On the structure of α′m-precipitates in Al-Zn alloys

The lattice parameter of the f.c.c. α′-phase in an Al-12at.% Zn alloy decomposed at ageing temperatures ranging between 170 °C and 210 °C was investigated by an analysis of Moiré fringe contrasts which appear inside the precipitates in the TEM-micrographs. Additionally information was gained concerning the relationship between the corresponding lattice of the α′_m-precipitates and the matrix. The analysis proved that the misfit between the lattice of the α′_m-precipitates and the adjoining matrix is fairly small, namely less than 1.5% in lattice spacing and less than 1° in the angular rotation. From these results it was concluded that the major part of the matrix planes passes the precipitates without interruption.     

On the Fit between the Precipitates Occurring in Al–Cu and Al–Cu–Mg Alloys and the Al-rich Matrix

The quality of the fit between neighbouring planes of the most important metastable phases occurring in Al–Cu and Al–Cu–Mg alloys and the Al-rich matrix is evaluated by means of geometric models. The results obtained are compared with experimental yields.     

Computerberechnete Atomanordnungen in elektronenmikroskopisch beobachteten facettenförmigen Phasengrenzflächen im gerichtet erstarrten Al?Al3?Ni-Eutektikum

The directionally solidified Al Al₃Ni eutectic consists of an Al-matrix and of facetted Al₃Ni-rods grown into it nearly parallel to each other and to the direction of heat flow. — The orientations of the interfacial planes observed by electron microscopy were determined relative to both crystal lattices. The atomic arrangement in crystallographic planes of both phases parallel to these observed interfaces has been computed and drawn by a special program. The mating projections of their atomic packing were compared together in order to estimate the lattice misfit, atomic matching and atomic densities of the boundary layers. While the most favoured habit plane (001)Al₃Ni is explainable in terms of attaining low interfacial free energy through low misfit, similar atomic arrangements and densities, the higher indexed planes could not be verified by an appropriate concept due to their complex, incoherent nature and similar size.     

Site occupation and solubility limit of Sc in Lu3Al5O12

Crystallographic study of the Lu₃Al₅O₁₂:Sc system shows existence of a wide range of compositions involving substitutions by Sc for both six-fold octahedral and eight-fold dodecahedral sites of the lattice. In comparison to other rare-earth garnets, the total amount of Sc that can be tolerated by this matrix is however much lower. The limited solubility of Sc is associated with the small size difference between the constituent atoms, which approaches a critical value with increasing the Sc content. Preparation of clear single-phase solid solution crystals requires an accurate account of site occupation preferences.     

High-resolution electron microscopy of microstructure of SrTiO3/BaZrO3 bilayer thin films on MgO substrates

SrTiO₃/BaZrO₃ heterofilms as buffer layers are deposited on (0 0 1) MgO substrates by an RF-sputtering technique. The atomic structure and the defect configuration at the interfaces are investigated by means of aberration-corrected high-resolution transmission electron microscopy. At the BaZrO₃/MgO interface, two types of interfacial structures, MgO/ZrO₂-type and MgO/BaO-type, are observed. Antiphase boundaries and dislocations are found at the BaZrO₃/MgO interface. The formation of these lattice defects is discussed in terms of film growth and structural imperfections of the substrate surface. At the SrTiO₃/BaZrO₃ interface, a high density of misfit dislocations is observed with different configurations. The formation of these dislocations contributes both to the relaxation of the large misfit strain and to stopping of the further propagation of lattice defects which are formed in the BaZrO₃ layer into the SrTiO₃ layer.     

An approximation to the description of growth of coherent precipitates

The tree-dimensional growth of a spherical coherent particle in an isotropic infinite matrix is considered in the frame of the stationary solution of the classical diffusion equation applying WAGNERS approximation in a varied manner and taking into account both the concentration dependence of the diffusion constant D and the influence of elastic energy. From the obtained results qualitative conclusions are drawn on the stability of the precipitates during the OSTWALD-ripening process in dependence on their size. The evaluations doubtlessly confirm the higher stability and the favoured growth of the larger precipitates.     

Study of doped higher manganese silicides crystals by transmission electron diffraction and electron backscatter diffraction

The microstructure of Al-, Ge-, and Mo-doped higher manganese silicide crystals (HMS), grown by the Bridgman method have been investigated by transmission electron diffraction, electron backscatter diffraction, scanning electron microscopy, and X-ray energy-dispersive spectrometry. It is shown that the matrix crystal has the Mn₄Si₇ structure. The introduction of Ge and Mo impurities into an HMS crystal results in the precipitation of Si-Ge solid solution and molybdenum disilicide. The size of precipitates varies in a wide range: from several nanometers to several hundreds of micrometers. The following orientation relationships between Ge-Si precipitates and the Mn₄Si₇ crystal were determined: (112) $[\bar 110]$ Ge-Si ‖ (010)[100] Mn₄Si₇. Polycrystalline MoSi₂ precipitates form a multicomponent texture along the [001] Mn₄Si₇ direction. Small amounts of cubic MnSi and Al-Mn-Si alloy precipitates were revealed. In addition, Al oxide was observed mainly in crystal pores. It is shown that 0.5–0.8 at % Al, 0.4–0.6 at % Mo, and 1.5–2.0 at % Ge impurities are incorporated into the Mn₄Si₇ lattice.     

The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al-Y binary alloys using a computer simulation. Up to 10⁷ Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 10²¹ m⁻³) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.     

On the Binding Energy of Clusters with a Small Number of Lithium Atoms and Varying Scope of Interaction Between the Atoms. Hückel Molecular Orbital Approximation

The effect of the scope of interaction between the atoms in two-dimensional and threedimensional clusters containing 6, 7, 9, 10, 15 and 16 lithium atoms, upon the binding energy (BE) of the clusters has been investigated. BE of the clusters was calcuated by the HÜCKEL Molecular Orbital (HMO) method. It has been established that for clusters with the same number of atoms BE has its maximum value when the interaction of each atom of the cluster with the atoms of its first and second coordination spheres in the lithium lattice are taken into account. After the maximum BE decreases and tends to a constant value which is attained when the interaction of each atom of the cluster with the atoms of its six coordination spheres is taken into account. The reasons for the above result should be looked for in the limits of applicability of the calculation procedure based on the HMO method to the study of properties of various models of clusters containing a small number of metal (e.g. lithium) atoms.     

Dislocations and dislocation reduction in space grown GaSb

The behaviour of dislocations in GaSb crystals grown in space both from a stoichiometric melt (floating zone method, FZ) and a Bi solution (floating solution zone, FSZ) respectively, is studied. Predominantly straight 60° dislocations with Burgers vectors of the type b = a/2 <110> in (111) glide planes are identified. In the 20 mm long FZ single crystal the linear growing out of the dislocations is observed which reduces the dislocation density in the centre of the crystal to values below 300 cm^–2. The Bi incorporation in the FSZ crystal results in a misfit between seed and grown crystal and in a network of misfit dislocations at the interface. Thermocapillary convection during growth as well as the surface tension may be the reasons for the presence of curved dislocations and the higher dislocation density within a 1 – 2 mm border region at the edges of both of the crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lattice strain and misfit dislocations in GaAs-GaAlAsP heterojunctions

The lattice strain and misfit dislocations in a GaAs-GaAlAsP heterojunction were examined. The change in lattice strain with the composition ratio or the position of the crystal was measured, and it was found that some of the misfit dislocations introduced in the heterojunction were edge-type dislocations. The critical thickness of the epitaxial layer for the generation of misfit dislocations was also measured. The critical differential strain between the atomic layers was on the order of several angstroms. The distribution of lattice strain was analyzed by a two-dimensional simple cubic lattice model, and the distribution of differential strain was examined.     

Formation and characterization of GaAs/As superlattice grown by molecular beam epitaxy at low substrate temperature

High-resolution X-ray diffractometer and transmission electron microscope (TEM) are used to characterize the redistribution of As precipitates in Si δ-doped GaAs grown by molecular beam epitaxy at low substrate temperature (230°C). The superlattice satellite peaks are observed for samples annealed at 700-800°C for 10 min, which is attributed to the formation of a GaAs/As superlattice during the annealing period. The degree of As precipitates confined on the δ-doped planes is revealed on the intensity of satellite peaks in the X-ray rocking curves, as confirmed by the TEM observations. The lattice expansion and contraction of the annealed low-temperature epitaxial layers can be easily observed from the asymmetry of the satellite peaks.     

Bandwidth narrowing in n-type many-valley semiconductors

A simple method that takes into account the many-valley effects of the host conduction band is outlined for the calculation of the impurity density of states. N random impurity sites were simulated with a computer, within a diamond lattice as a host. A Kohn-Luttinger donor wave function was associated with each impurity. It was found that the low energy side (or the high energy side when the effect of non-orthogonality is taken into account) of the density of states is strongly reduced to a small tail when compared to the case of neglecting many-valley effects. At the Fermi energy, the density of states is considerably enhanced which leads to an enhancement of the extrinsic specific heat. The inverse participation ratio varies with the impurity concentration, presenting different states which are responsible for different physical properties. An analytical calculation taking into account these effects was carried out for the sake of comparison.     

Revealing of lattice defects on (111) faces of gallium phosphide and indium phosphide by chemical etching

A solution is presented suitable for revealing lattice defects on (1 1 1 ) phosphorous faces of GaP and InP. It is possible to distinguish between pits originated by grown-in dislocations and microdefects such as perfect loops, faulted loops and precipitates and/or inclusions. Moreover it is also possible to reveal large stacking faults of Shockley type and microtwin lamellae. One to one correlations have been given by means of transmission electron microscopy.     

Annealing-induced relief of compressive misfit strain in liquid phase epitaxial yttrium iron garnet films

Films of yttrium iron garnet were grown on (111) gadolinium gallium garnet by liquid phase epitaxy from a PbO-B₂O₃ flux. Incorporation of Pb as a substitutional impurity produced an increase in film lattice parameter which resulted in initial compressive misfit strains in the films. The initial strains were relieved by annealing in O₂. The relief process was studied by X-ray double-crystal diffractometry and topography supplemented by optical and scanning electron microscopy. Strain relief was found to be associated with the occurrence of defects which were imaged in both film and substrate topographs. When the initial compressive misfit was sufficiently large, annealing eventually resulted in a tensile strain and the development of cracks which propagated through the films and into the substrates.     

Wet KOH etching of freestanding AlN single crystals

We investigated defect-selective wet chemical etching of freestanding aluminum nitride (AlN) single crystals and polished cuts in a molten NaOH–KOH eutectic at temperatures ranging from 240 to 400 °C. Due to the strong anisotropy of the AlN wurtzite structure, different AlN faces get etched at very different etching rates. On as-grown rhombohedral and prismatic facets, defect-related etching features could not be traced, as etching these facets was found to mainly emphasize features present already on the un-etched surface. On nitrogen polar basal planes, hexagonal pyramids/hillocks exceeding 100 μm in diameter may form within seconds of etching at 240 °C. They sometimes are arranged in lines and clusters, thus we attribute them to defects on the surface, presumably originating in the bulk material. On aluminum polar basal planes, the etch pit density which saturates after approx. 2–3 min of total etching time at 350 °C equals the density of a certain type of dislocations (presumably screw dislocations) threading the surface. Smaller etch pits form around annealed indentations, in the vicinity of some bigger etch pits after repeated etching, and sometimes also isolated on the surface area. Although alternate explanations exist, we attribute these etch pits to threading mixed and edge dislocations. This paper features etching parameters optimized for different planes and models on the formation of etching features especially on the polar faces. Finally, the issue of reliability and reproducibility of defect detection and evaluation by wet chemical etching is addressed.     

Strukturdefekte in GaP/GaAs-Heteroepitaxieschichten

Transmission electron microscope (TEM) observations and chemical etching were made on GaP material grown by vapour phase epitaxy on (1 1 1 ) GaAs substrates using water vapour as transport agent. This material was found to contain many defects as dislocations, stacking faults, grain boundaries and microtwin lamellae. The density of all these defects decreases with increasing layer thickness. The occurrence of several faults (principally microtwin lamellae) was attributed to the mechanical stress and the layer bending as a consequence of the different temperature dependence of the lattice constants and the lattice misfit.     

Electron microscopic investigations of precipitates in doped NaCl crystals

Precipitation morphology of NaCl:CdCl₂ and NaCl:SrCl₂ crystals is investigated by high voltage transmission electron microscopy. The volume fraction of the precipitates and the content of CdCl₂ has been evaluated using the theory developed by Hillard. Selected area diffraction patterns of the precipitates and investigations of the misfit dislocations around the precipitates is presented.     

Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

The results of TEM investigations are presented of the effects of various solutions and ageing treatments, cooling and quenching rates as well as of tensile stress at elevated temperatures on the microstructure of the grain boundary (GB) region of the examined Al Zn and Al Zn Mg alloys. A pronounced influence was found of partial coherency of GB stable β(Zn) precipitates with the α matrix in the Al Zn alloy on the asymmetry of Zn depleted GB region and on the shape of GB precipitates. The role of GB's as sources and sinks of vacancies and the effect of stress gradients in GB regions were emphasized in the establishment of differences in the nucleation and decomposition characteristics within the GB region as compared to those within the grain interior. The existence of a negative stress gradient towards the GB has been suggested to lead to the observed gradual retardation of transformation kinetics within the GB region in Al Zn alloys. A rapid quench from annealing to ageing temperature introduced large local stresses close to GB giving rise to “band of precipitates” on subsequent ageing within the former precipitate free region (PFZ) in Al Zn Mg alloy. Similar enhancement of local stresses accompanied by acceleration of decomposition kinetics was also observed in the aged Al Zn sample subjected to a prior tensile creep exposure within the one-phase region.