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.     

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.     

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.     

Local translational order in the icosahedral quasicrystalline phase of V41Ni36Si23

Abstract

The local translational order of the icosahedral quasicrystalline phase in rapidly solidified V₄₁Ni₃₆Si₂₃ has been investigated by means of selected-area electron diffraction and high-resolution electron microscopy. Experimental results show that translational order always occurs in the twofold direction and can be observed when the quasicrystal is examined along its fivefold and threefold as well as twofold axes. Sometimes it occurs in two or three coplanar twofold directions simultaneously. In these cases some crystalline islands exist in the icosahedral quasicrystal and this corresponds to the initial stage of transformation from a quasicrystal to the stable crystalline phase.     

Characterization of grain-boundary dislocation networks by a combination of large-angle convergent-beam electron diffraction and weak-beam techniques

The Burgers vector of very close intrinsic dislocations in a near-Σ = 11,{311} grain-boundary in nickel is identified using a geometrical method based on local and accurate measurements of the angular deviation from perfect coincidence Σ = 11 by large-angle convergent-beam electron diffraction and of the dislocation spacing from weak-beam images of the grain boundary.     

Dislocations in quasicrystals and their interaction with cluster-like obstacles

A dislocation moving through a quasicrystal leaves in its wake a fault denoted a phason wall. For a two-dimensional model quasicrystal the disregistry energy of this phason wall is studied to determine possible Burgers vectors of the quasicrystalline structure. Unlike periodic crystals, the disregistry energy is an average quantity with large fluctuations on the atomic scale. Therefore the dislocation core structure and mobility cannot be linked to this quantity, e.g. by a Peierls-Nabarro model. Atomistic simulations show that dislocation motion is controlled by local obstacles inherent to the atomic structure of the quasicrystal.     

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.     

Stable icosahedral quasicrystals in the Ag-In-Ca,Ag-In-Yb,Ag-In-Ca-Mg and Ag-In-Yb-Mg systems

On the basis of the idea that a quasicrystal is a compound in which the average number of valence electrons per atom is around 2.0 and that its formation is dominated by atom size, stable icosahedral quasicrystals have been successfully synthesized in Ag-In-Ca, Ag-In-Yb, Ag-In-Ca-Mg and Ag- In-Yb-Mg systems. The Ag-In-Ca and Ag-In-Yb icosahedral quasicrystals have stoichiometry around Ag 42 In 42 Ca 16 and Ag 42 In 42 Yb 16 . The Ag-In-Ca-Mg and Ag-In-Yb-Mg quasicrystals form in the composition ranges Ag42 x /2 In 42 x /2 Ca 16 Mg x ( x =0-7.5 at.%) and Ag 42 x /2 In 42 x /2 Yb 16 Mg x ( x =0-10 at.%). Electron diffraction studies confirmed that the icosahedral quasicrystals have a primitive lattice. The average number of valence electrons per atom in all these quasicrystals is 2.0.     

The atomic structure of the dislocation cores in a small-angle grain boundary in BaTiO3 thin films

The atomic structure of the cores of the dislocations forming a [110] tilt boundary of 10^o misorientation in a BaTiO₃ film is characterized by means of focal-series reconstruction. Along the small-angle grain boundary, facets with different inclinations are accompanied by different types of dislocation. The dislocation with a Burgers vector of a [001] appears as a perfect edge type. Those with a Burgers vector of a [111] are dissociated into three partials with a Burgers vector of (a/3)[111], leading to two segments of stacking faults. Structural models of these dislocation cores are suggested on the basis of the phase of the reconstructed wavefunction and tested by simulation of the wavefunction.     

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.     

The displacement field of a rectangular Volterra dislocation loop

The displacement field of a rectangular Volterra dislocation loop having three non-zero Burgers vector components is obtained in an analytical closed form. The solution is obtained via integrating the Burgers equation for the displacement field of any closed dislocation curve and is expressed in a relatively compact form. The current solution has utility in a number of problems including dislocation-particle interaction problems, where the boundary condition involved imposes certain restrictions on the displacements in the medium, and modelling of cracks of arbitrary shapes. The solution is also useful in benchmarking newly emerging dislocation dynamics codes which discretize a curved dislocation line in some form or another. Several verification steps of the solution correctness are made including a comparison with the displacement and stress fields of a circular Volterra dislocation loop of equal Burgers vector and comparable size.     

Characterization of room-temperature plastic deformation of ß-Si3N4 by atomic force microscopy and transmission electron microscopy

Dislocation microstructures induced by room-temperature microhardness tests have been investigated in silicon nitride. Surface analysis of the residual indent by atomic force microscopy reveals intragranular slip bands and demonstrates that room-temperature plastic deformation involves dislocation motion as well as cross-slip events. Cross-slip events have been found to occur between {1010} prismatic planes. Transmission electron microscopy shows that dislocations have a Burgers vector b = [0001] and are located along the screw direction. Based on these observations, specific dislocation core configurations are discussed.     

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.     

Stair-rod dislocations in perovskite films on LaAlO3 substrates

Two types of stair-rod dislocation formed at junctions of {111} stacking faults in SrTiO₃-SrRuO₃ two-layer films on LaAlO₃ substrates have been studied by high-resolution transmission electron microscopy. The first type, formed by the interaction of two stacking faults situated at the flat SrRuO₃-LaAlO₃ interface without any step, has a Burgers vector (a/3)<110>. The other type, formed by the interaction of two stacking faults, of which one starts from the SrRuO₃-LaAlO₃interface area involving an interface step, has a Burgers vector (a/3)<120>. The formation mechanism of the stair-rod dislocations and the effect of the step are discussed.     

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 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.     

Bending experiments on the xi'-(Al-Pd-Mn) quasicrystal approximant

Single crystals of the quasicrystal approximant phase xi'1-(Al-Pd-Mn) were deformed at a high temperature in three-point bending geometry. Two different mechanisms of plastic deformation were observed in this phase: one based on the motion of phason lines and the other based on dislocations. Line directions and Burgers vector directions of the dislocations were determined. The relative importances of the two mechanisms are discussed as a function of the sample orientation with respect to the bending geometry.     

HRTEM observation of interfacial dislocations at faceted Al–Pb interfaces

Al–Pb ribbons containing 1?at.%?Pb have been produced by melt-spinning and subsequently investigated by high-resolution transmission electron microscopy. It is shown that the lattice mismatch of about 22% between the nanometre-sized Pb inclusions and the surrounding Al matrix is accommodated by a periodic array of misfit dislocations at the Al–Pb interface. The closing failures of Burgers circuits drawn around misfit dislocations on {111} and {100} facets identify the corresponding Burgers vectors as (a ₀/4)? 211 ? and (a ₀/2)? 110?, respectively. The Burgers vector of (a ₀/4)? 211? corresponds to the projected edge part of a 60° (a ₀/2)? 110? dislocation. The Pb inclusions themselves appear to be free of defects.     

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.     

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.