Volume 95, Issue 7

Tantalum has a wide range of applications due to its high melting temperature, its corrosion resistance and its mechanical and electrical properties. One of these applications are fine wires which are needed for Ta-capacitors. During fabrication of such components, the wires are exposed to high temperatures and, thus, grain growth combined with embrittlement becomes a major problem. In the present work, the effects of doping with small amounts of Si and YN on microstructure and mechanical properties of annealed Ta-wires have been investigated. Doped samples show a higher hardness and strength than samples of pure powder metallurgical (PM) Ta; however, the grain growth kinetics are very similar. Samples doped with Si and samples doped with both Si and YN exhibit almost identical properties.

The molecular mechanisms responsible for the deformation of wood, as well as the mechanical interaction of cell-wall components, such as cellulose, lignin and hemicelluloses, are not well understood. In a recently published experiment [1], we have shown that wood foils and single cells of compression wood of spruce (Picea abies [L.] Karst.) could deform permanently under tensile load via a stick and slip mechanism at the molecular level occurring during shear of the matrix between cellulose microfibrils. The shear originates from the fact that microfibrils are spiralling around the central lumen of the wood cell which can be considered as a hollow tube. During stretching, the microfibril angle between the cell axis and the direction of the cellulose was found to decrease in synchrotron X-ray diffraction experiments, giving rise to shear deformation of the matrix and to a recovery mechanism after irreversible deformation. The corresponding stick and slip mechanism is treated here in the framework of a simple mechanical model which shows that the cellulose fibril orientation might define slip planes for shear deformation, in analogy to the slip planes in single crystals as described by Schmid’s law. Moreover, the effect of fibril reorientation upon straining as well as the possibility of cell torsion are considered in the model and discussed with respect to experimental data.

Intermetallic titanium aluminides exhibit attractive thermophysical properties, which give them the potential for extensive use as lightweight structural components. Novel design concepts are based on alloys with the general composition (in at.%) Ti-45Al-(5–10) Nb, which were subjected to precipitation hardening. Optimized compositions have been identified that are capable of carrying stresses in excess of 700 MPa at service temperatures of 700 °C and have superior creep properties. The alloys exhibit at room temperature yield stresses in excess of 1GPa combined with plastic tensile elongations of about 2%. Wrought alloys of this type can be an attractive alternative to the nickel-base superalloys in certain ranges of stress and temperature. The future and promise of these new TiAl alloys lies in innovative processing methods designed to achieve better performance.

A new model for the case of a superposition of a phase transformation and a microstructure transition from columnar plane front to equiaxed dendritic growth for peritectic alloys is presented. This model is applied to Fe–Ni alloys where a phase transition from γ to δ takes place when the solute content decreases. The results of the model are compared with predictions of the nucleation constitutional undercooling model. They show that the peritectic two-(solid) phase region is restricted and may completely disappear depending on nucleation undercooling and dendrite growth kinetics.

High strength and high ductility together are prime requirements for modern structural metallic materials. Important industrial examples are steels for car components such as body sheets, tubes and forgings. The present paper traces two lines of alloy development, stable austenitic stainless steels and intentionally metastable austenitic stainless steels. Although both groups have different advantages and disadvantages, each one exceeds all other presently available structural metallic materials in terms of strength to density ratio at equal ductility.

Es wurden die Härten für die Phasen Ferrit, Martensit und angelassener Martensit an DP-Stählen, sowie für Ferrit und Bainit an TRIP-Stählen ermittelt. Die Ferrithärten von DP- und TRIP-Stählen unterscheiden sich und liegen bei 3,0 bzw. 3,7 GPa. Angelassener Martensit zeigt ca. 6,5 GPa, nichtangelassener Martensit, je nach Kohlenstoffgehalt, ca. 8,5 bzw. 11,5 GPa. Bainit in TRIP-Stählen hat eine Härte von etwa 7,5 GPa. Über Röntgendiffraktometrie wurden die Volumenanteile von Restaustenit und dessen Kohlenstoffgehalt über die Gitteraufweitung bestimmt. Die Restaustenitanteile betragen ca. 4 Vol.-% bei der DP-Variante und ca. 7 Vol.-% bis 14 Vol.-% bei den TRIP-Varianten. Der Kohlenstoff im Restaustenit wurde an den TRIP-Varianten mit 0,9 Ma-% bis 1,1 Ma-% ermittelt. Dabei ist der Einfluss der Legierungselemente Al und Si auf die Gitteraufweitung zu berücksichtigen.

Kaltleiterbauelemente werden funktionsbedingt mit Temperaturwechseln beansprucht. Aufgrund des Wärmetransports und der inhomogenen Verteilung der Wärmequellen kommt es im Bauteil zu Temperaturunterschieden, die ihrerseits wieder mechanische Spannungen (Wärmespannungen) bewirken. Diese Spannungen können so groß werden, dass sie die Festigkeit des Kaltleiters überschreiten und zur Zerstörung der Bauteile führen. Es wird ein Modell vorgestellt, mit dessen Hilfe es möglich ist, die mechanischen Spannungen in keramischen Kaltleitern (bei Kenntnis der relevanten Materialparameter) in Abhängigkeit von den angelegten elektrischen Beanspruchungen zu berechnen. Dabei wird die Wechselwirkung zwischen den elektrischen Feldgleichungen und der Wärmeleitungsgleichung gelöst und das elektrische Feld und das Temperaturfeld in Abhängigkeit vom Ort bestimmt. Im Anschluss daran kann das mechanische Spannungsfeld bestimmt und eine Ausfallrate der Teile ermittelt werden. Dieses Modell wird verwendet, um durch Auswahl geeigneter Werkstoffe und über die Veränderung des Bauteildesigns (Form, Größe, Aufbau der Bauteile) die Temperaturunterschiede (Thermal Management) und damit auch die mechanischen Spannungen im Bauteil gezielt zu verringern (mechanisches Konstruieren). Dadurch wird die Zuverlässigkeit der Teile im Einsatz erhöht. Weitere Maßnahmen zur Verbesserung der Zuverlässigkeit sind die Verbesserung der Festigkeit der Kaltleiterwerkstoffe und ein elektrischer Proof-Test, durch den Bauteile mit zu geringer Festigkeit aussortiert werden können.

Experimental and numerical results on photonic crystals are presented for the frequency range 26–60 GHz. The material used is alumina where two techniques have been applied for fabricating the photonic crystals: manual assembly of alumina rods and rapid prototyping. The observed positions of the fundamental and higher photonic band gaps are in excellent agreement with the calculated results. A new type of defect in the 3D woodpile structure, is created by inserting interstitial rods. As a new 2D structure a square parquet lattice is investigated. The concept of a negative index of refraction is adressed including a model calculation and an experimental demonstration by the transmission through a slab of a 2D photonic crystal.

Thermal and mechanical loading induce phase specific strains/stresses in MMCs. At elevated temperature part of the phase-specific strains/stresses relax. The relaxation of the phase-specific strains/stresses is determined by in-situ experiments using white high-energy synchrotron radiation. The experiments reveal that such time-resolved strain measurements are possible and that short-time phenomena can be accessed using white high-energy synchrotron radiation. The elastic strain relaxation behavior is similar for all lattice planes accessed. The influence of the temperature on the characteristic relaxation time is determined. The characteristic relaxation time appears to be independent of the amount of plastic deformation the sample suffers before strain/stress relaxation.

The ferritic-austenitic duplex-steels have a very complex precipitation and transformation behaviour, which requires professional treatment. Especially the precipitation of the σ-phase causes considerable changes with regard to the mechanical as well as the corrosive properties, which are to be considered during the treatment of the duplex-steels.

This paper presents details on the evolution of texture in Alloy 80A during the initial ingot break-down process by means of repeated hot-working (cogging). The ingot was produced by vacuum arc re-melting showing equiaxed and columnar grains. Compression tests were performed with a Gleeble system covering the range of temperatures, strains, and strain rates experienced during processing on an industrial scale. Texture analysis (neutron and electron backscatter diffraction) was applied to all specimens before and after hot deformation to reflect the thermo-mechanical history. The ingot exhibited a well pronounced <100>-fiber texture of the initial microstructure. Low deformation ratios did not change the type of texture, but the degree of the preferred orientation. High strains partially result in the destruction of the fiber texture and lead to the appearance of additional texture components, such as cube- and Goss-components. With continued deformation, recovery and recrystallization become predominant.

Two complementary techniques, namely three-dimensional atom probe and small-angle neutron scattering, were employed to study precipitation phenomena in a stainless maraging steel (Fe-12.3 %Cr-8.9%Ni-0.6 %Si-1 %Mo-0.6 %Al-0.8%Ti, wt.%) during ageing at 475 °C. Atom probe investigations revealed the precipitation of a single Ni-rich phase exhibiting an average particle diameter of 2.5 nm after 12 h. After ageing for 100 h these precipitates had grown to an average size of 4 nm. In addition, needle-or plate-like Ni-rich precipitates larger than 15 nm were present. Their compositions differ mainly in the amount of Fe, Ni and Ti. Furthermore, Cr-rich precipitates were observed. The size ranges and the number densities of the precipitates match well with those observed by small-angle neutron scattering.

Reintitan und einige Titanlegierungen werden als Ersatz für beschädigte Teile in den menschlichen Körper eingebaut. Die positiven Eigenschaften wie exzellente Biokompatibilität und hoher Korrosionswiderstand werden in einigen Anwendungen durch unzureichende mechanische Stabilität eingeschränkt. Diese Problematik wird an Reintitan für den Zahnersatz und an Titanlegierungen für Gelenkprothesen aufgezeigt. Im Zahnersatz wird ein Unterbau aus Titan aus ästhetischen Gründen und zum Verschleißschutz durch eine Keramikauflage verblendet. Die dafür notwendigen Prozessschritte verändern das Gefüge und den Oberflächenzustand und sind damit kritisch für den Einsatz, weil sie die Eigenschaften der Ausgangswerkstoffe negativ beeinflussen können. Es wird gezeigt, dass sich das günstige Ermüdungsverhalten von handelsüblichem Stangenmaterial stark verschlechtert, sich die ungünstigen Eigenschaften von Gussmaterial dagegen verbessern. Bei Gelenkprothesen führt häufig der Verschleiß bei Reibung gegen den als Gegenkörper eingesetzten Kunststoff zum Ausfall. Durch Ionenimplantation von Stickstoff und Kohlenstoff wird der Verschleißwiderstand wesentlich erhöht. Forschungsergebnisse, die in Zusammenarbeit verschiedener Institutionen gewonnen wurden, lassen Schädigungsmechanismen und die positive Auswirkung der Ionenimplantation auf den die Lebensdauer begrenzenden Verschleiß besser verstehen. Die Ergebnisse zeigen das Zusammenwirken von Verschleiß- und Korrosionsbeanspruchung, wie sie für den Ausfall von künstlichen Gelenken typisch ist. Sie liefern die Grundlagen zum Einsatz der Titanlegierung TiAl6V4 in hoch beanspruchten Gelenkteilen und zum Ersatz allergisch wirkender Legierungselemente in neueren Titanwerkstoffen für die Humanmedizin.

Residual stresses play an important role in the production of forged components like turbine discs. Variations in the processing parameters can lead to different residual stress states, which complicate subsequent processing steps, e. g. turning to the final shape. Therefore, a deeper understanding of the residual stress evolution during thermo-mechanical processing and the resulting distortions during machining is essential in order to optimize the manufacturing process with respect to cost efficiency and quality of the product. Consequently, the development of residual stresses in a forged turbine disc made of nickel-based superalloy IN718 was simulated using a finite element (FE) model. The experimental verification of the model was done by independently performed neutron strain scanning. For the studied forged and subsequently water-quenched disc the obtained residual stresses agree well with the stresses predicted by the used FE.