The recovery of slip planes in quasicrystals is investigated on the basis of the formation of perfect tiles due to a flipping procedure. The procedure is focused on tilings with single translation classes. The minimum number of new vertices or the number of necessary flips to reform the tiles is taken as a measure of disorder of the slip process. Different ranges of the density of new vertices depending on the slip vector could be derived. Short displacements lead to the formation of a domain of ID quasicrystal.

Besides the six established decagonal states of the Al-Co-Ni quasicrystal two more modifications have been discovered by means of transmission electron microscopy. One is a pentagonal quasicrystal with a superstructure found in specimens with a very high Co-content and quenched from the highest possible temperature lying within the stability field of decagonal Al-Co-Ni. Its electron diffraction patterns are characterized by a 5-fold rotation axis as a unique symmetry element as well as superstructure reflections similar to those of a related decagonal phase. the other is a one-dimensional quasicrystal closely related to decagonal Al-Co-Ni. the modulation length of 61 Å along the periodic direction in its pseudo 10-fold diffraction patterns can be assumed to be caused by a strong linear, uniform, phason strain in the material.

SummaryTools for the description and investigation of ternary alloy systems are summarized. The quasicrystal-forming systems Al-Mn-Pd and Al-Co-Ni are described; liquidus projection, isothermal and pseudobinary intersections are represented. The significance of phase equilibria for the production of quasicrystals is outlined, where stable and metastable equilibria are considered.

The phase equilibria of alloys in the vicinity of icosahedral phase region of the ternary Al–Cu–Fe system were investigated by thermal and magnetothermal analysis, metallographical methods and X-ray diffraction. It was reconfirmed that the icosahedral phase forms by a ternary peritectic reaction as L+λ+ β⇔i. Its temperature was found at 882±2°C. The liquidus projection was established.

An overview of experimental methods for the determination of thermodynamic data of alloys is given. The significance of thermodynamic and kinetic data is outlined for stable and metastable states with respect to quasicrystalline alloys. The information on the stability of quasicrystals is summarized on the basis of the competitive formation of quasicrystals and of crystals with translation symmetry.

Concerning different variants of quasicrystalline phases we present quasiperiodic tilings which combine properties of superstructures with those of modulated structures or nanostructured domains of approximants.

The utilisation of labels in description of ternary systems is summarized. Different types of nonvariant three-phase equilibria including critical tie lines are described regarding the Cu-In-Se system. Explaining labels are proposed which are based upon examples found in the literature.

A physical displacement across a slip plane or a domain boundary may be resolved into a sequence of phason flips which restore the nearest neighbour configurations and the tile forms of the original quasicrystal. The number or density of required new vertices is correlated with the square of the implied internal space component of the physical shift. Examples are shown which are relevant to domain boundary widths and the ease of progress of physical slip in the relevant quasicrystal.

Highly faceted microholes (voids) in icosahedral Al-Mn-Pd quasicrystals form during annealing at temperatures between 750 and 830°C via classical Ostwald ripening. The specimens were single-phase icosahedral with a composition of Al71Mn9Pd20. If such a specimen containing voids is cooled to room temperature with a constant cooling rate of typically 0.6 to 5 K/min, the facets of the voids are frequently found to be decorated by a different material. The decorations have typically a morphology similar to fractals. Specimens rapidly quenched after isothermal heat treatments around 830°C never showed similar decorations. By X-ray microanalysis in a scanning electron microscope it has been found that this decoration material is considerably poorer in Al-content that the icosahedral phase, namely: Al60Mn10Pd30. Accordingly, these decorations are precipitations at the surfaces of voids inside a single-phase icosahedral matrix. This implies that these decorations are formed at the void surfaces by a diffusion induced local equilibrium. The local equilibrium can be understood on the basis of the equilbrium results in a single icosahedral phase.

A stable one-dimensional quasicrystal has been found for Al-Co-Ni which is related closely to the decagonal phase in this system. It exists as a high-temperature state above 900 C in a range of compositions between approximately 71 and 71.5at.% Al between 18at.% and 22at.% Co. The modulation length along the periodic direction in its pseudo tenfold diffraction patterns is approximately 61A. We show that the 20A edge length defective rhomb tiling superimposed on to a corresponding high-resolution image, defined by centres of atomic clusters, can be considered as a subset of a phason-strained, 2.5A edge length Penrose tiling that has the observed period. An ideal 20A edge length tiling based on special configurations of this Penrose tiling can be chosen as a prototype for the observed experimental tiling.

In as-cast samples of the decagonal Al-Co-Ni quasicrystal so-called crystalline approximants are usually not found, by contrast, they form due to insufficient homogenisation during annealing of as-cast samples at temperatures too far below (more than approx. 50 K) the solidus of the corresponding composition. By contrast, annealing of as-cast samples or samples containing approximants, rather close to the solidus, irreversibly resulted in quasicrystals. In this way, inhomogeneities like atomic segregation can be healed and equilibrium is reached. Hence, it can be concluded that the approximants in the Al-Co-Ni system are of metastable nature only.

A one-dimensionally periodic pentagonal quasicrystal with a superstructure has been found in the Al-Co-Ni system in a sample quenched from 1160oC at the composition Al71.5Co25.5Ni3. Its electron diffraction patterns are characterized by a fivefold rotation axis as a unique symmetry element as well as superstructure reflections similar to those of a related decagonal phase. The presence of this symmetry in electron diffraction patterns originates from strong dynamical scattering effects of electrons in relatively thick regions of the sample, in combination with the unique parallel arrangement of fivefold symmetric atom clusters. Differently, the superstructure reflections are caused by the long-range order of this new state which can be described by a random variant of a Penrose rhomb tiling.

A tool for prediction of conserved secondary structure of a set of homologous single-stranded RNAs is presented, For each RNA of the set the structure distribution is calculated and stored in a base pair probability matrix, Gaps, resulting from a multiple sequence alignment of the RNA set, are introduced into the individual probability matrices. These 'aligned' probability matrices are summed up to give a consensus probability matrix emphasizing the conserved structural elements of the RNA set. Because the multiple sequence alignment is independent of any structural constraints, such an alignment may result in introduction of gaps into the homologous probability matrices that disrupt a common consensus structure, By use of its graphical user interface the presented tool allows the removal of such misalignments, which are easily recognized, from the individual probability matrices by optimizing the sequence alignment with respect to a structural alignment. From the consensus probability matrix a consensus structure is extracted, which is viewable in three different graphical representations. The functionality of the tool is demonstrated using a small set of U7 RNAs, which are involved in 3'-end processing of histone mRNA precursors. Supplementary Material lists further results obtained. Advantages and drawbacks of the tool are discussed in comparison to several other algorithms.

Human Hepatitis B Virus (HBV) RNAs contain a cis -acting sequence, the post-transcriptional regulatory element (HPRE), which facilitates the cytoplasmic localization of intronless transcripts. Our previous studies have shown that the HPRE is composed of at least two independent sub-elements, HPREalpha and HPREbeta, which co-activate a reporter for RNA export in a greater than additive manner. Utilizing deletion, mutation and co-variational analyses, we have identified three regions important for full HPRE activity. The three separate regions of the HPRE function can function independently in a dose-dependent manner when multimerized. Two of these regions contain stem loops, HSLalpha and HSLbeta1, which are necessary for full HPRE function. These structures are conserved throughout the mammalian Hepadnaviruses. Disruption of either stem-loop structure by mutagenesis decreases HPRE function while compensatory mutations restore activity. The location of the stem-loops in the genome reveal that they are present in all of the HBV transcripts. HSLalpha and HSLbeta1 are likely to contain the binding sites for the cellular factor(s) which mediates HPRE function.

The phase equilibria of the ternary Al-AlCo-AlNi system are demonstrated using isothermal sections at 1170, 1100, 1050, 900, 850, 730, End 600 degrees C. These are based on metallographic and TEM investigations and on several temperature-concentration sections, which are investigated using thermal analyses. The Scheil reaction scheme is used for explanation of the data. Eleven phases from the binaries Al-Co and AI-Ni and the three ternary phases Y-2 (Co2NiAl9), X and D were found at room temperature. The binary high-temperature phase Co4Al13(h) is stabilized in the ternary down to room temperature by Ni addition (YI). The phase field of D is composed of several modifications which can hardly be distinguished by light microscopy. These modifications are not considered in the present paper. 45 three-phase regions can be derived from the reaction scheme in the Al-AlCo-AlNi subsystem.

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.

The phase equilibria of the ternary Al-AlCo-AlNi system were investigated around the decagonal phase. Isopleths with 70, 71.5 and 72.5 at.% Al and with 10 and 13 at.% Ni an elaborated with the aid of differential thermoanalysis, magnetothermal analysis, optical microscopy and transmission electron microscopy. These are based on results of samples of more than 80 different compositions and on data from other isopleths.