[en] Short communication

[en] The applicability of the vacuum plasma spray process for the fabrication of composites with graded structures is investigated experimentally and by model calculations of the internal stresses by the finite element method. The process proved to be applicable to the fabrication of graded coatings as well as of free standing components with graded structure. This applies to metal/metal composites and to metal/ceramics systems, too. The graded composition leads to a pronounced reduction of internal stresses. (orig.)

[en] The Ti-Al and the Ti-Si system are chosen as model systems to study the origin of metastable phase formation during mechanical alloying of elemental powder blends with a negative heat of mixing. The results demonstrate that kinetics play only a minor role in the pahse selection, in particular for the final state. Instead, the intermetallic phases are energetically destabilized with respect to the solid solutions and the amorphous phase due to the destruction of the chemical long-range order upon milling. Thus the latter phases tend to form during mechanical alloying of elemental powder blends as well as during milling of powders of the stable intermetallic compounds. The metastable phases can be predicted by the free energy curves which are calculated by the CALPHAD method. In case of the Ti-Al and Ti-Si alloys, metastable structures and grain sizes in the nm range can be obtained which promise improved mechanical behaviour with respect to the processing of light-weight intermetallic compounds for high-temperature applications. (orig.)

[en] The velocity of DSC grain-boundary dislocations, with Burgers vector b^g=1/18[41anti 1], is measured for temperatures ranging from 823 to 1023 K and stresses ranging from 10 to 30 MPa, in silicon Σ=9 bicrystals. Screw segments are heavily kinked and much faster than edge ones, parallel to the [011] tilt axis of the bicrystals, which look very rigid. The apparent activation energy for the glide of fast screw segments is ≅1.3 eV and the stress exponent m≅2.5. Results are discussed in terms of the kink-pair model and estimates of kink formation and migration energies, based on the core structures of kinds, satisfactorily account for the observed anisotropy of DSC dislocation motion. (orig.)

[en] N-type conductivity and the ultimate Fermi-level position F_lim ≅ E_v + 1.0 eV are obtained in neutron bombarded InP. In the heavily defective samples the dc-conductivity is measured in the temperature range from 350 to 77 K. The neutron transmutation doping efficiency of InP is estimated after post-irradiation annealing up to 900degC. (orig.)

[en] Moessbauer spectra of the dimer of a bimetallic Eu-Al complex, [(CF₃COO)₃EuHAl(C₂H₅)₂.2 THF]₃ are measured at different temperatures (81 to 166 K) and some Moessbauer parameters, such as isomer shift, electric quadrupole splitting and asymmetric parameter, are derived from the experimental spectra. The Debye temperature of the compound determined by a Debye model is 128 K. The results indicate that europium in the organo-europium compound is trivalent and that a bridging hydrogen atom between two europium atoms exists in the dimer. The low Debye temperature implies that the weaker binding force between the europium atom and the lattice may be related to the structure and the chemical bonding in the organometallic compound of europium. (orig.)

[en] The formation of metastable phases in non-equilibrium processing techniques is stll not understood quantitatively. This is mainly a result of the lack of detailed information on kinetic constraints and the thermodynamics of the systems investigated. In this paper a thermodynamic approach for the interpretation of metastable phase formation is illustrated on the Ni-Ti system which is based on the determination of free energy curves by electromotive force measurements and the calculation of phase diagrams for metastable phases by applying the CALPHAD method. Special emphasis is placed on the thermodynamics of the undercooled liquid and the amorphous phase in order to predict the formation of amorphous alloys and supersaturated solid solutions by rapid quenching of the melt as well as during co-deposition and solid-state reaction of the elemental components. (orig.)

[en] Two types of icosahedral quasicrystals are observed in Al₆₅Cu₂₀Fe_15-xCr_x (0 ≤ x ≤ 15) alloys, the face-centred AlCuFe-type icosahedral phase with dissoluted Cr and the primitive AlCuCr-type icosahedral phase with dissoluted Fe. In the vicinity of Al₆₅Cu₂₀Fe₈Cr₇ a stable decagonal phase (a=0.45 nm and c=1.23 nm) forms competitively with the icosahedral quasicrystals. All these three quasicrystalline phases can be regarded as Hume-Rothery phases stabilized by the energy band factor. The density is measured to be 4.57, 4.44, and 4.11 g/cm³ for the icosahedral Al₆₅Cu₂₀Fe₁₅, the decagonal Al₆₅Cu₂₀Fe₈Cr₇, and the icosahedral Al₆₅Cu₂₀Cr₁₅ alloys, respectively. Depending on the composition in the range between Al₆₅Cu₂₀Fe₈Cr₇ and Al₆₅Cu₂₀Cr₁₅, several crystalline phases are observed during the transormation of the AlCuCr-type icosahedral phase: the 1/1-3/2-type orthorhombic (o) and the 1/0-3/2-type tetragonal (t) approximants of the decagonal phase, a hexagonal (h) phase, as well as a long-range vacancy ordered τ₃-phase derived from a CsCl-type structure with a=0.2923 nm. The structures of all the crystalline phases are closely related to those of the icosahedral (i) and decagonal (d) quasicrystals, which leads to a definite orientation relationship as follows: i5 parallel d10 parallel o[100] parallel t[100] parallel h[001] parallel τ₃[110]. (orig.)

[en] The microstructure of a crept Ti-52Al-3C sample was studied using transmission electron microscopy. Both primary precipitates (formed when the alloy was molten) and secondary precipitates (formed by a solid state reaction) are present; the shapes of both precipitates are plate-like. Ordinary large angle grain boundaries and ledges at twin boundaries are preferential sites for heterogeneous nucleation of secondary Ti₂AlC precipitates, whereas dislocations and small angle grain boundaries are not. (orig.)

[en] Coarsening of γ'-precipitates in nickel-base superalloys and of δ'-precipitates in aluminium-rich aluminium-lithium alloys is investigated by analytical transmission electron microscopy. The growth rate of the particles increases with their volume fraction. This increase is described best by Ardell's model. The γ'-volume fractions of the superalloys are independent of the annealing time t. In the case of the Al-Li alloys the analogous statement holds only if t is long enough to yield average δ'-particle radii of 9.4 nm or more. For shorter heat-treatments the δ'-volume fraction increases strongly with t. (orig.)