Ragnvald Høier

Microstructure of a pressure die cast magnesium—4wt.% aluminium alloy modified with rare earth additions

Abstract Addition of cerium-rich mixtures of rare earth (RE) elements to aluminium-containing magnesium pressure die cast alloys is known to improve the creep properties at elevated temperatures. In the present investigation, a detailed description of the microstructure of a magnesium-4 wt.% aluminium alloy containing 1.4 wt.% of a cerium-rich mixture of RE elements is presented. Particle types occurring and their distribution in the microstructure, as well as the distribution of elements in solid solution, are described. Heat treatment is carried out to verify if solid state precipitation occurs. Some qualitative arguments for the beneficial effect of RE elements on the creep properties are presented.

Microstructural characterization and microstructural effects on the thermal conductivity of AlN(Y2O3) ceramics

Abstract Two different high density aluminum nitride (AlN) ceramic materials with Y2O3 as a sintering additive have been sintered at 1880°C, and characterized using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The lattice dissolved oxygen content, grain size and amount of secondary phase were fairly similar for the two materials, however, the thermal conductivity was quite different. SEM and TEM studies show that the secondary phase distribution along the grain boundaries or at grain junctions may have a major influence on the thermal conductivity. Microstructural changes were found to disrupt the connection between the high thermal conductivity AlN grains, resulting in a decrease in the thermal conductivity of the material as a whole. No amorphous layer was observed by high resolution transmission electron microscopy (HRTEM) at the cleanest grain boundaries in any of the materials.

Efficient beam-selection criteria in quantitative convergent beam electron diffraction

Abstract New criteria to identify significant beams in many beam dynamical calculations are introduced. The new beam selection criteria are used in a structure factor refinement procedure where the correct identification of the most significant beams is crucial. Beams are divided into two categories. Strong beams are included in the dynamical calculations through diagonalisation. The effects of weak beams are successfully described by Bethe-perturbation of the dynamical matrix. The new method is compared to existing algorithms and found favourable, both in terms of computing time and stability in calculations.

Preparation and characterisation of aluminium nitride-titanium nitride composites

Abstract Aluminium nitride–titanium nitride (AlN–TiN) composites were prepared to increase the fracture toughness of AlN-based materials. Two methods were used to achieve particle-reinforced materials; TiN (0.3–3.4 vol.%) was formed in-situ or TiN particles (0–21 vol.%) were added. The resulting composites were dense and homogeneous, even at high TiN content. The Vickers hardness and Youngs modulus increased when adding 21 vol.% TiN, about 8 and 5%, respectively. SENB measurements showed a 33% increase in fracture toughness when 21 vol.% TiN was added. The toughening mechanism was mainly crack deflection around TiN grains. The fracture toughness was also calculated using various models based on Vickers indentation. The different models underestimated the KIC values compared to the SENB method. The electrical resistivity for materials with low TiN content was high, 108–1014 Ω cm. In the 21 vol.% TiN material the percolation limit of TiN was reached and the resistivity dropped to 10−1–10−2 Ω cm.

Use of quantitative convergent-beam electron diffraction in materials science

Methods for quantitative convergent‐beam electron diffraction are outlined and some results of our applications of convergent‐beam electron diffraction are shown, with emphasis on quantitative analysis of crystal structures in materials science. Examples of thickness measurements and determination of lattice parameters are presented. Measurements of low‐order structure factors to obtain information on bonding charge‐density distributions are reviewed, with examples from TiAl intermetallics. For non‐centrosymmetric crystals, a method to determine three‐phase structure invariants is given. Determination of polarity is also discussed. Microsc. Res. Tech. 46:130–145, 1999.

Channelling effects on oxygen-characteristic X-ray emission and their use as reference sites for ALCHEMI

Abstract Many oxides consist of a framework of oxygen atoms on known crystal sites together with metal atoms distributed over other sites. In this paper we report the first observation of a channelling effect on oxygen K X-ray emission and discuss its use as a reference atom for ALCHEMI.

Determination of Crystal Symmetry from Electron Channelling Patterns

The application of selected area channelling patterns in crystal symmetry determination has been investigated. In addition to rotation axes and mirror planes diffraction effects have been observed which may be ascribed to violation of Friedels law. It has thus been possible for the first time with this technique to distinguish uniquely between centrosymmetric and noncentrosymmetric point groups. This is demonstrated with examples taken from the cubic and the hexagonal crystal systems, of which some show very detailed zone-axis absorption HOLZ patterns. Owing to the large tilt angle represented in each diagram the directions of double scattering may easily be localized. It is shown that this may be utilized in determination of the extinctions and the space group. The sensitivity to deviation from centrosymmetry is discussed and found to be somewhat less than for the convergentbeam technique. Crystal sizes from micrometres to bulk may be used. The strength of the channelling method is its simplicity and in particular the possible use of bulk specimens which may also be directly studied by other techniques, for example by X-rays.

Reactions between La1−x CaxMnO3 and CaO-stabilized ZrO2 Part II Diffusion couples

The chemical stability of diffusion couples and coarse grain powder mixtures of calcium substituted lanthanum manganite and cubic calcia stabilized zirconia have been studied. The aim was to investigate the chemical stability of these materials as a model system for respectively the cathode and the electrolyte in solid oxide fuel cells. With increasing amount of Ca in lanthanum manganite, the major secondary phase was shifted from La2Zr2O7 to CaZrO3, and the thickness of the reaction layers of secondary phases was increasing with increasing heat treatment time. Precipitation of La2O3 had taken place in the perovskite containing low amounts of Ca (0 and 20 mol %). The transport mechanisms of the cations were strongly dependent on the interface geometry. La0.7Ca0.3MnO3 was observed to give the most stable interface to zirconia both in air and in reducing atmosphere (pO ∼ 10−6 atm). A-site deficiency of LaMnO3 was also observed to increase the stability. However, we conclude that a thin film of an electrode material consisting of lanthanum manganite on a zirconia substrate is unstable, regardless of A-site deficiency, because the solubility limit of Mn in the zirconia is not reached. From the experimental data, a reaction mechanism has been proposed, based on observations of relative diffusion rates.

On the minimum number of beams needed to distinguish enantiomorphs in X-ray and electron diffraction

The minimum strategy for distinguishing enantiomorphs by dynamical diffraction is determined. It is found that, in the absence of anomalous dispersion, it is possible to determine the absolute hand of an enantiomorphic crystal by three-beam dynamical X-ray or electron diffraction in a general orientation only if a fourth noncoplanar reciprocal-lattice point can be identified. Three-beam dynamical diffraction alone is unable to distinguish enantiomorphic forms. Identification is possible using four or more dynamical beams, in general, unless all relevant structure factors lie on a plane in reciprocal space passing through the origin. Supporting computations are given.

Determination of structure-factor phase invariants and effective structure factors in non-centrosymmetric crystals

Many-beam diffraction effects in non-centrosymmetric crystals have been studied with emphasis on three-beam interactions and determination of three-phase structure invariants in electron diffraction experiments. The effective structure factor has been determined both by numerical many-beam calculations and from the second Bethe approximation. The dependence of this factor on the phase invariant, the excitation errors and the magnitude of the structure factors involved has been discussed in detail. From the values of the effective structure factors at symmetrical positions on each side of a three-beam condition an asymmetry ratio is introduced. By a comparison of the observed variation in this ratio with theoretical profiles, it has been shown that the magnitude of three-phase invariants can be determined in the non-centrosymmetric case. This method may in principle be applied in any type of electron or X-ray three-beam experiments where variations in the effective structure factor are projected out. An example from electron channelling patterns is given.