Volume 89, Issue 2

Das Ausgas- und Sinterverhalten von WC–Co Hartmetallen wird mittels komplexer thermischer Analyse besonders in Hinblick auf Schwindung (Dilatometrie), Masseverlust (Thermogravimetrie), Gasfreisetzung (Massenspektrometrie) und Eutektikumsbildung (DSC) untersucht. Hinsichtlich der Schwindung ist bekannt, daß mehr als 75 % der Gesamtverdichtung durch Sintervorgänge in der festen Phase bestimmt werden und daß diese Vorgänge stark von den Eigenschaften und der Vorbehandlung der Ausgangspulver abhängen. Ein Modell zur Beschreibung des Festphasensinterns wird vorgeschlagen. Das Modell erlaubt Schlußfolgerungen zum Verständnis der ablaufenden Prozesse und der zugrunde liegenden Mechanismen (Teilchenannäherung und Teilchenumlagerung) auf der Basis diffusionsgesteuerter Kriechprozesse an der Oberfläche der Teilchen.

The melting behaviour of pseudobinary (Ti, Mo)C–Co and (Ti, W)C–Co alloys was investigated by thermo-analytical and metallographic methods including compositional measurements by energy dispersive spectroscopy (EDS). As a result of these investigations with respect to their melting behaviour, it was concluded that the pseudobinary (Ti, Mo)C–Co and (Ti, W)C–Co alloys behave in an analogous way as TiC–Co alloys do. The solidification terminates with a pseudobinary eutectic reaction: melt Co (ss) + (Ti, Me)C (Me = Mo, W) at an eutectic temperature influenced by the [Me]/([Ti] + [Me]) ratio of the alloys. The nature of the starting materials, whether they are carbide mixtures or pre-alloyed carbides, affects neither the phase equilibria nor the solidification behaviour.

The melting behaviour of alloys from Ni-rich portions of the quaternary systems Ni–Ti–Mo–C and Ni–Ti–W–C, where the monocarbide phases δ-(Ti, Mo)C and δ-(Ti, W)C coexist with metal melts, was investigated by means of differential thermal analysis (DTA), metallography, X- ray diffraction and SEM/EDS microanalysis. In a first approximation, the systems can be treated as pseudo-ternary with the components Ni–TiC–MoC and Ni–TiC–WC in the compositional range studied. Pseudo-ternary eutectic reactions: melt ⇔ Ni (ss) + δ-(Ti, Me)C + C (Me = Mo or W) have been observed and graphite can be in equilibrium with Ni (ss) , δ-(Ti, Me) C and melt.

Swelling effects taking place during reaction sintering of Ti–Al elemental powder mixtures were investigated in detail. In order to correlate the macroscopic swelling with microscopic effects of phase formation several samples were studied using dilatometry, metallography, XRD (X-Ray Diffraction), SEM (Scanning Electron Microscopy), and in- situ-SEM. A model is presented describing four different stages of swelling. An approach has been developed to calculate the maximum swelling of the samples depending on the aluminium content and the formation of Al 3 Ti layers with different porosity levels. Finally a powder metallurgical route allowing pressureless sintering of pre-treated Ti–Al elemental powder mixtures is introduced.

A capacitive deflection measurement has been applied to investigate the stress formation during drying of granular ceramic films. The films were prepared from aqueous zirconia suspensions and cast via spin coating on a 3″ Si wafer. Drying stresses (σ̄ ≈ 10 6 Pa) were recorded for different film thickness and relative humidity. The sensitivity of the stress measurement is better than lOkPa, while the time resolution has been reduced to 10 −2 s. The relative humidity was controlled using a climate chamber. The Young’s modulus of the green films ( E = 45GPa) was determined using ultrasonic surface waves excited by short laser pulses. The drying process appears to be separated into three different drying regimes characterized by the rate of stress formation. In the first stage stress-free drying is observed which becomes more pronounced for thicker films. In the second stage the rate of stress formation is the largest and nearly constant with time. It decreases with increasing thickness. The third period starts with a turnover to a lower rate of stress formation which decreases with time until the stress reaches a saturation level. A 1-d model of the drying induced stress formation in a porous, visco-plastic film was applied and solved numerically by a Finite Differences Method. The model predictions are in good agreement with the experimental results. A microscopic model for the stress formation at the particle neck region explains the large stress increase in the drying body when a constant contact zone at the necks has been formed after a period of free shrinkage of the liquid pendular rings.

Im Mittelpunkt der Forschungstätigkeit des Jubilars stand die Aufklärung der Vorgänge, die bei der Verdichtung des Pulverpreßlings auf dem Wege zum kompakten Sinterkörper ablaufen. Hierzu kamen moderne diagnostische Verfahren zum Einsatz, u. a. auch die KOSSEL-Technik, an deren Färderung er maßgeblich beteiligt war. Ausgehend vom Prinzip der Entstehung und des Informationsgehaltes von elektronenstrahl- bzw. synchrotronstrahlangeregten Kossel-Aufnahmen werden Anwendungen im Mikrobereich, wie die Präzisionsgitterkonstantenbestimmung, die Einzelkorn-Orientierungsbestimmung, die Eigenspannungsanalyse sowie die Diagnose von Abweichungen von der kubischen Symmetrie, an ausgewählten Beispielen beschrieben.

In the binary system Sm–Co the region between Sm 5 Co 19 (79.2 at.% Co) and Co is discussed. SmCo 5 is not stable at low temperatures; it is transformed in the anomalous eutectoid reaction SmCo 5 → Sm 5 Co 19 +Sm 2 Co 17 , and the experimental evidence does not support other assumptions about this process. In this reaction, the crystallization of Sm 5 Co 19 is suppressed or greatly retarded, leaving the appearance of simple precipitation of Sm 2 Co 17 . This precipitation, in any form, destroys the high coercivity that is otherwise obtained in sintered magnets. The conditions for possible spinodal decomposition are described, and it is shown why it cannot be responsible for the coercivity mechanism in binary SmCo 5 . The partial replacement of Co by Cu in SmCo 5 also leads to the precipitation of Sm 2 Co 17 but now, contrary to the behavior in binary Sm–Co, it actually enhances coercivity.

Copper alloys with high electrical and thermal conductivities in combination with high strength at elevated temperature have been developed by including very fine thermodynamically stable TiC dispersoids into a Cu or an age-hardenable CuTi4 matrix. TiC dispersion strengthened copper alloys were prepared by mechanical alloying in a planetary ball mill using a blend of atomized prealloyed CuTi(X) powder and graphite powder. The mechanism of the formation of TiC dispersoids during milling and heating at various temperatures was investigated using X-ray diffraction and by TEM investigations. The processing technique was found to result in TiC dispersoids of different sizes (diameter 10 to 50 nm) preferentially localized on the grain boundaries. The matrix of the alloys is microcrystalline (grain size 100 to 500 nm).The observed creep behaviour of the alloys is described in dependence on size and volume fraction of the dispersoids and the composition of the matrix. It is demonstrated that these alloys – despite their microcrystalline microstructure – show a high creep strength, provided numerous small dispersoids are located on the grain boundaries, thus effectively preventing grain boundary sliding.

Purpose of thermomechanical treatments (TMT) is the specific modification of a microstructure by lattice defects and/ or particles of a second phase. This is discussed in a systematic manner using NiTi-based and Cu-based shape memory alloys as examples. Due to the martensitic phase transformation characteristics three principal temperature ranges should be distinguished with different deformation behaviour: T ≥ M d (stable austenite, ausforming), T ≤ M f (completely transformed martensite, marforming) and an intermediate range M s < T < M d with transforming austenite (strain-induced, stress induced, partially transformed). It is shown how microstructures can be obtained which lead to the best combination of strength with desired functional properties of shape memory alloys.

Low alloy PM steel Fe-1.5 %Mo-0.7 %C was prepared from elemental powders by pressing and sintering. Unnotched specimens were fatigue tested in the push-pull mode at 20 kHz up to N > 10 8 . It showed that heat treated materials offer better fatigue endurance strength than as-sintered ones especially at high numbers of cycles. The total porosity lowers the fatigue limit, the relative decrease being quite similar to that found with PM iron. Sintering temperature and time also strongly affect the fatigue behaviour. For predicting the fatigue limit, the load bearing cross-section A c was found to be a well suited microstructural parameter which can be measured in low-deformation fracture surfaces.

The behaviour of spherical and cylindrical voids in a solid under irradiation is investigated. The irradiation is assumed to result only in the creation of point defects – vacancies and interstitials. In the stationary stage of the process, i. e. for the time when the point defects have already taken their stationary distributions, the irradiation tends to increase the sizes of the voids. Special attention was paid to the nonstationary stage of the void evolution. Because the diffusion coefficient of interstitials is larger than the diffusion coefficient of vacancies, a fast decrease of the void size is observed during nonstationary stage. A critical void radius is found: voids with radii smaller than the critical one disappear during the nonstationary stage; voids with radii larger than the critical one will fail to disappear during the nonstationary stage. The radii of these voids will increase during the stationary stage later on. The influence of the finite size of the sample on the rate of the void evolution is investigated.

Self-Propagating High-Temperature Synthesis (SHS) of the Si-N 2 system is a typical SHS using a gas-solid system to obtain one of the most important advanced ceramics – silicon nitride. For improving the properties and reliability of Si 3 N 4 -based ceramics, SHS of Si–TiC–N 2 and Si–TiO 2– N 2 are investigated in this work. Stability diagrams at combustion temperatures are established through thermodynamic calculations. In contrast to the conventional idea of “diluent”, a new idea of “active diluent” is proposed. In these two systems TiC and TiO 2 not only act as diluents, but also participate in the reactions during SHS. Two new kinds of in-situ particulate reinforced ceramic composites, Si 3 N 4 /SiC/TiCN, Si 3 N 4 /Si 2 N 2 O/TiN, are developed.

Ausgehend von Betrachtungen zu den sich zwischen den Oberflächen medizinischer Implantate und dem Umgebungsgewebe ausbildenden Wechselwirkungen werden neue Ansätze zu deren optimierter Gestaltung vorgestellt. Für Implantate aus Titan und Titanlegierungen wird für den Fall von Knochen als Umgebungsgewebe eine Legierungsbildung zwischen Implantatoberfläche und Hauptkomponenten des Knochens (Kollagen I und Hydroxylapatit) unter Nutzung eines elektrochemischen Verfahrensschrittes vorgestellt. Dieser Verfahrensschritt arbeitet unter weitgehend physiologischen Bedingungen (Temperatur, pH-Wert), so daß auch Proteine zur optimierten Gestaltung von metallischen Implantatoberflächen genutzt werden können. Darauf basierende weitere Entwicklungen werden diskutiert.