The mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30 have been investigated by measuring the elastic constants, coefficients of thermal expansion (CTE) and plastic deformation behavior in compression. The feasibility of these two clathrate compounds as a thermoelectric material in terms of mechanical stability under possible thermal stresses is evaluated by calculating thermal stresses that are expected to develop within these compounds when used as thermoelectric devises.

Deformation behavior of (Nb)/α-Nb5Si3 two-phase alloys is examined by high temperature compression tests. The alloys exhibit brittle fracture behavior at temperatures up to 1473 K, while reasonable compressive deformability at 1673 K. Upon high temperature compression of the alloys, the flow stress gradually decreases after the peak stress due to the recrystallization/recovery in the (Nb) phase, as well as the increase in the density of mobile dislocations within the α-Nb5Si3 phase. Two types of slip systems that operate in α-Nb5Si3 have been identified as {011)<11-1] and {001)<100] in the present study.

The ordered alloy phase of Ti3Al shows a rather wide solid solution range in aluminum and also in vanadium. Several Ti-Al-V ternary alloys have been prepared to investigate the alloy composition effect upon microstructure, crystallography, and mechanical characteristics. The materials containing 75, 70, and 65 at.% titanium, and 0 or 5 at.% vanadium were prepared by arc melting. Metallographic observation has revealed that the binary Ti-Al alloys contained somewhat coarse grains with about 100 μm grain diameter. In contrast to this, ternary alloys containing 5 at.% vanadium showed smaller grained microstructures with grain diameters around 15 μm. The grain size could not be adjusted to a unified value in the present study. X-ray diffraction study and microanalysis showed that the alloys contained single phase α2. Not every possible diffraction peak of the D019 ordered structure has been observed in the XRD patterns. The lattice parameters, a and c, were observed to decrease as the aluminum content increased and also when vanadium was added. Compression tests have been performed at various temperature ranging from an ambient temperature up to 1300K on rectangular parallelepiped specimens with 2mm×2mm×3mm dimensions. Alloys containing more aluminum showed higher strength, and vanadium addition enhanced the strength of the alloys. In some alloys deformability and strength are both enhanced by vanadium addition in some alloys. Temperature dependence of strength showed a little variation upon chemical compositions.

To improve the insufficient oxidation resistance of Titanium Aluminides at temperatures above 750°C the fluorine effect offers an innovative way. The focus of this paper is to define the fundamental material variables for the fluorine effect related to the macroscopic behaviour (oxidation resistance) and its long time stability. The thermodynamic model predicted the fluorine effect for the TiAl within a corridor of total fluorine amount in terms of partial pressures. To realize the fluorine effect the required F-concentration [in at.-%] within the near surface region had to be found. Using fluorine ion implantation several fluences within 5e15 and 5e17 F cm-2 were implanted with an energy of 20 keV. The implantation depth profiles were calculated by using the Monte Carlo simulation code T-DYN and verified experimentally by using the non-destructive PIGE - technique (Proton Induced Gamma-ray Emission). After oxidation tests at 800°C – 1000°C a value of 2e17 F cm-2 / 20 keV was determined as an optimal implantation parameter set. Following these results the maximal fluorine concentration was identified to be a fundamental material variable for starting the alumina formation with a required fluorine amount of about 40-45 at.-%. However this maximum fluorine concentration showed a rapid decrease to values less than 5 at.-% only after a few hours of oxidation (900°C and 1000°C) followed by a slow decrease. Therefore the maximum fluorine concentration Cmax – now located at the metal/oxide – interface – was identified to be a fundamental parameter for the long time stability. An exponential decay function containing a constant term of about 1 at.-% was found to describe the time behaviour of Cmax for isothermal and cyclic oxidation (900°C, 1000°C). Because the alumina scale on the surface acts as a diffusion barrier for fluorine, the stability of Cmax is strongly influenced by the F-diffusion into the metal. From the F-depth profiles the diffusion coefficient of fluorine into the TiAl at 900°C was determined as a fundamental parameter for the long-term stability of Cmax showing a value of 1.56e-15cm2/s.

A special feature of the Co-Nb system is the occurrence of the three different types of Laves phase with the ideal composition NbCo2. The C36 and the C14 phases are stable only at high temperatures and exhibit small homogeneity ranges, whereas the C15 phase forms with a broad homogeneity range enclosing the ideal composition. In case of C36 and Co-rich C15 the additional Co atoms substitute Nb atoms (Nb1-xCox)Co2. In the C36 phase the Co atoms preferentially occupy one of the two crystallographic Nb sites and are locally displaced by approx. 20 pm from the original Nb positions allowing the formation of favorable short Nb-Co bonds. In Nb-rich C14 only one of two crystallographic sites is occupied by Nb. The Kagomé layers of the Co atoms are distorted in the crystal structures of the hexagonal Laves phases. The deviation from the idealized crystal structure is mainly governed by the valence electron concentration. Quantum mechanical calculations show that the distortion is already an inherent feature of the point defect-free structures.

The intermetallic compound MoSi2 crystallises in the body-centred-tetragonal C11b structure and while it is brittle when loaded in tension, it deforms plastically in compression even at and below the room temperature. The ductility of MoSi2 is controlled by the mobility of 1/2〈331] dislocations on {013) planes but the critical resolved shear stress for this slip system depends strongly on the orientation of loading and it is the highest for compression along the 〈001] axis. Such deformation behaviour suggests that the dislocation core is controlling the slip on the {013)〈331] system. Since the most important core effect is dissociation into partial dislocations connected by metastable stacking faults the first goal of this paper is to ascertain such faults. This is done by employing the concept of the γ-surface. The γ-surfaces have been calculated for the (013) and (110) planes using a method based on the density functional theory. While there is only one possible stacking fault on the (110) plane, three distinct stacking faults have been found on the (013) plane. This leads to a variety of possible dislocation splittings and the energetics of these dissociations has been studied by employing the anisotropic elastic theory of dislocations. The most important finding is the non-planar dissociation of the 1/2〈331] screw dislocation that is favoured over the planar splittings and may be responsible for the orientation dependence of the critical resolved shear stress for the {013)〈331] slip system.

Laves phases comprise a large group of intermetallic compounds with general composition AB2 and multi-component derivatives. The crystal structures of Laves phases are often regarded as closest packing of spheres. This observation, beginning with very early work on Laves phases, has led many researchers over the years, to emphasize the role of geometrical factors in the formation of Laves phases. In order to develop a firm understanding of chemical bonding in Laves phases and assess the importance of geometrical factors, we undertake a first-principles-electronic structure-based chemical bonding analysis for several representatives. As a first step towards this goal we concentrate on the K-Cs system which contains the Laves phase CsK2 and the hexagonal Cs6K7 compounds. In such alkali-metal-only compounds it is generally expected that chemical bonding effects are minimal. Atom volumina and charge transfer investigations reported here, however, suggest that even in alkali metal-alkali metal Laves phases chemical bonding plays a non-negligible role.

The coarsening kinetics of γ’ precipitates in binary and ternary Al3Sc1-xZrx alloys is studied by using the two- and three-dimensional phase-field simulations. Our focus is on the influence of diffusion coefficients of Sc and Zr atoms on the transformation path kinetics from disordered f.c.c. matrix to two phases equilibrium state with γ’ precipitates and f.c.c. disordered matrix. Our simulation results demonstrate that in the case of binary alloys taking into account the concentration dependence of the mobility of atoms decreases the coarsening rate. In the case of ternary alloys we show that the Al3Sc particles precipitate first following by appearance of a Zr-rich shell. Our simulations results are in good agreement with experimental observations.

B2-RuAl as a potential intermetallic for thin-film applications at high temperatures is studied with respect to thin-film synthesis and cyclic oxidation behaviour. Using the multilayer approach, single phase RuAl thin films were fabricated. The phase sequence from the elements Ru and Al goes through RuAl6 to the final product RuAl. To understand the reaction mechanism calorimetric as well as kinetic experiments were performed. The cyclic oxidation behavior is characterized at 1200 °C up to 47 h. The morphology of the grown alumina shows no cracks or regions of spallation which indicates the good cyclic oxidation behavior. Compressive stresses in the oxidation scale of about 1.2 GPa at maximum were determined.

Since the ternary intermetallic compound Co3(Al,W) with the L12 structure was discovered, two-phase Co-base alloys composed of the γ-Co solid-solution phase and the γ'-Co3(Al,W) phase as a strengthening phase have been investigated as promising high-temperature materials. Some Co-base alloys have been reported to exhibit high-temperature strength greater than those of conventional Ni-base superalloys. Although the excellent high-temperature physical properties of the Co-based alloys are considered to result from the phase stability and strength of Co3(Al,W), the pristine physical properties of Co3(Al,W) have not been fully understood, supposedly due to the difficulties in obtaining single-phase Co3(Al,W). In the present study, we examine the effect of heat treatment on the microstructure of alloys with compositions close to single-phase Co3(Al,W) as well as their mechanical properties, e.g. elastic modulus, thermal expansion, etc., in hope of deriving the pristine properties of the Co3(Al,W) phase. A single crystal with the composition of Co-10Al-11W grown by floating-zone melting exhibits a thermal expansion coefficient of 10×10-6 K-1 at room temperature, which is virtually identical to those of the commercial Ni-base superalloys. However, it increases with increasing temperature followed by a discontinuity at around 1000°C, inferring the phase transformation from γ' to γ. The investigated thermal expansion behavior indicates that the lattice mismatch between the γ' and γ phases is reversed from positive at room temperature to negative at high temperatures above around 500°C. The results of elastic property measurement and environmental embrittlement investigation of polycrystalline Co3(Al,W) will also be presented.

The authors have developed a new atomization model enabling direct numerical simulation of the simultaneous flow of compressible atomizing gas jets and a weakly compressible liquid metal stream. It has been used to simulate the atomization of a Ni-50wt.%Al melt stream by argon gas jets in a closed-coupled atomizer. The 2D simulation results show that the presence of the liquid intermetallic stream significantly influences the field variables, particularly the aspiration pressure. At the gas plenum pressure used, the gas nozzles are choked and hence the gas flow upstream of the tip of the liquid delivery tube is not influenced by the presence of the liquid intermetallic stream, whereas the downstream gas flow is affected. Significant differences between model predictions assuming either incompressible or compressible gas are reported. Besides the atomization of liquid intermetallic stream by argon gas, this unified atomization model is available for use to simulate a variety of different twin-fluid atomization processes.

The influence of a heat-treatment on the plastic deformation behavior in Mg12ZnY with a long-period stacking ordered (LPSO) structure was investigated by using crystals grown by the Bridgman method. Annealing of the crystal at 798 K for 3 days induced the change in the crystal structure of Mg12ZnY from the 18-fold rhombohedral structure (18R) to the 14-fold hexagonal structure (14H). The plastic behavior of those LPSO crystals showed a large variation depending on the loading axis in both crystals, because of the limitation of operative deformation modes in them. The change in the stacking sequence in the LPSO crystals did not show a large influence on the plastic deformation behavior at room temperature.

Single-phase Fe-Nb and Co-Nb Laves phase alloys were produced by arc melting and levitation melting. By casting the levitation melted alloys in a preheated mould and subsequent slow cooling to room temperature, solid rods of 15 mm in diameter and about 100 mm length of the brittle Laves phases were obtained. Within the extended homogeneity ranges of the NbFe2 and NbCo2 Laves phases, the Vickers hardness was measured in dependence on composition. The results show that the hardness has a maximum at the stoichiometric composition in both systems, indicating defect softening. Nanoindentation measurements on a Co-Nb diffusion couple confirm the dependence of the hardness on composition. In addition, these measurements indicate that the crystal structure of the Laves phase polytype – cubic or hexagonal – seems to have no effect on the hardness. Indentation fracture toughness KIC-IF data for the different polytypes of the Laves phases were evaluated from the Palmquist cracks originating from the edges of the Vickers indentations.

Four kinds of L12-type Ni3(Si,Ti) intermetallic alloys with a quaternary element X (X: Al, Cr, Co and Mo) were warm rolled accompanied by intermediate annealing and then cold rolled to thin foils. The effects of alloying element on microstructure, tensile properties and oxidation resistance of the cold-rolled Ni3(Si,Ti) foils were investigated. The Al-added Ni3(Si,Ti) alloy showed an L12 single-phase microstructure, while the Cr-, Co- and Mo-added Ni3(Si,Ti) alloys exhibited a two-phase microstructure consisting of L12 and fcc Ni solid solution phases. Room-temperature strength of the Ni3(Si,Ti) foils was slightly enhanced by the addition of quaternary element, whereas high-temperature strength was significantly enhanced especially by the addition of Mo and Co. High-temperature tensile elongation was remarkably improved by the addition of all the elements investigated. On the other hand, oxidation resistance was improved by the addition of Al and Cr.

Thermal vacancy formation correlated with atomic ordering was modelled in B2-ordering A-B binary intermetallics. Ising Hamiltonian was implemented with a specific thermodynamic formalism for thermal vacancy formation based on the phase equilibria in a lattice gas composed of atoms and vacancies. Extensive calculations within the Bragg-Williams approximation [1] were followed by Semi-Grand Canonical Monte Carlo (SGCMC) simulations. It has been demonstrated that for the atomic pair-interaction energies favouring vacancy formation on A-atom sublattice, equilibrium concentrations of vacancies and antisite defects result mutually proportional in well defined temperature ranges. The effect observed both in stoichiometric and non-stoichiometric (both A-rich and B-rich) binary alloys was interpreted as a tendency for triple defect formation. In B-rich alloys vacancy concentration did not extrapolate to zero at 0 K, which indicated the formation of constitutional vacancies. Energetic conditions for the occurrence of the effects were analysed in detail. The modelled temperature dependence of vacancy concentration in the B2-ordering A-B binaries with triple defects will be included in the Kinetic Monte Carlo (KMC) simulations of chemical ordering kinetics in these systems with reference to the experimental results obtained for NiAl [2].

Titanium aluminides containing high niobium additions emerged as an attractive alloy family for automotive and aero-engine applications. Their processing by near net shape casting is rather demanding, not only due to easy contamination but also due to the fact that microstructure formation during solidification and subsequent solid state transformations sensitively depends on alloy composition and the applied processing conditions. The sequence of phase formation during solidification of the ternary alloy Ti-45Al-8Nb was analyzed based on non-equilibrium thermodynamic calculations and solidification experiments. This alloy solidifies completely via the β(Ti) phase, because the nucleation undercooling for the α(Ti) phase is high enough to prevent its formation. Thermodynamic calculations and experiments are shown to converge at last, with only minor improvements being necessary to correctly describe the metastable eutectic reaction “Liquid → β(Ti) + γ-TiAl” that occurs at the end of the solidification path.

In as-cast ingots produced by arc-melting, several metastable polytypic modifications of NbCr2 were found additional to the cubic C15 phase stable at room temperature: C14, C36 and 6H-type structures, often highly faulted and/or intergrown. Strikingly, these phases had formed at locations of the specimen which had experienced a relatively low cooling rate, whereas the C15 phase was formed preferentially in regions which had experienced the highest cooling rates.

A two-step treatment, oxidation in air followed by reduction in hydrogen, was carried out to modify the smooth Ni3Al foil surface into Ni particles supported on the oxide structure. The surface structure significantly changed depending on the oxidation temperature. A layer of granular NiO formed on the outer surface and inner oxide zone (IOZ) over Ni3Al foil surface after oxidation at 973 K. The IOZ was a mixture of Al and Ni oxides. In contrast, a large amount of faceted NiO particles formed on the outer surface after oxidation at 1173 K. Beneath the NiO particles, NiAl2O4 thin layer formed on IOZ over Ni3Al foil surface. And then, these NiO was selectively reduced to Ni after reduction treatment, constituting an oxide supported Ni particles structure. These results suggest that it is possible to modify the surface structure of Ni3Al foils simply by oxidation-reduction treatment.

Three intergovernmental research organizations from the EIROforum collaboration: the European Space Agency (ESA), the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL), are cooperating to perform advanced experimental characterization in the field of materials science within the framework of the IMPRESS Integrated Project. This project aims to develop and test two distinct prototype-based intermetallic materials: (i) γ-TiAl turbine blades for aero-engines and stationary gas turbines, and (ii) Raney-type Ni-Al catalytic powder for use in hydrogen fuel cell electrodes and hydrogenation reactions. The opportunity to carry out investigations combining the use of both synchrotron radiation at the ESRF and neutrons at the ILL provides unique experimental data to complement other benchmark experiments performed on the ground and in microgravity. We present an overview of the different synchrotron X-ray and neutron characterization techniques implemented at ESRF and ILL to study the solidification and subsequent processes leading to the final products for these two materials.

For higher fuel efficiency and greater thrust to weight ratios, there is a continuous drive for higher performance turbine engine components. Nb-silicide intermetallics, owing to their high melting point and high-temperature strength, are potential candidates for high temperature applications. These intermetallics when precipitated in the metal matrix of a (Nb) solid solution, result in intermetallic-strengthened metal matrix composites that have a good combination of room temperature toughness and high temperature strength. The microstructures of these in-situ composites can be complex and vary significantly with the addition of elements such as Ti and Hf. Hence an improved understanding of phase stability and the microstructural evolution of these alloys is essential for alloy optimization. In the present paper we describe binary alloy microstructural evolution modeling of dendritic and eutectic solidification obtained using phasefield simulations. The effect of parameters such as heat extraction rate, the ratio of the diffusivity of the solute in liquid to solid, and the liquid-solid interfacial energy, on microstructural evolution during solidification is discussed in detail.