H.-U. Nissen

The existence regions of structural modifications in decagonal Al-Co-Ni

A subdivision of the stability region of decagonal Al-Co-Ni quasicrystals into eight different structural modifications is presented for the temperature against composition section along the maximum extension of the decagonal phase. The only methods identifying this subdivision are transmission electron microscopy techniques. Besides two decagonal superstructures (one of which also exists as a high-temperature modification), two variants of the basic decagonal state were observed. Also, a one-dimensionally periodic fivefold quasicrystal (together with its high-temperature superstructure modification) and a one-dimensional quasicrystal are parts of the quasicrystalline stability region. Both have a close structural relationship to the decagonal states. All samples investigated were obtained by quenching from the corresponding temperatures. The eight modifications can be differentiated by a set of attributes, especially by their characteristic diffraction patterns taken with the electron beam parallel to the unique periodic axis.

Stable Al-Mn-Pd quasicrystals

Abstract A stable decagonal quasicrystal phase in the Al–Mn–Pd system is described. Its composition is Al70·5Mn16·5Pd13, as determined by X-ray microanalysis attached to a scanning electron microscope. The temperature range of stability has been found to be around 800°C by investigation of differently annealed specimens in a transmission electron microscope. An icosahedral quasicrystal phase with composition Al70·5Mn8·5Pd21 and a face-centred icosahedral quasilattice has also been found, partly in coexistence with the new decagonal phase. The icosahedral phase is also stable at temperatures around 800°C. The Al–Mn–Pd system is the first system in which stable decagonal quasicrystals as well as stable icosahedral quasicrystals occur.

Highly perfect decagonal Al-Co-Ni quasicrystals

Conventionally solidified samples of decagonal Al70Co11Ni19 annealed at 1050degreesC for 12h, studied by electron diffraction and high-resolution transmission electron microscopy (HRTEM), display sharp spots in their diffraction patterns and almost no diffuse background scattering. Detailed geometric analysis of tilings constructed from and superimposed onto HRTEM images show that the structure of these samples is very close to the perfect quasiperiodic structure of a Penrose pentagon tiling. This conclusion is corroborated by the observation that, for this structure, tilings in three sizes can be constructed using pattern matching based on semi(de)composition steps with a scaling factor tau . Moreover, these three tilings have a perpendicular space projection located in a window which is only very slightly larger than the decagonal window of an ideal Penrose pentagon tiling.

Two different superstructures of the decagonal Al-Co-Ni quasicrystal

Abstract In a decagonal quasicrystal with composition AI70Co15Ni15 two different types of superstructure have been found by electron microscopy techniques. One of these superstructures has previously been observed. The other newly characterized superstructure causes a ring-like zone of low diffraction intensities and, additionally, reflections at positions having half-integer indices. Both types of superstructure have a doubled period of 8·2 A along the unique tenfold axis, which is suggested here to be due to Al/TM (TM= Co, Ni) ordering on different layers. The main structural elements are columns with a diameter of approximately 20 A. During irradiation with a 300 keV electron beam both superstructures are transformed to a ‘basic’ decagonal structure with a 4·1 A period, in which the wheel-like contrast features with 20 A diameter are absent. For the more disordered basic decagonal structure no conspicuous tiling can be found which can be superimposed onto the electron micrographs, while the new superst...

Growth morphology of icosahedral Al-Mn-Pd single quasicrystals

Abstract Stoichiometric as-cast ingots of composition Al70Mn9Pd21, Al70Mn15Pd15 and Al70Mn17Pd13 have been heat treated in several different ways and have subsequently been investigated by scanning electron microscopy (SEM) and electron channelling studies in order to characterize the growth morphology as well as the degree of structural order of the stable icosahedral quasicrystal phase. Icosahedral single-quasicrystal units, typically 2 mm in size, are formed for the first of these three compositions, while in the two other specimens up to 0.5 mm units have been found. Icosahedral Al-Mn-Pd single quasicrystals have a strongly developed cleavage normal to the fivefold axes and to a lesser degree also normal to the twofold and threefold axes. Curved surfaces of crushed single quasicrystals are microscopically rough, i.e. they are composed of steps. The ideal growth shape of stable icosahedral Al70Mn9Pd21 quasicrystals is found to be the Archimedian {4,6,10} polyhedron. This shape has been found on the bas...

Electron microscopy of crystalloid structure in Ni-Cr small particles

Abstract The crystalloid structure, an aperiodic arrangement of small structural units, observed in Ni-Cr small particles was studied by selected-area electron diffraction, dark-field imaging, high-resolution electron microscopy and optical diffraction analysis using the high-resolution micrographs. The electron and optical diffraction patterns show an arrangement of sharp diffraction spots with twelvefold rotational symmetry and an intensity distribution which is (at least approximately) consistent with this symmetry. The high-resolution electron microscopy revealed that the crystalloid structure is non-periodic but has bond orientational order over a range of 60 nm. From these experimental results, as well as from those for crystalline phases coexisting in the same particles, a random tiling model of three local structural units (A15-type, Zr4Al3-type and 30° rhombus prism type) is proposed as a possible structural model of the crystalloid structure. An indexing scheme using five integers is also propos...

Dislocations, domains and quenched phonon and phason strains in Al-Mn quasicrystals observed by electron microscopy

Abstract Several kinds of edge dislocations and domain structures as well as some structural irregularities interpreted as arising from quenched phasons and phonon strains have been observed in Al-Mn quasicrystals by HREM and subsequent image processing. Noise-reduced pictures were obtained by pseudo-optical filtering of selected diffraction peaks. The line vectors and Burgers vectors of the dislocations have been determined and are indexed in a six-dimensional coordinate system. The results indicate a relatively strong phonon-phason coupling in the quasicrystals.

Crystallization in amorphous NiZr2, studied by HRTEM

Abstract The crystalline textures of amorphous NiZr 2 heated in a differential scanning calorimeter (DSC) at very low heating rates were studied by high-resolution transmission electron microscopy. Three differently heat-treated specimens were compared. One of these was taken out of the DSC just after a strong endothermal peak had occurred, which is unexpected for amorphous binary alloys. The remaining specimens were taken out of the DSC after termination of crystallization. All specimens have a polycrystalline texture, with an average grain size of 100 nm. The grains have the tetragonal Al 2 Cu-type structure, and each of these grains has a nanocrystalline domain texture consisting of rotation twin domains characterized by the following orientation relation: [001] 1 |[111] 2 ; [110] 1 |[110] 2 . The boundary between any two nanocrystals intergrown according to this relation is a planar interface with index (110). The typical distance between such interfaces is 1–3 nm. It is concluded that all specimens are fully crystallized after the occurrence of the strong endothermal peak and that the orientation of the twin domains in the nanocrystalline texture results from medium-range bond orientational order assumed to be present in the amorphous starting material.

Orientation relation between icosahedral and crystalline phases in Al-Mn alloys

Abstract A transmission-electron-microscope investigation of the orientation relations between faceted icosahedral quasicrystals and crystalline f.c.c. matrix in rapidly cooled alloys of Al-Mn reveals three different orientation relations: In the third relation the f.c.c.[111] axis cannot be exactly parallel to the twofold axis i2 because of geometrical restrictions. Most of the observed relations can be ideally classified into these three classes (with deviations smaller than 4°). The point-symmetry elements common to the two structures in these three orientation relations are subgroups of m35 (icosahedral phase) as well as of m3m (f.c.c. phase). For the three orientation relations, the corresponding subgroups are 2,222 and 32 respectively, whereby the last case is slightly idealized.

Electron Microscopy and Surface Investigations of Quasicrystals

A review of recent results on electron-microscopy investigations of quasicrystalline materials is presented with special emphasis on thermodynamically stable quasicrystals. Also, an introduction into the technique of high-resolution transmission electron microscopy (HRTEM) as well as into convergent beam electron diffraction and Kikuchi line pattern techniques is given. Additionally, techniques sensitive to thin surface layers and especially their application to quasicrystals are discussed: secondary electron imaging, X-ray photoelectron scattering, electron backscattering diffraction and scanning tunneling microscopy.