Volume 94, Issue 5

The fatigue life of plain ferritic carbon steels fatigued in stress-controlled (plastic-strain-controlled) tests exhibits a maximum (minimum) at a temperature of approx. 300 °C as a result of dynamic strain ageing (DSA). In the regime of DSA, an enhanced cyclic hardening related to an increased dislocation density is observed, and the dislocation arrangement changes from a dislocation cell structure (at intermediate stress amplitudes) at room temperature to a dense edge di-/multipolar bundle/wall/vein structure in the temperature range of DSA and back to a dislocation cell/ subgrain structure at higher temperatures. Related effects occur also in α-iron containing some carbon when fatigued at room temperature at a very low strain rate. This behaviour is considered to be typical of body-centred cubic materials. In the present study, the dislocation mechanisms responsible for this behaviour are discussed in detail, and a dislocation glide model is developed which can explain both the enhanced dislocation density and the change of the dislocation arrangement in the regime of DSA. The model is based on the different mobilities of mobile edge and less mobile screw dislocations in body-centred cubic metals and on the strong tendency towards multiple slip leading to a dislocation cell structure at lower temperatures. In the regime of DSA, it is proposed that the edge dislocations are hindered more strongly in their glide motion by the solute carbon atoms than the screw dislocations so that screws and edges are then more or less equally mobile. Taking into account that screw dislocations of opposite sign annihilate mutually by cross slip, this explains the observed dislocation arrangement of dense edge di-/multipolar bundles/ walls/veins of high dislocation density which, in turn, leads to an enhanced cyclic hardening rate. The dislocation sub-grain structure observed at higher temperatures reflects the increasing tendency for the transformation from dislocation cell to subgrain structures, as the temperature is increased.

Three single-crystalline Ni-base superalloys with varying content of the γ′ phase are investigated with respect to their microstructure and creep behaviour at 1000 °C. It is shown by convergent beam electron diffraction, X-ray diffraction and energy-dispersive X-ray spectroscopy that the composition of the matrix and precipitates as well as the lattice misfit between both phases are similar for all three model alloys despite a variation of the γ′ content from about 50 to about 65 %. This allows to study the influence of the precipitate volume fraction on high-temperature behaviour in separation from other microstructural parameters. Highest creep strength is found for the alloy with intermediate particle content (about 58 %), which is significantly below the particle volume fraction of contemporary single-crystal superalloys such as CMSX-4.

Den durch thermo-mechanische Vorbehandlung erzeugten Titankarbidauscheidungen vom Typ TiC sowie anderen Phasen wird in dem austenitischen Stahl X10 CrNiMoTiB 1515 mit der Werkstoffnummer 1.4970 eine positive Rolle bei der Reduzierung der bestrahlungsinduzierten Porenbildung und He-Versprödung zugesprochen. Die Untersuchungen zeigen jedoch, dass Titankarbide unter Bestrahlung in diesem Werkstoff nur oberhalb etwa 520 °C stabil bleiben, dort zur Rückhaltung von Helium in der Matrix beitragen und damit zu einer Minderung der Hochtemperaturversprödung durch Helium führen können. Unterhalb dieser Temperaturgrenze wird die Auflösung von Titankarbiden und die Bildung neuartiger, unter Bestrahlung stabiler Phasen, wie Ni 3 Si und NiSi, beobachtet. Sie haben einen entscheidenden Einfluss auf die Porenbildung und das Schwellverhalten unter Bestrahlung.

The behaviour of steels at multi-step cyclic loading was explored up to now almost exclusively in fatigue-life-oriented investigations. Thus, only few works exist dealing with the cyclic deformation behaviour at two- and multi-step loading. Therefore, the cyclic deformation behaviour at two-step experiments with a single amplitude change (2-block experiments) and with multiple changes between two blocks of certain length and different amplitudes (multi-block experiments) was investigated in this work at the technically important steel AISI 4140 (German grade 42CrMo4).

Eine zutreffende Lebensdauervorhersage für gekerbte Proben mit dem örtlichen Konzept auf der Basis von Daten, die an glatten Proben ermittelt wurden, setzt voraus, dass bei identischer zyklischer Beanspruchung ein vergleichbarer Schädigungsablauf an der glatten und der gekerbten Probe auftritt. Die an einem normalgeglühten Stahl Cm15 durchgeführten Untersuchungen deuten darauf hin, dass die an glatten Proben beobachtete Bildung und Ausbreitung von Ermüdungslüdersbändern und die damit verbundene Aufrauung der Probenoberfläche bei inhomogener Beanspruchung eingeschränkt wird, was zu einer Verletzung des oben angeführten Äquivalenzprinzips und damit zu deutlichen Unterschieden der Anrisslebensdauern bei gleicher örtlicher Beanspruchung führen kann. Durch eine Grobkornglühung des Werkstoffes gewinnt die Rissbildung an den Korngrenzen oder an Ermüdungsgleitbändern an Bedeutung. Dadurch wird der Effekt der Lüdersdehnung verringert, wodurch sich eine Verbesserung der Vorhersagegenauigkeit mit dem örtlichen Konzept erzielen ließ.

Vollräder und Radreifen gehören zu den höchstbeanspruchten Komponenten von Schienenfahrzeugen. Während höhere Fahrgeschwindigkeiten insbesondere die dynamischen Kräfte erhöhen, führen zunehmende Beschleunigungs- und Bremsmomente zu einer enormen Schlupfbeanspruchung der Räder. Eine Optimierung des Systems Rad/Schiene ist nur bei genauer Kenntnis des Ermüdungsverhaltens der eingesetzten Rad- und Schienenwerkstoffe in technisch relevanten Wärmebehandlungszuständen möglich. In der vorliegenden Arbeit werden neben der licht-, raster- und transmissionselektronenmikroskopischen Charakterisierung der Mikrostrukturen, die zyklischen Eigenschaften ausgewählter Radstähle unter Berücksichtigung von Mittelbeanspruchungen vorgestellt. Dabei werden ergänzend zu Spannung – Dehnung-Hysteresismessungen thermometrische, resistometrische und Giant-Magneto-Resistor Messverfahren genutzt.

This paper presents results on the evolution of microstructure in 12% Cr tempered martensite ferritic steels during heat treatment and creep. Prior austenite grain size and micro grain size measurements were performed using optical microscopy, and scanning and transmission electron microscopy. Moreover, orientations of micro grains are determined using TEM and SEM. Prior austenite grain sizes, micro grain dimensions and orientation relationships between micro grains are interpreted in terms of processes associated with the formation of martensite and ferrite during heat treatment and creep. The elongated micro grains (in the interior of prior austenite grains) can have high- or low-angle boundaries. The hierarchical evolution of internal interfaces is described. Micro grain boundaries most often represent (i) variant boundaries separating individual former martensite variants and (ii) subgrain boundaries (associated with recovery processes during tempering and creep). The present results also show that there is a need to study (i) the evolution of dislocation density during heat treatment and creep and (ii) the precipitation of carbides during tempering and creep in relation to the different types of internal interfaces.

Für den Karosserieleichtbau wurden in den letzten Jahren erfolgreich verschiedene Gruppen von höherfesten, kaltgewalzten Stählen entwickelt. Neben dieser Werkstoffentwicklung wurden auch neue Leichtbaukonzepte erarbeitet, wie zum Beispiel maßgeschneiderte, geschweißte Platinen (so genannte Tailored Blanks). Diese Platinen, die aus unterschiedlichen Werkstoffen und/oder aus Feinblechen mit unterschiedlichen Blechdicken zusammengesetzt sind, werden in dieser Arbeit bezüglich ihrer Umformbarkeit bewertet. Dazu werden Grenzformänderungskurven experimentell ermittelt und mit einem Finite-Elemente-Modell berechnet. Es zeigt sich, dass das Versagensverhalten durch die Ausbildung sowie durch die Position der Schweißnaht während des Umformversuchs beeinflusst wird. Dabei ist die Breite der Schweißnaht eine wichtigere Einflussgröße als die lokale Aufhärtung.

The strength of B2-ordered Fe 60 Al 40 has been investigated with respect to the influence of vacancies and interstitial impurities. Preconditioned states with different ratios of the concentration of vacancies and interstitial impurities have been established, which then were recovered by subsequent annealing treatments. The evolution of the recovered states with temperature has been monitored by measuring the strength (ultra-microhardness) at room temperature as a function of the annealing temperature. The observed dependencies are discussed on the basis of the resistance of vacancies and impurities to dislocation motion, of vacancy – interstitial interactions and also of the possible formation and dissolution of precipitates (primarily perovskites). The yield stress maximum which, independent of the stoichiometry, appears near 800 K is found to be controlled by vacancy migration in combination with strain ageing by interstitial agglomerates and by the formation of precipitates.

The high-temperature fatigue behaviour of smooth cylindrical specimens of a near-γ-TiAl alloy with duplex microstructure was studied performing total-strain-controlled low-cycle-fatigue tests in the temperature range 500 – 750 °C. The tests were run in air and high vacuum. Mean stress, environmental degradation (oxygen- and hydrogen-induced embrittlement) and plastic strain range were identified to be the most significant parameters affecting damage evolution. The cyclic stress– strain response was found to depend strongly and characteristically on temperature. Below the brittle-to-ductile-transition temperature ( T BD ) of about 650 °C, cyclic deformation leads to a pronounced initial hardening manifesting itself in an increase of stress amplitude and a decrease of plastic strain range. Conversely, cyclic deformation curves from tests performed above T BD show a marked state of saturation which is established already after few cycles. With increasing temperature, fatigue life decreases in vacuum. However, testing in air not only decreases fatigue life tremendously as compared to vacuum, but also leads to an inverse temperature effect, i. e., the number of cycles to failure increases with temperature.

Fatigue crack growth in 2024-T3 has been studied in ambient air and in vacuum at load ratios R = –1, R = 0.05 and R = 0.5 using ultrasonic equipment (cycling frequency 20 kHz) and servo-hydraulic equipment (20 Hz). In vacuum, no strain rate influences were found and similar growth rates and threshold stress intensities were measured at both frequencies. In ambient air, threshold stress intensities were similar at 20 Hz and 20 kHz and were 53 –62% of the respective values measured in vacuum. Above threshold, fatigue crack growth rates at ultrasonic frequency are slower (at R = –1) or similar (at R = 0.5) to growth rates at 20 Hz. Ultrasonic fracture mechanics tests in Ti-6Al-4V at load ratios R = 0.1, R = 0.5 and R = 0.8 in ambient air delivered threshold values similar to cycling frequency 50 Hz, whereas growth rates above threshold are approximately a factor of 3 higher at 20 kHz. The compressive part of a load cycle under fully reversed loading condition causes additional fatigue damage, and the maximum stress intensity factor at threshold is lower at R = –1 than at R = 0.05 or R = 0.1.

The structural evolution in NiTi shape memory alloys subjected to pseudoelastic cycling is examined in the present study. Single crystals with [100] and [111] orientations were subjected to repeated compressive cycles and then studied by transmission electron microscopy (TEM). TEM observations were made at cycle numbers 1, 2, 5, 10, and 20 since the majority of degradation occurs during these initial cycle numbers. Under compression, single crystals with [111] orientations degraded much faster than crystals with [100] orientations. Under tension, single crystals with [100] orientations fractured in the elastic region, and crystals with [111] orientations showed considerable degradation as a function of cycling. Intermittent TEM observations on single crystals oriented along the [111] direction showed an increase in dislocation density on multiple active slip systems as a function of cycling. Single crystals oriented along the [100] orientation show a less dramatic increase in dislocation density as a function of cycling. TEM observations have revealed that dislocation structures formed near martensite plates have a similar periodicity as internal twin modes within the martensite. This observation implies that, although the interface between the martensite and parent phase is a low-energy boundary, the local disruptions due to internal twins create preferential nucleation sites for the formation of lattice defects.

Magnesium alloys containing rare earths (Gd, Y, Ce, ≈3– 10 wt.%), Sc (< 1 wt.%) and Mn (≈1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20 –580 °C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn 2 Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg 12 Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn 2 Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.

The low-cycle and high-cycle fatigue behavior of the Mg-base wrought alloy AZ31 is investigated in the temperature range 20– 300 °C using total-strain-controlled push – pull tests. It is shown that the mechanical properties of the material are substantially influenced by the direction dependent formation of deformation twins and resulting deformation asymmetry. As a consequence tensile mean stresses develop during total-strain-controlled fatigue tests without mean strain.

The microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibrium τ phase (Mg 32 (Al, Zn) 49 ). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubic τ phase Mg 32 (Al, Zn, Ca) 49 as well as Al 2 (Ca, Zn) phase, were detected in addition to the I phase. In the as-cast condition, regions supersaturated with Al and Zn were found to surround the α-Mg grains and to become τ phase precipitation sites during high-temperature annealing in all alloys investigated. TEM and SANS measurements revealed no influence of Ca on the size distribution and the coarsening rate of the precipitates, which are formed by a continuous precipitation process.

The dislocation structures developed during plastic deformation in the regimes of discontinuous glide (Portevin – Le-Chatelier effect, PLC) as well as viscous glide at elevated temperatures (600 – 800 K) have been studied by transmission electron microscopy in polycrystals of Cu-5Al, Cu-10Al, Cu-15Al and Cu-4.1Mn alloys (at.%). Discontinuous glide is characterized by correlated dislocation movement in few active slip planes due to breakaway of aged and rapid multiplication of fresh dislocations, while beyond the transition, when solute diffusion is fast enough to continuously reconstitute the solute cloud during dislocation motion, a homogeneous distribution of smoothly moving dislocations is observed in all alloys. Owing to the decrease of stacking-fault energy and increase of short-range order in the Cu–Al alloys a change from cell structure (Cu-5Al) to planar slip (Cu-10Al and Cu-15Al) occurs in the discontinuous glide regime; in the Cu-4.1Mn alloy with a stacking-fault energy like Cu and negligible short-range order, development of cells prevails also during discontinuous glide. The observations are discussed in terms of current views on deformation in the PLC regime.

YBa 2 Cu 3 O 7– δ (YBCO) is a high-temperature superconductor (HTS) with unique characteristics for use in electrical engineering. The structure and the transport properties of the cuprate superconductor are reviewed. The key issues in HTS wire technology, viz. texture tailoring and grain boundary engineering of the polycrystalline material to be used in technical devices, are discussed. A production route is described to fabricate long lengths of highly aligned YBCO films on cube-textured nickel and nickel alloy substrates. Electron backscatter diffraction in the scanning electron microscope is used to control the process → texture relationship in developing the sheet material as well as the epitaxy and phase stability of the YBCO film.

Novel oxide dispersion-strengthened (ODS) silver alloys were synthesised by mechanical alloying and subsequent consolidation via hot pressing in protective atmosphere. The effect of dispersoid size and volume fraction on the electrical and mechanical properties is exemplified with Cr 2 O 3 , CaO and Y 2 O 3 as model candidates for a systematic study of the principles of particle selection. The results for the development of the microstructure under different processing conditions show that control of the impurity level is of primary importance for achieving high-quality, fully dense material. Hence, a modified cryo-milling technique at liquid-N 2 temperature was applied. Data at ambient temperature are presented, revealing that the mechanical and electrical properties can be tailored within a wide range as a function of the microstructure. These results are explained by microstructural concepts for room temperature yielding and electrical conductivity.

In order to achieve higher accuracy and predictive capability in finite element simulations, the implementation of physically based microstructure models becomes inevitable. The underlying reason for this is that the dislocation microstructure and the evolution of texture largely influence the considered forming processes. The present article presents recent results of research activities on 3-dimensional rolling simulations of Al alloys and their experimental validation. The finite element simulations were carried out taking dislocation density evolution into account.

Investigations on ceramic microspecimens made of Y 2 O 3 - stabilized ZrO 2 produced by slip casting or micro powder injection moulding are introduced. During the production of the microspecimens, feedstocks and sintering conditions were varied. Differently moulded specimens were examined with respect to their microstructure and surface topography using light microscopy, scanning electron microscopy (SEM) and confocal white light microscopy. Additionally, the mechanical characteristics were investigated by three-point bending tests using a micro universal testing device. The statistical analysis was realised by means of the Weibull theory and interpreted by the aid of SEM images of fracture surfaces. This research allowed to understand correlations between different feedstocks used, process parameters like the sintering conditions applied and the resulting characteristics as well as material properties of the microspecimens. These results could be used to improve the production process.

Der Beitrag beschreibt das Potential und wesentliche Einflussgrößen von thermisch leitfähig modifizierten Thermoplasten, die als Funktionskunststoffe zur direkten Wärmezu- und -abfuhr beispielsweise im Spritzgießverfahren zu komplexen Bauteilen geformt bzw. darin lokal integriert werden können. Durch eine gezielte Zugabe sehr gut wärmeleitender metallischer bzw. keramischer Füllstoffe kann die Wärmeleitfähigkeit solcher gefüllter Kunststoffe deutlich verbessert werden. Um Wärmeleitfähigkeiten im technisch interessanten Maßstab zur erreichen, sind Füllstoffanteile von bis zu 60 Vol.-% notwendig. Entscheidende Einflussgrößen neben dem Füllgrad sind die Eigenleitfähigkeit und die Geometrie der Füllstoffpartikel. Durch anisotrope Partikel, wie z. B. plättchenförmiges Kupferpulver mit einem hohen Länge/Durchmesser-Verhältnis, lässt sich die Wärmeleitfähigkeit gegenüber kugeligen Partikeln deutlich verbessern. Verarbeitungsbedingte Orientierungen führen bei anisotroper Füllstoffgeometrie zu einer auch für die Praxis relevanten Vorzugsrichtung beim Transport der Wärme.

The high interface area in nanocrystalline materials leads to advanced structural and functional properties that are interesting for a variety of applications and products. Three distinct examples for potential applications are given: Nanocrystalline and submicron-sized light-weight intermetallics based on γ-TiAl exhibiting favourable deformation behaviour at reduced temperatures, nanocrystalline cermet coatings produced by thermal spray process exhibiting improved hardness and wear resistance, and nanocrystalline Mg hydride-based composites for hydrogen storage in future mobile applications exhibiting extremely high, reversible storage capacity and fast kinetics.

Cu 47 Ti 34 Zr 11 Ni 8 , Cu 47 Ti 33 Zr 11 Ni 8 Fe 1 and Cu 47 Ti 33 Zr 11 Ni 8 Si 1 bulk glassy alloys were prepared by injection copper mold casting. Mechanical properties, glass-forming ability, thermal stability and microstructural characteristics of as-cast rods were investigated. Calorimetric studies indicate a beneficial role of small Si or Fe addition on the thermal stability of Cu–Ti – Zr –Ni bulk glassy alloys. Compression tests reveal fracture strengths of 2040 to 2190 MPa, Young’s moduli of 100 to 109 GPa and elastic strains up to 2.4 %. The Si-containing glassy alloy exhibits a plastic elongation of 2.2%. The significant increase in plasticity observed for the Si-containing alloy is due to a special bimodal composite structure consisting of nano-scaled Cu crystals homogeneously dispersed in the bulk metallic glass matrix. The increase of the global plasticity can be explained by the formation of multiple shear bands that make the glassy alloy resistant to crack propagation.

Nanoskalige Schichten für tribologische Anwendungen können unterteilt werden in nanostrukturierte Viellagenschichten, nanomodulierte Übergitterschichten, nanokristalline metastabile Schichten, nanostabilisierte Ein- und Mehrlagenschichten sowie nanogradierte Filme. Grenzflächenanteile, Korngröße, Einzellagendicke, Grenzflächenenergie, Textur, und Spannungsverteilung sind neben der Stoffauswahl und Herstellungstechnologie maßgebend für Eigenschaften und Verhalten dieser Schutzschichten. Durch ein funktionales und strukturelles Design können so maßgeschneiderte Schichten für hochbeanspruchte Werkzeuge und Bauteile erhalten werden. Durch Einfrieren metastabiler Zustände bei der Gasphasenabscheidung erhält man neuartige Filmmaterialien mit multifunktionalen Eigenschaften. Eine thermodynamische und kinetische Modellierung des Stoffsystems und der Abscheideparameter ergibt hierbei die Möglichkeit, Schichten in Bezug auf Konstitution, Eigenschaften und Verhalten gezielt zu optimieren. Verschiedene nanoskalige Schichtverbunde und Verbundkonzepte, die in jüngster Zeit entwickelt bzw. hergestellt wurden, werden beschrieben und diskutiert.

Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. Since the strain is not a state variable of crystal plasticity, however, there is no critical strain, rather there are critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.