A new stable icosahedral quasicrystal in the Cd-Mg-Dy system

A new stable icosahedral quasicrystal has been found in annealed Cd-Mg-Dy alloys. The composition of the icosahedral phase was determined to be approximately Cd₆₆Mg₂₁Dy₁₃. Powder X-ray and electron diffraction patterns revealed that the phase has a primitive icosahedral lattice with a quasilattice parameter a_R = 0.5634 nm. The electron diffraction study confirmed that the phase has a well ordered primitive icosahedral structure.     

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.     

Stability of the F2-(Al-Pd-Mn) phase

The stability of the F2-(Al-Pd-Mn) phase has been studied by in-situ neutron diffraction on a single quasicrystal with composition Al_69.8Pd_21.4Mn_8.8. We find that the F2 phase is not stable and corresponds to a transient state in the process of the transformation of the icosahedral phase to the F2_M phase. The icosahedral-to-F2_Mphase transition occurs at around 715^oC. In the F2 phase a large amount of diffuse scattering is located close to the icosahedral Bragg reflection in place of the S ₁ superstructure reflections characteristic of the F2_m phase.     

Determination of the Burgers vector of dislocations in icosahedral quasicrystals by a high-resolution lattice-fringe technique

A technique for the determination of the full six-dimensional Burgers vector characterizing a dislocation in an icosahedral quasicrystal is presented. It is based on the lattice-fringe analysis of two high-resolution transmission electron microscopy images taken at two different sample orientations. As an example we present the analysis of a dislocation in a bent icosahedral Al-Pd-Mn quasicrystal. We obtained a Burgers vector B = A₀[-2,0,3,-2,3,0] where A₀ = 0.645nm is the six-dimensional hyperlattice constant. This result is consistent with previous results obtained by diffraction contrast analysis and convergent-beam electron diffraction techniques.     

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.     

Solid and liquid thermal expansion and structural observations in the quasicrystalline Cd84Yb16 compound

The structure of single-grain Cd₈₄Yb₁₆ samples aligned along the twofold and fivefold axes has been followed from 300 to 1050?K using high-energy synchrotron X-rays. The quasicrystal phase is stable up to its melting temperature of 914?K and has a large linear thermal expansion of 37.1?ppm?K^?1 over this temperature range. The samples melt congruently over a temperature range of less than 1?K. The liquid is 7% less dense than the solid and, upon cooling from the melt, the quasicrystal phase directly solidifies within a 1?K interval. The amount of undercooling achieved, about 5–25?K, was dependent on the cooling rate. The total scattering function of the liquid is consistent with a dilute liquid Cd structure. These results agree with suggestions that the structure of the liquid must undergo reordering in order to form the solid phases. However, there is no compelling evidence for icosahedral short-range order in the liquid prior to the formation of the quasicrystalline structure.     

Morphological study of micropits formed by anodic etching of an Al-Pd-Mn icosahedral quasicrystal

Faceted micropits obtained by electrochemical etching of a single quasicrystal of an Al-Pd-Mn icosahedral phase have been examined by optical and electron microscopy. Anodic etching of the flat surface of a fivefold plane in a solution composed of CH₃OH and HNO₃(3:1 in volume ratio) reveals a variety of icosahedrally faceted micropits which originated from pre-existing microvoids and etch pits of pentagonal pyramid shape due to the other origins, probably structural defects inside the specimen. As the dissolution progresses, the micropits make successive changes in their shapes, ending in pentagonal pyramids before they vanish.     

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.     

A new highly ordered Al-Ni-Ru decagonal quasicrystal with 1.6 nm periodicity

A new decagonal quasicrystal (the D phase) with a period of about 1.6 nm was found to form in conventionally solidified and heat-treated Al₇₅Ni₁₅Ru₁₀ alloys. The electron diffraction patterns of the Al-Ni-Ru D phase exhibit a large number of quite sharp diffraction spots located at precise decagonal symmetry positions, indicating a highly ordered decagonal quasicrystal with a long-range quasiperiodic correlation. The D phase is formed with a composition close to Al₇₄Ni₁₅Ru₁₁, as determined by energy-dispersive X-ray analysis. By means of high-resolution electron microscopy, the structural features of the Al-Ni-Ru D phase, which are obviously different from that of the Al-Pd D phase (a typical decagonal quasicrystal with 1.6 nm periodicity reported previously), have been revealed.     

Structure of a cubic phase related to Cd?Mg?rare-earth quasicrystals

The structure of a Cd₆₈Mg₁₂Dy₂₀ crystalline phase denoted as the φ-phase, which has a composition close to that of the Cd₆₆Mg₂₁Dy₁₃ icosahedral quasicrystalline phase, has been investigated by electron diffraction and scanning transmission electron microscopy (STEM). The φ phase has a fcc lattice with a = 21.6Å. High-angle annular dark-field STEM with Z contrast confirms that the phase has the Cd₄₅Sm₁₁-type structure. The atomic cluster in the structure is shown to be characterized by a Friauf polyhedron with tetrahedral symmetry.     

Nucleation of misfit dislocations in strained-layer InGaAs on GaAs

Abstract

Austenitic 316L stainless steel alloys annealed at 550°C for 100 h present a few ferrite precipitates surrounded by a new interfacial phase, here called the I-phase, that develops at the level of the austenite/ferrite interface. The I-phase presents the typical patterns of an icosahedral quasicrystal with a primitive hypercubic lattice of parameter A = 0.63 nm. The marked orientation relationships between the I-phase and the ferrite precipitates strongly suggest that this phase results from a decomposition of the ferrite and not of the austenite. The I-phase is metastable and transforms eventually after annealing at 700°C to the stable crystalline σ-phase.     

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     

Microstructural evolutions and hardness during heat treatment of Al64Cu20Fe12Si4 quasicrystal alloy

The microhardness and microstructural characteristics and subsequent heat treatment of conventionally solidified Al₆₄Cu₂₀Fe₁₂Si₄ quasicrystal were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS), differential thermal analysis (DTA), and Vickers microhardness tester. XRD analysis indicated that the conventionally solidified samples showed a quasicrystalline icosahedral phase (i-phase) together with cubic β-AlFe, tetragonal θ-Al₂Cu, and monoclinic λ-A₁₃Fe₄ crystal phases. However, the i-phase together with cubic β-AlFe and monoclinic λ-A₁₃Fe₄ phases observed heat threaded samples. As-cast and subsequently heat-treated quasicrystal samples were measured using a microhardness test device. Vickers microindentation tests were carried out on the heat-treated quasicrystal samples with the load ranging from 1 to 500?mN at room temperature. The melting point of the i-phase was determined as 900°C by DTA examinations.     

Formation of the icosahedral quasicrystalline phase in Zr70Pd30 binary glassy alloy

It is found that a single icosahedral quasicrystalline phase is formed as a primary precipitation phase in the melt-spun Zr₇₀Pd₃₀ binary glassy alloy with a two-stage crystallization process. The onset temperature of the transformation from the amorphous to the icosahedral phase is 701 K at the heating rate of 0.67 K s-1. The size of the icosahedral particles lies in the diameter range below 10 nm and the particles are distributed homogeneously. The second-stage crystallization reaction results in the formation of a Zr₂Pd phase through a single exothermic reaction. The formation of the nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the Zr-Pd binary glassy state. Comparison with the thermal stability of an icosahedral phase in the Zr-Ni-Pd system shows that the icosahedral phase is stabilized by the addition of Ni. The stabilization is due to the restraint of the long-range rearrangement of the constitutional elements resulting from the strong chemical affinity between Zr and Ni.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

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.     

A cubic approximant in the Zn-Mg-Er alloy

A cubic approximant for the icosahedral phase is found in the Zn-Mg-Er system. The preparation of the Zn-Mg-Er ternary phase (the so-called R phase) through quenching and annealing is described in detail. The R phase was found by means of scanning electron microscopy in combination with wavelength-dispersive X-ray analysis. The composition of the R phase varies around Zn₆₃Mg₂₃Er₁₄ in different samples depending on the initial composition. The structure has been studied by a combination of high-resolution transmission electron microscopy (HRTEM) and X-ray powder diffraction. The R phase is cubic with a ₀ = 2.02 nm and crystallizes in the space group F 43m. The close relationship of this phase to the icosahedral phase in the Zn-Mg-Y, RE system (RE = rare earth) is demonstrated through electron diffraction features and HRTEM images.     

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.     

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.     

Structural study of a superlattice Al-Ni-Ru decagonal quasicrystal using high-resolution electron microscopy and a high-angle annular dark-field technique

A high-quality superlattice Al-Ni-Ru decagonal quasicrystal with 0.4nm periodicity, formed in the conventionally solidified Al₇₀Ni₂₀Ru₁₀ alloy, has been studied by high-resolution electron microscopy and a high-angle annular dark-field (Z-contrast) technique. It has been clearly revealed that its structure is characterized as an aperiodically ordered arrangement of decagon-shaped atomic columnar clusters which have a diameter of 2nm and show pentagonal symmetry. On the basis of high-resolution electron microscopy structure images, and the atom-resolution Z-contrast observations which highlight the transition-metal sites, the arrangement of atoms in the superlattice decagonal quasicrystal are proposed.