Volume 95, Issue 9

The orientation relationship (OR) between fcc and bcc lattices are described by crystallographic fundamentals using the example of Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W). Complete pole figures containing all variants will be used to distinguish even between slightly different ORs. EBSD on iron meteorites and duplex steel has been used to analyse a large number of crystal orientations with regard to a high statistical significance and a high probability to capture all variants in a single measurement. It is shown that the use of fixed OR models like K–S, N–W, Bain, Pitsch, or Greninger–Troiano does not satisfacturally reflect the observed experimental pole distributions. It is not convenient to use high-indexed lattice planes and directions to describe the small deviations from the given models. The Euler subspace representation offers a readily comprehensible tool to get an idea about the characteristic of the experimentally detected OR.

The metastable nanocrystalline γ form of gallium oxide has been prepared and its microstructure and thermochemistry have been studied for the first time by employing X-ray and electron diffraction, high-resolution transmission electron microscopy, adiabatic and differential scanning calorimetry. The randomly oriented crystallites of maximum 5 nm in size have been observed. The sponge-like morphology of γ -Ga 2 O 3 particles may explain the high specific surface area, previously reported for this material. The defect spinel-type structure of γ -Ga 2 O 3 is similar to that of γ and η -Al 2 O 3 . Up to 5.7 wt.% of water can be stored in γ -Ga 2 O 3 and subsequently released at elevated temperatures. Dry γ -Ga 2 O 3 specifically absorbs atmospheric water at room temperature. The transformation of γ -Ga 2 O 3 into stable β -Ga 2 O 3 occurs in two steps. In the range 650–800 K, γ' -Ga 2 O 3 is formed in the course of a reversible higher-order phase transition. The latter irreversibly transforms into β -form above 873 K. The enthalpy of this exothermic transformation is determined as –19.3 ± 0.4 kJ · mol –1 . The coefficients of the Gibbs energy equation for γ and γ' -Ga 2 O 3 have been assessed.

Aluminiumalloy (AA2014) based metal matrix composites were produced by spray atomization and co-deposition. The mass median diameter and mass flow rate of SiC were varied between 6 and 58 μm and between 10 and 30 wt.% of the melt flow rate, respectively. Microstructures of deposits and overspray powder particles were examined. Porosity and SiC content were measured at different positions in the preform. The fraction of SiC particulates incorporated in the preform varies between 4 and 11 vol.%, which is caused by the low compaction efficiency lying within the range of 30 to 50% of injected secondary particulates. Porosity is observed to increase with increase in deposited SiC content. Optical microscopy shows a uniform distribution of particles in the microstructure. Microstructural examination of overspray powder particles shows that the basic reason for the incorporation of smaller particles is the collision between reinforcement particles and the liquid droplets during flight. The results are presented and discussed in light of process parameters used.

This paper presents the first study of the plastic behavior of NiAl thin films with thicknesses in the micron and sub-micron range. The influence of geometrical and microstructural constraints was studied for different Al contents (45 to 50 at.% Al) and film thicknesses (0.2 to 3.1 μm) in the temperature range from 20 to 700 °C. The films were deposited on Si substrates and subjected to temperature changes resulting in thermal strains of up to 0.84%. The stress evolution in the films was measured by the substrate curvature method. The residual stress at room temperature of nearstoichiometric NiAl films significantly increased with decreasing film thickness indicating an increase of the film strength. At temperatures above 400 °C strong stress relaxation was found, which was more pronounced in thinner films suggesting the contribution of diffusional creep processes. The existence of a critical film thickness is suggested at which a transition from diffusion-controlled to dislocation-controlled plasticity occurs. The results have potential implications for the reliability of NiAl films as protective coatings in thermal engines.

Fracture toughness tests were performed on Al 2 O 3 /Nb sandwich specimens using chevron-notched bending bars. These tests allow the independent determination of the fracture toughness K c at a defined crack velocity and the total work of fracture J c . The J c values are much greater than the data for the energy release rate G c following from K c . The fracture surfaces were studied by optical and scanning electron microscopy. The results are discussed in terms of the fracture toughness of cleavage in the Al 2 O 3 and severe plastic deformation in the Nb sheet.

In situ straining experiments on α -Al 2 O 3 /Nb/α-Al 2 O 3 single crystal sandwiches have been performed in a high-voltage electron microscope in order to observe the dislocation motion connected with the fracture of these samples. The tricrystals had two different orientation relationships at the interfaces, both possessing high fracture energies. The in situ studies showed that the fracture occurs along the interfaces but more frequently inside the Nb sheet. The fracture is connected with strong plastic deformation. The crack opening and the dislocation processes are analysed.

The Kapoor-Frohberg cell model and two-sublattice ionic liquid model were used to describe liquid phase in the system MgO–Al 2 O 3 –SiO 2 . Solid solution phases were described by the compound energy formalism. Two data sets have been created based on the same description of solid phases and different models of liquid. It is shown that both models of liquid reproduce experimental data in binary systems very well. Thermodynamic parameters of ternary compounds cordierite and sapphirine were assessed from subsolidus reactions. The liquidus surface for the MgO–Al 2 O 3 –SiO 2 system is calculated for both models of liquid. The comparison of calculated phase diagrams with experimental data shows that the Kapoor-Frohberg cell model reproduces experimental data better than the ionic sublattice model.

Margules equations are used to express the thermodynamics of a ternary system containing one solvent and two solutes in the vicinity of solvent, in terms of first order interaction coefficients of binary and ternary systems. Considering just the first order Margules coefficients, the resultant excess Gibbs energy function was convergent and the derived logarithmic activity coefficients of solvent and solutes were thermodynamically consistent. In the present study Margules equations are modified to get consistent equations. Partial functions of a ternary system are deduced using these modified equations via the excess Gibbs energy function. Derived partial functions are thermodynamically consistent and also deduced results are the same as those obtained using Maclaurin infinite series. Using the activity coefficient expressions of solvent and solutes, the activity coefficients of solvent and solute are calculated in Ni– Cr–Fe and Fe–Ti –C ternary systems, which are in excellent agreement with the experimental data.

Diffusion couples and equilibrated alloys have been investigated in order to contribute to the knowledge of Ti –Zn phase diagram. Differential scanning calorimetry, X-ray methods and scanning electron microprobe analyses are applied. Diffusion couples studies show that TiZn 3 layers grow directly from the Ti–Zn liquid phase at 700 °C. Eight thermal peaks were observed in the interval 400–620 °C. Six of these peaks belong to stable three-phase equilibria and two are probably associated with metastable equilibria. Anew version of the Ti–Zn phase diagram, in the zinc-rich region, has been constructed.

The effect of the droplet size on the accuracy of surface tension measurement by the sessile drop method is discussed for liquid metals through a simulation by using the Laplace equation. It is found that with increasing size of the droplet, a higher accuracy of the measured value of the surface tension can be obtained. In order to make a large droplet of liquid metals, the constrained drop method with a special crucible shape was applied to measure the surface tension of liquid Ga, Sn, Bi, In, and Pb. The uncertainty of the measured surface tension was within 1%. The temperature dependences of the surface tension of liquid Ga, Sn, Bi, In, and Pb were obtained in the present experiment as follows: Ga: σ Ga = 737 − 0.062 T  mN/m (823 ≤ T ≤ 993 K ) Sn: σ Sn = 579 − 0.066 T  mN/m (723 ≤ T ≤ 993 K ) Bi: σ Bi = 417 − 0.070 T  mN/m (773 ≤ T ≤ 873 K ) In: σ In = 600 − 0.082 T  mN/m (673 ≤ T ≤ 993 K ) Pb: σ Pb = 499 − 0.089 T  mN/m (757 ≤ T ≤ 907 K )

The mathematical description of the kinetics of a layer development is subjectal to a continuous evolution connected with a growing amount of empirical data. Commencing with attempts to describe selected stages of this process, attempts are made at present to mathematically represent phenomena occurring in a gas atmosphere, on the boundary of gas-metal, and in a nitrided material, together with a description of mutual dependencies between these phenomena. The model proposed in this paper, which describes the growth of the zones of a layer nitrided in conditions of a diffusion quasi-equilibrium, is a simple and convenient tool allowing for both the determination of nitrogen diffusion coefficients in the layer’s monophase zones and for forecasting the growth of the layer’s zones as a function of the time of the process and the nitrogen potential.

Die Beständigkeit und das Anodenverhalten verschiedener Stahlarten, die Molybdän in „Chromispel-C“-Elektrolyten festhalten, wird untersucht. Ziel ist es, geeignete Anoden zu finden, da die klassischen Bleianoden sehr schnell in diesen Elektrolyten unter unkontrollierter Freisetzung von Chlorgas zerstört werden. Es wurde der Einfluss der Verweildauer in der Wanne, die Natur des Anodenmaterials sowie die Anodenstromdichte untersucht. Mit zunehmender Verchromungszeit und Stromdichte nimmt die Geschwindigkeit der chemischen und elektrochemischen Zerstörung der Anoden zu. Ein höherer Gehalt an Molybdän führt zu keiner Verbesserung der chemischen Beständigkeit. Diese lässt sich jedoch durch die Anwesenheit von Silizium in Stählen positiv beeinflussen. Die Schutzwirkung des Molybdäns zeigt sich deutlicher, wenn der Wanne elektrischer Strom zugeleitet wird. Wenn Stahl mit anderen leicht passivierenden Metallen (Ti, Al, u. a.) kombiniert wird, ist die Geschwindigkeit der elektrochemischen Korrosion am niedrigsten. Die Ergebnisse werden im Rahmen gegenwärtiger Theorien über den passiven Zustand der Metalle diskutiert.

A series of Hastelloy C tubes were solidified under a centrifugal field with various conditions of bulk liquid flow obtained with a controllable electromagnetic field. Their macrostructures show that the columnar-to-equiaxed transition was promoted both by increasing electromagnetic stirring and by increasing the superheat, and the application of electromagnetic stirring at different times can also influence the percentage of the total area occupied by the equiaxed region. Mechanical damage of dendrites is the most probable mechanism for the promotion of the columnar-to-equiaxed transition with high velocity flows. The detachment of the nuclei originated in the chill zone can also promote the columnar-to-equiaxed transition.