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.     

Titanium segregation mechanism in deformed vanadium-titanium alloys

Deformed V-Ti alloys with 0.3-5at.% Ti have been characterized by transmission electron microscopy and X-ray diffraction. A mechanism for Ti segregation during annealing, related to the formation of deformation twins, has been identified. Energy-dispersive X-ray spectroscopy indicates strong local changes in Ti content along the twins. Samples post-deformation annealed at 1473K show Ti segregation and the formation of Ti-rich precipitates which have been identified using electron microdiffraction and X-ray spectroscopy. X-ray diffraction measurements reveal changes in the lattice parameter of the alloys induced by the Ti segregation and the precipitation processes.     

Electron diffraction patterns from the Al–Mn decagonal phase

Abstract

The variety of electron diffraction patterns arising from the decagonal phase has been explored using a stereographic analysis for generating the important zone axes as intersection points corresponding to important relvectors. An indexing scheme employing a set of five vectors and an orthogonal vector has been followed. A systematic tilting from the decagonal axis to one of the twofold axes has been adopted to generate a set of experimental diffraction patterns corresponding to the expected patterns from the stereographic analysis with excellent agreement.     

Debonding criterion for the piezoelectric fibre push-out test

An Al-Ni-Co pentagonal quasicrystal, showing electron diffraction patterns with pentagonal symmetry and including so-called superlattice reflections, has been examined by the high-angle annular detector dark-field technique with scanning transmission electron microscopy. Columnar clusters with a pentagonal arrangement of transition-metal atoms around their centres are arrayed in a rhombic tiling with a bond length of 2nm, all the clusters having the same orientation of pentagonal symmetry.     

Transmission electron microscopy study of a new silicon nitride phase

An abnormally large phase, which was found in the precursor-derived Si 3.0 B 1.1 C 5.3 N 3.0 ceramics after crystallization under a nitrogen pressure of 100bar at 1800C for 3h, has been characterized by means of transmission electron microscopy and electron-energy-loss spectroscopy (EELS). EELS analysis shows that this phase consists of only silicon and nitrogen, no other elements being detected. The analysis of selected-area diffraction and convergent-beam electron diffraction in conjunction with EELS reveals that the unknown phase is a variant of silicon nitride. It has a hexagonal structure with lattice parameters a =0.737nm and c =0.536nm, and the space group P62c. .     

Polarity determination of zinc oxide nanorods by defocused convergent-beam electron diffraction

The crystal polarity of wurtzite zinc oxide nanorods grown hydrothermally parallel to the c?=?[0001] direction has been determined by convergent-beam electron diffraction using a defocused probe. The method enables a simple comparison of the diffracted intensities in ±0002 reflections across side-on nanorods, demonstrating growth in this case is in the c?=?[0001] direction. The method is shown to be viable for nanorods down to about 11?nm in diameter.     

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.     

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.     

Electron back-scatter diffraction study of inoculation of Al

The microstructure of cast commercial-purity aluminium inoculated by addition of an Al-Ti-C grain refiner has been studied by scanning electron microscopy and electron back-scatter diffraction (EBSD). It is found that a small fraction of the TiC particles present in the melt act as nucleation centres for grains. EBSD shows that the aluminium grains have a crystallographic cube-cube orientation relationship with the particles on which they nucleate. Nucleation occurs only on the largest particles, consistent with model predictions. Despite potential thermodynamic instability, TiC particles are effective nucleating agents.     

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.     

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.     

Structure images of unaged martensite containing a high carbon content

Abstract

A highly tetragonal (large c/a ratio) martensite in a Fe-4 wt% Al-1·6 wt% Mn-2 wt% Calloy, which is not aged and considered to be as ‘fresh’ as possible, has been observed by high-resolution electron microscope. From the arrangement of atom rows in the [010] structure image and the corresponding electron diffraction pattern, it is concluded that carbon atoms are clustered on the (305) plane. The unit structure of these carbon-clustered regions is similar to that of the modulated structure observed in Fe-C alloys, although a periodic layer distribution of carbon atoms is not observed.     

Twinning in the structure of the ‘decagonal’ phase Al65Cu20Co15

Abstract

The existence of twinning in the pseudo-decagonal phase Al₆₅Cu₂₀Co₁₅ has been established through TEM studies. From electron diffraction patterns, a rhombic crystalline lattice with a very large unit cell is easily identified. Diffraction patterns for microtwinned regions exhibit splitting and distortion of the reflections which is a result of the overlap of lattices which are rotated by 36° with respect to one another. While patterns from single components of the twins exhibit only pseudo-tenfold symmetry, those from the microtwinned regions exhibit nearly perfect tenfold symmetry.     

A new modulated structure in GaN nanoparticles

A new modulated structure with a superlattice having parameters a = 2.209nm, b = 3.826nm, c = 1.037nm and f = g = n = 90 has been found in GaN nanoparticles synthesized by a dc arc plasma method. The nanoparticles transformed further into particles with holes at their centres under electron-beam irradiation during high-resolution electron microscopy observations. At the same time, Ga atoms were extruded on to the surface of the nanoparticles and formed an amorphous layer. A series of simulations of high-resolution images and electron diffraction patterns revealed that the modulation could be attributed to aggregations of N vacancies founded during the electron bombardment. Molecular mechanics calculations show that the aggregation of N vacancies is far more energetically favourable than that of Ga vacancies. The stability of the GaN particles is discussed.     

Structural phase transitions in epitaxial SrRuO3 thin films

The microstructural evolution of epitaxial SrRuO₃ thin films from ambient temperature (about 293K) to about 900K has been studied by in-situ transmission electron microscopy. Upon heating from the ambient temperature, the intensities of h, k, 2n+1 and h, -h+2n+1, 0 reflections in selected-area electron diffraction patterns decrease with increasing temperature. Two structural phase transitions were observed at about 673and about 783K, revealed by the vanishing of the h, k, 2n+1 and h,-h+2n+1, 0 reflections respectively. The examination of electron diffraction patterns along several different zone axes, taking into account the possible tilting configurations of RuO₆ octahedra, indicates that, upon heating, the orthorhombic structure of SrRuO₃ transforms into a tetragonal structure at about 673K, and further transforms into a cubic structure at about 783K. Possible structural models for the high-temperature phases are considered.     

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.     

Atomic and electronic structure of a carbon nitride compound prepared by magnetron sputtering

The atomic structure, chemical composition and chemical bonding state of a carbon nitride compound, synthesized by rf magnetron sputtering, have been studied by high-resolution electron microscopy, nanobeam diffraction and electron energy loss spectroscopy. It is revealed that the grain sizes of a crystalline compound in an amorphous matrix lie in the range 5-10 nm, and that the compound has a hexagonal structure with a = 0.324 nm and c = 0.404 nm. The C and N K edges are observed in the high-energy region of the electron-energy-loss spectrum. Only features corresponding to C sigma* bonds are found in the C K edge spectrum, which provides evidence for sp³ hybridization of C orbitals in the compound. Quantifying the electron-energy-loss spectra suggests that the compound is C₇N₁₀.     

Demonstration of the g · b x u = 0 edge dislocation invisibility criterion for electron channelling contrast imaging

Dislocations with a large edge component do not exhibit a full loss of diffraction contrast in the transmission electron microscope unless the conditions g b = 0 and g b x u = 0 are simultaneously met. Because of a similarity between transmission electron microscopy (TEM) diffraction contrast and electron channelling contrast in the scanning electron microscope, the same contrast rules would be expected to apply for electron channelling contrast imaging (ECCI). Using the characteristic edge dislocations formed in annealed FeAl, it is demonstrated that the same extinction conditions apply for TEM diffraction contrast and ECCI.     

An ordered arrangement of atom columnar clusters in a pentagonal quasiperiodic lattice of an Al-Ni-Co decagonal quasicrystal

The structure of an Al-Ni-Co decagonal quasicrystal (called the S1-type superstructure), which shows diffraction patterns with superlattice reflections, has been studied by atomic-scale observations of electron microscopy. The structure of the decagonal quasicrystal can be characterized as an ordered arrangement of two kinds of atom columnar cluster with different directions of pentagonal symmetry. A fundamental lattice, which is constructed by connecting all the atom clusters, shows a pentagonal quasiperiodic lattice with a bond length of about 2 nm. The pentagonal lattice is divided into two superlattices, which are constructed by connecting atom clusters with the same directions of pentagonal symmetry, with a bond length of 2     

Formation of decagonal and approximant phases in the Al-Ni-Ru system

Al-Ni-Ru alloys with a wide composition range have been synthesized and examined by X-ray and electron diffraction experiments. In this system, two types of decagonal (d) phases are formed, namely basic-type and superlattice-type d phases. Annealing experiments have shown that the basic-type d phase is stable at high temperatures and that its single-phase region is located in a small composition range around Al 75 Ni 15 Ru 10 . On the other hand, the superlattice-type d phase is metastable and has been found to form only in the samples before annealing. Besides the d phases, a high-order crystal approximant phase with lattice parameters a = 99.5 Å and b = 84.6 Å has been found. The structural relation between the approximant phase and the d phase is discussed in terms of a phason tensor.