Icosahedral quasicrystal in annealed ZnMgSc alloys

A new icosahedral quasicrystal has been found in Zn 85-x Mg x Sc 15 ( x = 3,4,5) alloys annealed at 922K for 5-100h. Electron diffraction and powder X-ray diffraction experiments indicate that this quasicrystal has a primitive icosahedral quasilattice (P type) and a six-dimensional lattice parameter a 6D =0.7115nm. The stoichiometric composition of the icosahedral quasicrystal was estimated to be close to Zn 80 Mg 5 Sc 15, since the icosahedral quasicrystal coexists in the x = 5 samples with small amounts of a MgZn 2 -type Laves phase and a Zn 17 Sc 3 -type cubic phase with lattice parameter a =1.3854nm. The latter is interpreted to be a 1/1-type approximant crystal of the icosahedral quasicrystal. The atomic cluster included in the Zn 17 Sc 3 -type cubic phase indicates a structural similarity between the Zn-Mg-Sc quasicrystal and the Cd-Yb (and Cd-Ca) icosahedral quasicrystals recently reported.     

Evolution of superlattice order in Al–Mn quasicrystals and its relation to face-centred icosahedral quasicrystals

Abstract

We have established a simple icosahedral to face-centred icosahedral ordering transformation in Al–Mn quasicrystals. This result strongly supports the view that the recently discovered face-centred icosahedral quasicrystal in ternary Al–Cu–Fe and related alloys represents a long-range superstructure of simple icosahedral quasicrystals     

Relations between quasicrystals and crystalline phases in Al─Li─Cu─Mg alloys: A new class of approximant structures

Abstract

From an electron-diffraction study the crystalline c-phase, observed together with the quasicrystals in an Al─Li─Cu─Mg alloy, has been analysed as exhibiting an almost perfect icosahedral symmetry. Orientation relationships with quasicrystals are determined and serve to define a new class of approximant structures, including, beyond the c-phase, the Ti₂ Ni and Ti₂ Fe structures.     

Cu-based icosahedral quasicrystal formed in Cu-Ga-Mg-Sc alloys

The first-reported Cu-based icosahedral quasicrystal has been found as an almost single phase in Cu 48 Ga 34 Mg 3 Sc 15 alloy annealed at 1043K for 61h. On the basis of our experiments, this icosahedral quasicrystal is expected to be one of the equilibrium phases in this alloy system. Powder X-ray diffraction and electron diffraction experiments revealed that the quasicrystal exhibits a high degree of structural perfection and has a primitive type quasilattice with a six-dimensional lattice parameter a 6D =0.6938nm. The existence of a Cu 3.7 Ga 2.3 Sc-type structure, which is a bcc structure with diffraction symmetry m3, suggests that the Cu-Ga-Mg-Sc quasicrystal is to be classified into a new structural type to which Cd-based icosahedral quasicrystals and the Zn-Mg-Sc quasicrystal belong. This new type has a characteristic local atomic configuration different from both Mackay-type and Bergman-type quasicrystals.     

Production of single quasicrystals and their electrical resistivity in the Al-Pd-Re system

Single Al-Pd-Re icosahedral quasicrystals with a maximum diameter of 5mm have been grown by a slow cooling method on the basis of a partial phase diagram determined in the present study. Laue X-ray and electron diffraction verified the highly ordered structure of the single icosahedral quasicrystals. The electrical resistivity rho of the single quasicrystals was measured to be 2000- 4000muOmegacm at 300K and 3000-6000muOmegacm at 2K, revealing a negative temperature dependence with a rho_4.2K/rho_300K value smaller than 2.     

Arrangement of atomic clusters in a 2/1 cubic approximant in the Al-Zn-Mg alloy system

An arrangement of the so-called Bergman clusters of atoms in a 2/1 cubic crystalline approximant phase, which is closely related to Frank-Kasper-type icosahedral quasicrystals, in an Al-Zn-Mg alloy system is discussed in detail. The 2/1 cubic structure has eight Bergman clusters in its unit cell and the positions of the clusters can be understood as twelvefold sites in the 2/1 cubic approximant of the three-dimensional Penrose lattice. The atomic clusters are located at vertices of two types of rhombohedron, which are formed by six rhombic planes with an edge length of 3 1/2 a 2 /( 2 + 1) 1/2 and a shorter diagonal of 2 a 2 /( 2 + 1) 1/2 ( is the golden ratio and a is a quasilattice constant), which correspond to half a body diagonal and an edge length respectively of the 1/1 cubic approximant unit cell. The present results provide an important key for understanding the arrangements of atomic clusters in icosahedral quasicrystals.     

Dislocation density evolution upon plastic deformation of Al-Pd-Mn single quasicrystals

Dislocation density studies have been performed on icosahedral Al-Pd-Mn single quasicrystals after plastic deformation and after subsequent heat treatment. The deformation tests were carried out at a constant strain rate of 10- 5 s-1 at temperatures between 695 and 820 C. The heat treatments were performed at 730 C, corresponding to one of the deformation temperatures. The development of the dislocation density during heat treatment and that during plastic deformation are compared. The experimental data are interpreted using a kinetic equation, which describes the evolution of the dislocation density during deformation. Numerical values for the dislocation multiplication constant and the annihilation rate for icosahedral Al-Pd-Mn are presented.     

Nanoprecipitates of icosahedral phase in quasicrystal-strengthened Mg-Zn-Y alloys

Extensive microstructural studies have been performed with respect to the formation of the icosahedral quasicrystalline phase and its relationship to other phases in Mg 95 Zn 4.2 Y 0.8 alloy. The icosahedral phase forms as an intergranular eutectic phase as well as precipitates in the matrix. The precipitates are nanosized (typically 50 nm) with a definite orientation relationship with the matrix, sharply faceted on twofold planes which are on the basal and prismatic planes of the matrix. The detailed crystallographic relationship with the matrix is described. The icosahedral phase is occasionally found to coexist with the cubic W-Zn 3 Mg 2 Y 3 phase with a definite crystallographic relationship.     

Formation of Ga–Pd–Sc icosahedral quasicrystal and approximant phases

ABSTRACT

We report the formation of an icosahedral quasicrystal and its approximant in the Ga–Pd–Sc alloy. The primitive-type quasicrystal with a six-dimensional lattice constant of 0.713?nm formed in the melt-quenched Ga₅₃Pd₃₀Sc₁₇ alloy, with a similar composition to that of the Ga₅₅Pd₃₀Sc₁₅ 1/1 approximant. The atomic-scale observation and chemical analysis of the 1/1 approximant by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that the approximant consisted of Tsai-type clusters with a characteristic chemical ordering. Furthermore, a series of approximants were observed in the Ga₅₅Pd₃₀RE₁₅ alloys by replacing Sc with other rare-earth elements (REs) (RE?= Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). These approximants, which have relatively small lattice constants and consequently smaller RE–RE distances compared with those for other RE-containing Tsai-type approximants, may be candidates for materials with novel electronic and magnetic properties similar to those observed in Au-based quasicrystals and approximants.     

A new orthorhombic approximant of the icosahedral Al–Cu–Fe quasicrystal

We report on the formation of a new crystalline approximant phase of the icosahedral (i-)Al–Cu–Fe quasicrystal. This phase is formed during sintering of Al-based composites reinforced with i-AlCuFeB quasicrystalline particles. The structure of this phase has been characterized by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). TEM revealed that it is a B-centred orthorhombic phase with lattice parameters a = 1.166 nm, b = 1.195 nm and c = 3.44 nm. Its chemical composition, as determined by electron energy loss spectroscopy (EELS), is close to Al_76.9Cu_2.7Fe_20.4, with an average number of valence electrons per atom e/a of 1.92, similar to the value in all other approximants of the i-phase discovered thus far. Initial results on local atomic arrangements along one of its pseudo-5-fold axes are also presented.     

Phason elastic constants of the icosahedral Al-Pd-Mn phase derived from diffuse scattering measurements

The diffuse scattering in the diffraction pattern of the icosahedral Al-Pd-Mn quasicrystalline phase has been measured on an absolute scale by X-ray and neutron scattering on single-grain samples. Most of the diffuse scattering can be interpreted in the framework of the elasticity theory of icosahedral quasicrystals considering only phason fluctuations. At room temperature the absolute values of the K₁/k_BT and K₂/k_B T phason elastic constants are of the order of 0.06 and 0.031atom^-1. The amount of diffuse scattering intensity is insensitive to the sample annealing treatment.     

Use of electron channelling contrast imaging to assess near-surface mechanical damage

The crystal structure of a cubic phase in the system Zn-Mg-Er has been solved by a combination of high-resolution electron microscopy and X-ray powder diffraction. This phase is considered to be related to that of the quasicrystalline phase. The structure consists of 448 atoms in the unit cell with lattice constant of a ₀ = 20.20Å and the space group is F43m. Important structural elements in the cubic structure are interpenetrating icosahedral units around Zn and Mg atoms and Frank-Kasper polyhedra around the Mg atoms. No giant icosahedral atomic cluster, such as the 136-atom Bergman cluster, was found in the stucture.     

Effects of Ag and Pd on the nucleation and growth of the nano-icosahedral phase in Zr65Al7.5Ni10Cu7.5M10 (M = Ag or Pd) metallic glasses

The nucleation and growth of a nano-icosahedral phase from a supercooled liquid region of Zr₆₅Al_7.5Ni₁₀Cu_7.5M₁₀ (M = Ag or Pd) glasses have been examined by differential scanning calorimetry and transmission electron microscopy. The growth rate of the icosahedral phase is nearly constant at the initial stage and much slower than that of the Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass. The homogeneous nucleation rate has a maximum value of 4.4 x 10²⁰ m^-3 s^-1 at 695 K in the Zr₆₅Al_7.5Ni₁₀Cu_7.5Ag₁₀ glass, which is approximately 10² times higher than that for the formation of quasicrystalline phase in the Zr_69.5Al_7.5Ni₁₁Cu₁₂ glass and 10⁴ times higher than that of the Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 glass. With increasing Pd content, the nucleation rate of the primary phase increases significantly and the growth rate decreases at the crystallization temperature. Thus, the addition of Ag and Pd is effective for an increase in the number of nucleation sites and the suppression of grain growth, which is the main reason for the formation of icosahedral nanoparticles. The significant increase in the nucleation rate is due to an increase in the number of nucleation sites resulting from the short-range ordering consisting of Zr-(Ag or Pd) strong pairs. It is implied that the strong pair Zr-(Ag or Pd) also contributes to the restraint of the long-range rearrangements of the constitutional elements. The formation of the nanoicosahedral phase suggests that icosahedral short-range order exists in the glassy state in the present alloys.     

Frank's 'cubic' hexagonal phase: An intermetallic cluster compound as an example

Frank introduced in 1965 the novel idea of projection from a four-dimensional cube to recover the points of a hexagonal lattice with a special c / a ratio of ( 3 - 2 ½. This was called a cubic hexagonal crystal, as there was a similarity to the conventional cubic crystals in that directions were perpendicular to planes with the same Miller-Bravais indices. While a number of crystals in the NiAs-Ni 2 In family have been reported with this special axial ratio, the number of atoms in the unit cell is small. As the first example of an intermetallic cluster compound, we identify µ-Al 4 Mn, µ-Al 4 Cr, Zn-Mg-Sm and a host of related intermetallics featuring a special aggregate of icosahedra as Frank's 'cubic' hexagonal phase or its variant. The metric is generated by the Friauf polyhedra and the icosahedral linkages and leads to a multimetric crystal and several interesting connections with hexagonal quasicrystals, hexagonal phases and derivative orthorhombic and lower-symmetry structures.     

Sign determination of the 57 Fe electric-field gradient in Al-based crystals and icosahedral quasicrystals

Abstract

The icosahedral quasicrystals i-AIMn, isomorphically substituted by 28 at.% Fe or by a mixture of (CrFe) atoms, have been studied for the first time by in-field Mössbauer spectroscopy in order to determine the sign and asymmetry parameter of the dominant electric-field gradient (EFG) term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si crystalline phases have been studied. We show that the previous Mössbauer results are inadequate for determining whether there are two sites in the quasicrystalline structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with an asymmetry parameter of about 0·6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. One crystalline phase, the hexagonal β phase, is found to have a very similar quadrupole effect to that found in i-Al(MnFe). In addition it is found that this phase undergoes a change which is at least partly of magnetic origin.     

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     

X-ray study of phason strain in Al-based quasicrystals

Abstract

X-ray powder diffraction spectra have been measured for melt-spun Al-Si–Mn, Al-Cu–Fe and Al-Li–Cu and Bridgman-grown Al-Li-Cu quasicrystals to investigate the phason strain frozen during the solidification process. For the melt-spun samples, most of the peaks have a shoulder or a tail which is attributed to the anisotropic linear phason strain. The peak shapes for the three melt-spun samples are essentially the same, which indicate that the same type of linear phason is quenched in these samples. In contrast, symmetric peak shapes without shoulders nor tails are seen for the Bridgman-grown Al-Li–Cu, indicating that it contains little linear phason strain.

Our findings are discussed with respect to the three linear phason models which can be derived by degradation of the symmetry from ideal icosahedral group m35 to its maximal subgroups 3m, m3 and 5m. Of these three, the model for 3m reproduces best the observed shapes and widths of the peaks.     

Crystal structure of a hexagonal phase and its relation to a quasicrystalline phase in Zn-Mg-Y alloy

The crystal structure of a Zn-Mg-Y hexagonal phase, considered to be related to that of the quasicrystalline phase, has been determined by single-crystal X-ray diffraction. The atomic model, refined to a final R value of 0.027, has the composition Zn65.22Mg27.92Y6.86 and 92 atoms in a unit cell with lattice constants a = 14.579(2) A and c = 8.687(1) A and the space group P63/mmc. The Y atoms are situated at two different sites; one is fully occupied by Y atoms, while the other is shared with Mg atoms. The structure can be characterized by a layer structure stacked along the c axis, and also viewed as a columnar structure composed of fused Friauf polyhedras sharing hexagonal rings. The similarity of this hexagonal phase to the MgZn2 phase is shown. The structural relationship between this hexagonal phase and the icosahedral phase is also discussed.     

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.     

Decagonal quasicrystal with ordered body-centred (CsCl-type) hypercubic lattice

An ordered structure, which was found in a decagonal quasicrystal with 0.4 nm periodicity in a conventionally solidified Al₇₀Ni₂₀Ru₁₀ alloy, has been examined theoretically by the projection of an ordered body-centred (CsCl-type) hypercubic lattice. The ordered structure can be characterized as an ordered arrangement of two kinds of atom columnar cluster with different directions of pentagonal symmetry. Two lattices, which are constructed by connecting the atom clusters in the same directions, can be interpreted as ordered sublattices formed by each of vertex and body-centred positions of the CsCl-type hypercubic lattice. Diffraction patterns calculated from the ordered and disordered body-centred hypercubic lattices replicate well the electron diffraction patterns observed from the Al-Ni-Ru and other decagonal quasicrystals.