[en] The chemical interactions of the ceramic high temperature superconductors (HTS) of the type YBa₂Cu₃O_7-x at the interface to silicon and metals (Ag, Pt, Au, Cu, Ni, Al, W, WSi₂) as well as with components of atmospheric air (O₂, CO₂, H₂O, D₂) were investigated. Such reactions can occur in the course of manufacturing and processing of these materials or, as the case may be, during storage at room temperature. The results provide fundamental data for technical applications. These pertain on the one hand to the microstructure of the contacts and the morphology as well as thermal stability of interfaces in composites and multilayer structures, while on the other hand to the optimization of the manufacturing conditions as well as aging behavior of thick layers. Indications to the selection of suitable contact materials and to storage conditions are given. (orig.)

[en] The binary Al-Gd and the ternary Al-Gd-Mg systems were calculated using the Calphad method. It is demonstrated that previous interpretation of ternary liquidus temperatures below 700 C must be related to other phase equilibria. The actual ternary liquidus temperatures are much higher, up to some 600 C above the previous interpretation in literature. They are widely governed by the high-melting compounds Al₂Gd and Al₃Gd with liquidus surfaces stretching far into the ternary system. A small number of key experiments in this work confirmed the calculated liquidus temperature and the phase relations. The available experimental data in literature fit excellently with the calculation in the binary Al-Gd system. In the ternary Al-Gd-Mg system, which is shown in several sections of the phase diagram, a good agreement can be observed too, considering the necessary reinterpretation of the liquidus temperatures suggested by Rokhlin et al. Ternary solubilities were not found experimentally. The ternary compound Al₄GdMg (τ) forms in a ternary peritectic reaction at 761 C. (orig.)

[en] The Mn - Sc binary system has been investigated experimentally in the entire composition range. Two new phases Mn₂₃Sc₆ and MnSc₄ have been identified. Three invariant reactions have been determined: L <=> Mn₂₃Sc₆ + (Mn), L + Mn₂Sc <=> Mn₂₃Sc₆ and L <=> Mn₂Sc + MnSc₄. Based on the experimental data from this work and the sparse literature data, a thermodynamic assessment has been performed. The calculated phase diagram is in fair agreement with the measured values. (orig.)

[en] In the course of our systematic study, covering a number of metals, the reactions of metal copper with YBCO were examined with Cu/YBCO diffusion couples for a variety of annealing conditions and first results are reported here. The electrical properties of contacts formed will certainly be strongly influenced by M/YBCO interface reactions. In most of the studies of the electrical contact properties, however, the metallurgical aspects of an interphase formation has not been revealed. This metallurgical aspect is in focus of our study. We intend to couple this in a later study of the electrical properties with another sample setup that allows for reproducible electrical conditions. (orig./BHO)

[en] Molybdenum-base alloys have excellent high temperature mechanical properties and joining of these alloys as heat sink materials is envisaged in many structural applications of fusion reactors. TZM is an important Mo-base alloy and diffusion bonding experiments were carried out in a hot vacuum press by sandwiching interlayers, Ti, Ni, Mo and Ta between two pieces of the TZM alloy in the temperature range 900 to 1200 C. Detailed metallurgical analysis was carried out using optical microscopy, scanning electron microscopy and by electron probe microanalyser. All the sandwiched diffusion couples consisting of TZM/X/TZM, (X = Ti, Ni, Mo, Ti) showed defect-free microstructures at the bonded interface at the optimised bonding conditions. Ti seems to be a more suitable interlayer compared to other interlayers in terms of bonding temperature and applied pressure. The TZM/Ni interface was marked by the presence of the intermetallic compound NiMo. The concentration profiles for diffusion couples TZM/Ti were asymmetric, whilst TZM/Mo and TZM/Ta were symmetric about the Matano Interface. The interdiffusion coefficients calculated using the Boltzmann-Matano method for TZM/Ti diffusion couples showed decreases in the D values with increase in the Mo concentration. (orig.)

[en] A systematic search for metal-tin systems which show a potentially high remelting temperature in diffusion-soldered bonds was done by investigation of phase formation and reaction kinetics in M-Sn systems (M=Zr, Hf, Nb, Ta, Mo). Phase equilibration experiments were done with powder samples prepared from pure elements, heated in evacuated silica capsules between 300 and 1000 C for 1 h to 200 days, and studied by X-ray diffraction analysis. Diffusion couples M/Sn (M=Zr, Nb, Ta, Mo) were prepared from the foils of pure elements and heated in a bonding furnace or in evacuated silica tubes at 300-700 C for 6-95 h. The couples were cross-sectioned and studied by scanning electron microscopy with energy-dispersive X-ray analysis. The rates of M-Sn reactions can be semiquantitatively ordered in the sequence Zr>Nb>Hf>Ta>Mo(-Sn). The activation energy of the better defined reactions Zr-Sn, Nb-Sn, Hf-Sn are in the range of 67-93(±15) kJ/mol. A Ta-Sn phase diagram was constructed with similarities to the V-Sn system. Only Zr-Sn is a potential candidate for diffusion soldering of high-temperature stable bonds, Mo is suggested as an effective diffusion barrier against the attack of liquid tin. (orig.)

[en] A consistent thermodynamic data set for the V-N system is obtained by a computer-aided least squares method applied to all of the experimental phase diagram and thermodynamic data available from the literature. The sublattice model V₁(N, Va)_a is used to model the phases: fcc (a = 1), bcc (a = 3), and hcp (a = 0.5). The liquid phase is described by the Redlich-Kister formula, and the gas phase is treated as an ideal gas. Special attention is paid to the modeling of the fcc phase with its exceptional bulk of experimental data. This phase is first analyzed by an ideal solution, then by a regular, and finally by a subregular interaction in the nitrogen sublattice. The other solution phases are analyzed with similar modeling procedures. This step-by-step analysis procedure permits insight into reliable estimations for the parameters at each of the higher level models. Comparisons between the calculated and measured phase diagram and thermodynamic quantities show that most of the experimental information is satisfactorily accounted for by the thermodynamic calculation. Inconsistent experimental information is identified and ruled out. The thermodynamic property of the fcc phase in the V-N system is compared with those in the Cr-N and Ti-N systems and related to the Neumann-Kopp's rule. (orig.)

[en] The eutectic microstructure in hypoeutectic Al-Si cast alloys is strongly influenced by AlP particles which are potent nuclei for the eutectic (Si) phase. The solidification sequence of AlP and (Si) phases is, thus, crucial for the nucleation of eutectic silicon with marked impact on its morphology. This study presents this interdependence between Si- and P-compositions, relevant for Al-Si cast alloys, on the solidification sequence of AlP and (Si). These data are predicted from a series of thermodynamic calculations. The predictions are based on a self-consistent thermodynamic description of the Al-Si-P ternary alloy system developed recently. They are validated by independent experimental studies on microstructure and undercooling in hypoeutectic Al-Si alloys. A constrained Scheil solidification simulation technique is applied to predict the undercooling under clean heterogeneous nucleation conditions, validated by dedicated experimental observations on entrained droplets. These specific undercooling values may be very large and their quantitative dependence on Si and P content of the Al alloy is presented. (paper)

[en] Reactions at the interface between YBa₂Cu₃O_6.8 (Y123) and metals (Cu, Ni or Al) were studied using quasi-infinite diffusion couples metal/Y123 which were encapsulated and annealed at 350 to 800 C for 5 to 150 h. The phase formation at the interface was analyzed in cross-sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. In addition, bulk powder mixtures of Y123 with Cu, Ni or Al were annealed up to 600 h at 800 C (500 C for Al) and phase analysis was performed using X-ray diffraction. At the Cu/Y123 interface a Cu₂O layer grows in an initial stage and eventually a multiphase reaction zone develops. These complex reaction products are consistent with thermodynamic considerations and the bulk powder results. At the Ni/Y123 interface a thin NiO layer grows slowly, and at the seemingly inert Al/Y123 interface an extremely thin Al₂O₃ layer is assumed. Both oxide layers act as an efficient diffusion barrier, blocking the further reaction which occurred vigorously in the bulk powder mixtures Ni + Y123 and, even stronger, in Al + Y123. A reaction mechanism via a transient formation of Al₂O₃ is proposed. The kinetics of the layer growth is diffusion controlled with a parabolic law for Cu and Ni. A quantitative comparison with the oxide growth in air reveals that the growth is much faster for the contact reaction metal/Y123 compared with metal/gas. The activation energies for the Cu/Y123 and the Cu/gas reactions are similar, whereas the activation energy for the Ni/Y123 reaction is about half the value of the Ni/gas reaction. (orig.)

[en] The Growth Restriction Factor, Q, proved to be useful to analyse and control grain refinement during solidification of alloys. It is known that in multicomponent alloys a simple summation of the Q_i values of the individual constituents taken from the binary phase diagrams can lead to grossly wrong results and that the ternary or higher-level phase diagram needs to be evaluated. This work demonstrates that the actual evaluation of Q using the liquidus gradient and partition coefficients of the multicomponent phase diagram requires some precautions and may be cumbersome. More importantly, this approach entirely fails if an intermetallic phase turns out to be the primary solidifying phase even in tiny amount. A very simple and general solution of this problem is illustrated for Al-Si-Mg-Cu alloys.