finally not to vanish with increasing model size. Our model is compared with other cluster models for quasicrystalline materials and is shown to possess a density and connectivity very close to those of Elser's (best) model and those predicted by Henley for a canonical tiling. The relation of this model to recently discovered icosahedral phase alloys with resolution limited diffraction peak widths, which are essentially free of phason strain, is also discussed.

From the as-quenched to the annealed state in icosahedral AlCuFe. effect of phasons on the local order

Abstract Rapidly solidified Al63Cu25Fe12 icosahedral alloys have been studied by extended X-ray absorption fine structure (EXAFS) in the as-quenched state and after annealing. Moreover, the temperature dependence of the EXAFS spectra have been studied in detail for the two types of states during in-situ time resolved experiments.

Atomic dynamics of metallic glasses and icosahedral alloys (quasicrystals)

The most recent results from the investigation of the atomic dynamics of amorphous and icosahedral alloys using neutron inelastic scattering techniques are briefly presented. Short wavelength collective excitations, low energy modes, and structural relaxation in metallic glasses are covered. The dynamics of the disorder-dominated metastable rapidly quenched icosahedral alloys is contrasted with the first very detailed results obtained most recently for the phonon density of states and the dispersion of acoustic phonons in stable slowly cooled icosahedral alloys.

Site of ruthenium in icosahedral AlMnRuSi

In order to investigate the sites occupied by ruthenium atoms in the Al 76Mn 14Ru 7Si 3 and Al 68Mn 20Ru 8Si 4 icosahedral alloys, the differential structure factors (DSF) and the differential pair distribution functions (DPDF) concerning ruthenium were obtained by the anomalous X-ray scattering technique with synchrotron radiation. On the assumption that ruthenium atoms occupy aluminum or manganese sites in the atomic structure model (Mackay-type; Quasiperiodic Configuration of Icosahedral Clusters (QCIC) model) of the AlMn icosahedral phase, it is concluded that the ruthenium atoms are preferentially substituted at manganese sites which almost correspond to the vertices of the three-dimensional Penrose tiling (3D-PT) in i-Al 76 Mn 14 Ru 7 Si i3. In the case of i-Al 68 Mn 20 Ru 8 Si 4, ruthenium atoms are substituted for two kinds of atomic sites, one of which is the manganese site and the other is that of aluminum atoms at the innermost shell of Mackay icosahedra.

Equilibrium faceting shape of auasicrystals at low temperatures: cluster model

Abstract We calculate the zero-temperature eauilibrium crystal shape (ECS), in a model specialized for the atomie structures of known icosahedral alloys such as Al-Cu-Li and Al-Cu-Fe. We use the approximations of assuming a cluster model of the atomie structure, taking a lattice-gas description of the interface and representing the non-periodic structure by a large cubic celi (rational approximant) of 32 clusters. We find that the facets normal to the fivefold and twofold icosahedral symmetry directions dominate the ECS, with additional smaller facets normal to the threefold directions.

Mössbauer measurements and susceptibility measurements on crystalline and icosahedral AlCuFe alloys

In order to obtain more information on the atomic decoration and short range order of the icosahedral AlCuFe alloys, we compared the stable icosahedral phase with the crystalline phases of the parent composition and four icosahedral alloys around the thermodynamical stable composition by means of magnetization and Mössbauer measurements. The measurements of the magnetic susceptibility show that in the AlCuFe icosahedral alloys as well as in some of the parent crystalline compounds the magnetic moment of iron is suppressed, the magnetic behaviour being dominated by the electronic structure of aluminium and copper. It seems that the short range order in the studied icosahedral phases has some similarity with the one occurring around the 4(i) iron sites with 10 aluminium nearest neighbours in the crystalline structure of Al 13Fe 4.

Magnetic properties of Al-Ge-Mn and Al-Cu-Ge-Mn icosahedral alloys.

${\mathrm{Al}}_{52.5}$${\mathrm{Ge}}_{22.5}$${\mathrm{Mn}}_{25}$, ${\mathrm{Al}}_{52.5}$${\mathrm{Ge}}_{22.5}$${\mathrm{Mn}}_{24.85}$${\mathrm{Fe}}_{0.15}$, ${\mathrm{Al}}_{40}$${\mathrm{Cu}}_{10}$${\mathrm{Ge}}_{25}$${\mathrm{Mn}}_{25}$, and ${\mathrm{Al}}_{40}$${\mathrm{Cu}}_{9.94}$${\mathrm{Fe}}_{0.06}$${\mathrm{Ge}}_{25}$${\mathrm{Mn}}_{25}$ icosahedral alloys and AlGeMn and ${\mathrm{AlGeMn}}_{0.995}$${\mathrm{Fe}}_{0.005}$ crystalline alloys have been studied with x-ray diffraction, differential thermal analysis, magnetization, and $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy. It is found that AlGeMn, a ferromagnet, is the major second phase present in the samples of icosahedral Al-Ge-Mn and Al-Cu-Ge-Mn systems, and is thus partially responsible for the observed ferromagnetism in these alloys. This ferromagnet is also the main crystallization product of these systems. Room-temperature $^{57}\mathrm{Fe}$ M\"ossbauer spectra show that Fe atoms in these systems bear no magnetic moment and are distributed among a multiplicity of sites, which is interpreted as evidence of intrinsic disorder present in icosahedral alloys. Analysis of 4.2-K M\"ossbauer spectra shows that less than half of the Fe atoms possess a very small magnetic moment in both systems, which is the first direct evidence supporting the notion of two classes of transition-metal sites in Al-based icosahedral alloys. It is argued that the magnetic moment of Mn atoms in these systems as well as the Curie temperatures must be very small. It is noted that annealing of rapidly quenched Al-Ge-Mn and Al-Cu-Ge-Mn alloys leads to the occurrence of high values of coercive force and saturation magnetization.

Band-structure effects on the electronic properties of icosahedral alloys.

Distinct deviations from smooth trends of the Hall coefficient, thermopower, and electronic specific heat with the composition in icosahedral AlCuMg and GaZnMg alloys are observed. These arise in regions where the free-electron Fermi wave vector is predicted to lie near Bragg planes determined from strong x-ray-diffraction maxima. Our results provide the first experimental evidence for anomalies in the electronic density of states near the Fermi level in these alloys, which may be important in the stabilization of this class of icosahedral phases via Hume-Rothery considerations.

Anomalous X-ray scattering study of amorphous and icosahedral Al75Cu15V10 alloys

Atomic structures of amorphous and icosahedral Al75Cu15V10 alloys were precisely examined by the ordinary and anomalous X-ray diffraction techniques. Some distinct features of intensity profiles, and coordination numbers and interatomic distances determined from the RDF analysis, indicate the presence of the icosahedral short-range ordering clusters in the as-quenched amorphous sample and other annealed amorphous samples, which are similar to the basic atomic structure constructing the icosahedral phase. The intensity differences of the amorphous and icosahedral phases obtained in the anomalous X-ray scattering (AXS) at the CuK-absorption edge suggest that copper atoms are homogeneously dispersed in the icosahedral short-range ordering clusters.

High-temperatures magnetism in icosahedral Al52.5Ge22.5Mn25 and Al52.5Ge22.5Mn24.85Fe0.15

Magnetization and 57Fe Mössbauer-effect measurements are reported for the magnetically ordered icosahedral alloys Al52.5Ge22.5Mn25 and Al52.5Ge22.5Mn24.85Fe0.15. The lack of saturation of magnetization suggests a noncollinear magnetic structure. It is shown that transition-metal atoms are distributed among a multiplicity of sites, which is interpreted as evidence for atomic disorder characteristic of icosahedral alloys. The reported and literature data show that the two-site model leads to unphysical Mössbauer parameters. It is shown that 57Fe Mössbauer spectra are due to the simultaneous presence of the electric-quadrupole interaction and the magnetic-dipole interaction, and that the former is significantly stronger than the latter.

Icosahedral phase formation in high silicon and manganese content AlMnSi alloys

Single-phase icosahedral AlMnSi alloys containing up to 30 at.% Mn and 33 at.% Si have been prepared directly by quenching from the melt. Alloys of the series Al 70− x Mn x Si 30 (10⩽ x⩽30), Al 80−xMn 20Si x (6⩽ x⩽40) and Al 67−xMn xSi 33 (28⩽ x⩽32) were investigated. The regions of x for the formation of single-phase icosahedral alloys by direct quenching are x ≈ 26, x ⩽ 20 and x ≈ 29 for the three series respectively. These alloys have quasi-lattice constants in the range 0.437–0.461 nm. Details of the structural properties of these alloys as well as their thermal stability as a function of composition are discussed.

Matching rules and growth rules for pentagonal quasicrystal tilings.

We show that finite-range matching rules and growth rules exist for an infinite set of 2D pentagonal tilings other than the original Penrose tilings. There is a natural ordering of these tilings based on the range required for the rules. The results have implications for the determination of the atomic structure of icosahedral alloys.

Highly symmetric Mn sites in icosahedral Ti-Mn.

We find that the $^{55}\mathrm{Mn}$ NMR linewidth and Knight shift of icosahedral ${\mathrm{Ti}}_{63}$${\mathrm{Mn}}_{37}$ (i-TiMn) are identical to those of the crystalline phases obtained upon annealing. Comparison of the linewidth in i-TiMn and a bcc Ti-Mn solid solution shows that the Mn sites in both phases have a nearly cubic or higher symmetry. These measurements and our analysis provide strong evidence for a large density of structurally ordered, highly symmetric sites in an icosahedral alloy and demonstrate that a high degree of structural disorder is not an intrinsic property of the icosahedral phase.

New icosahedral alloys with superlattice order in the Al–Pd–Mn system prepared by rapid solidification

Abstract An icosahedral quasicrystalline phase was found to be formed in rapidly solidified Al–Pd–Mn alloys. The quasicrystalline structure was confirmed to have a superlattice structure of the simple icosahedral quasicrystal, similar to the face-centred icosahedral structure for Al–Cu–Fe quasicrystals. The intensity of the superlattice reflections increases successively with increasing Pd content, and hence Pd atoms seem to occupy predominantly the superlattice site rather than the fundamental lattice in the icosahedral structure.

Local order description of an icosahedral Al–Cu–Fe quasicrystal

Abstract A rapidly solidified Al65Cu20Fe15 icosahedral alloy has been studied by extended X-ray absorption he structure above the Cu and Fe K absorption edges. The local order has been determined around Cu and Fe atoms and compared with results obtained previously in other icosahedral alloys

Short-range quasicrystalline structure of Al–Cu–V icosahedral alloys

Abstract Extended X-ray absorption fine structure experiments are reported above the Cu and V K absorption edges in an icosahedral Al82Cu7V11 alloy. The radial distributions of neighbours around Cu and V atoms are compared with previous results obtained in other icosahedral alloys.