Volume 92, Issue 8

In order to improve the machinability of titanium alloys, a better understanding of the chip formation process is needed. Microstructur alanalysis of chips produced at high cutting speeds can lead to an estimate of temperatures and the deformation state in the chips if structural changes can be observed. Two-dimensional cross-sectional analyses are performed by means of optical microscopy (LM) and transmission electron microscopy (TEM). To study the deformation state of the finished workpiece, a scratch-grid method in combination with a scanning electron microscope (SEM) is used. The results of the different analyses for Ti 6A14V are presented in this paper and are compared with a finite element simulation of an orthogonal cutting process.

An Schweißsimulationsproben eines vergüteten Feinkornbaustahls S960QL wurde der Abbau von Eigenspannungen unter zügiger und kombiniert zügiger und zyklischer Belastung bei unterschiedlichen Beanspruchungshöhen untersucht. Die Untersuchungsergebnisse zeigen einen vorwiegend durch Streckgrenzenüberschreitung gekennzeichneten quasistatischen Abbau der Makroeigenspannungen, der sich weitgehend proportional zur Beanspruchungshöhe bzw. zur Oberspannung verhält. Signifikante Veränderungen der Mikroeigenspannungen treten dabei nur auf, wenn die Lastspannungen die Streckgrenze überschreiten. Ein Nachweis eines zyklischen Eigenspannungsabbau ist nur unter Rückgriff auf die aus den verschiedenen Eigenspan-nungskomponenten und den Lastspannungen berechneten Vergleichsspannungen möglich.

Niedriglegierte Qualitätsstähle mit hoher bis höchster Festigkeit bis 1100 MPa und sehr guter Zähigkeit und Schweißeignung sind heute verfügbar. Für Schweißkonstruktionen im Stahlbau, im Druckbehälterbau, für Pipelines, Schiffe und Ölbohrplattformen können diese Stähle wirtschaftlich eingesetzt werden, wenn die Sprödbruchsicherheit gewährleistet ist. Die bisher gültigen, meist empirischen Berechnungsmethoden zur Sprödbruchsicherheit behindern jedoch den Einsatz dieser modernen Stähle. Im Zuge der Harmonisierung von europäischen Bemessungsnormen wurden daher bruchmechanische Sicherheitsnachweise eingeführt. Der Anhang C des Eurocode 3 (Stahlbau) und der Anhang D der prEN 13445 (Druckbehälter) sind Beispiele der erfolgreichen Anpassung bruchmechanischer Sicherheitskonzepte für den Sprödbruchsicherheitsnachweis. Im vorliegenden Beitrag werden die Ergebnisse dieser Entwicklung vergleichend gegenübergestellt.

Start-stop and load change procedures of combustion engines result in inhomogenous and instationary temperature fields, which induce cyclic mechanical loadings, e. g., in solid and cooled turbine blades and vanes. The present study shows results of so-called “complex thermal-mechanical fatigue” (CTMF) tests carried out with a two specimen testing system in order to simulate the interaction of the “hot” outer and the “cold” inner side of a cooled turbine blade. The test samples were made from the austenitic steel AISI 316 L. Specimen 1 represents the “hot” and specimen 2 the “cold” side of the blade, respectively. Both specimens were loaded by individual thermal cycles with closed loop control of the temperature. The mechanical interaction of the “hot” and the “cold” side was simulated by keeping the total strains of both specimes at identical values and keeping the sum of the forces at a value of zero at every moment throughout the tests. The results are discussed on the background of the microstructures resulting from the individual loadings of specimen 1 and 2.

The present developments of rail-mounted traffic aim at higher speeds in the passenger and higher axle load in the freight traffic. As a consequence, a substantial increase of the loading of the wheels has to be regarded. Geometric changes at wheels and rails cause high cyclic loading in the dynamic system wheel/rail as a function of the course rate [1– 6]. Since for wheel steels only a few systematic investigations of the fatigue behavior are available so far, the actual investigations concentrate on the relation between the local microstructure and the description of the fatigue behavior of highly stressed wheel steels. As test materials monoblock wheel steels and tire steels were selected in accordance with UIC specification in technically relevant heat treatment conditions. The cyclic deformation behavior is characterized and evaluated by stress – strain hysteresis as well as thermometrical and electrical measurements.

Die kontinuierliche Beschreibung des Werkstoffverhaltens in großen Bereichen der Dehngeschwindigkeit (10 –3 –10 4 s –1 ) bei gleichzeitig hohen Temperaturen erfordert die Kombination verschiedener Formulierungen für die unterschiedlichen physikalischen Vorgänge während der Verformung. Im Bereich hoher Temperaturen bei gleichzeitig niedrigen Dehnraten sind Kriechprozesse maßgeblich, für den Bereich erhöhter Dehnraten (< 10 3 s –1 ) bei niedrigeren Temperaturen findet plastisches Fließen statt. Im dynamischen Bereich (>10 3 s –1 ) kann linear viskoses Werkstoffverhalten angenommen werden. Der Übergang zwischen diesen Prozessen wird mit Hilfe einer kontinuierlichen Funktion beschrieben. Anhand von Fließkurven an AA7075, Ck45N und TiA16V4, die bei Dehnraten von 10 –3 –10 4 s –1 und Temperaturen von Raumtemperatur bis zu 900 °C ermittelt wurden, wird eine neue Beschreibung verifiziert, die diese drei Verformungsmechanismen in einer kombinierten Formulierung berücksichtigt.

Ein stark vereinfachtes Modell für die Kinetik der Rekristallisation wird vorgestellt und mit der Avrami-Gleichung verglichen.

For the quality assessment of coatings there is an increasing need of test methods which are able to fulfil the requirements to a high reliability of the test results. The exact knowledge of the coating properties and their objective assessment helps the coating industry for an optimum coating development as well as the user which will use the coated parts under a specific purpose. As part of an European project four laboratories deal with the determination of elastic moduli of hard coatings by means of quantitative acoustic microscopy. On titanium nitride coatings and C doped chromium coatings on M2 tool steel basic and continuing research studies were accomplished, guidelines have been developed and proofed in round-robin tests.

Nb–TiO 2 alloys, strengthened by oxide dispersion (ODS), have good properties for application in surgical implants. These alloys have been processed by a powder metallurgy technique. Compaction and sintering tests were carried out in order to understand the microstructural evolution of these materials during processing. Oxygen degassing takes place due to the volatilization of niobium suboxides during highvacuum sintering. Additions of TiO 2 up to 1 wt.% caused an increase in the sinterability of niobium, especially during the first stage of sintering. An explanation for the role of TiO 2 particles during sintering is also proposed.

The pitting corrosion behaviour of the austenitic stainless steel alloys 31 and A 351 with different surface treatments in a technical MgCl 2 solution was investigated. Potentiometric current density-potential measurements were carried out, and critical repassivation temperatures were determined, using a galvanometric method. Pickled, ground, abrasively blast cleaned and shot-peened surfaces, respectively, were analysed concerning roughness, residual stress, work hardening and near-surface structure. The effects of the surface properties on the local corrosion behaviour was investigated. Basically, with increasing roughness, amount of residual stress and work hardening, one can see a tendency to decreasing resistance against pitting corrosion. On the other hand, those effects are superimposed by special properties of the passive layer, e. g., caused by pickling, or from the processing-specific surface texture.

Auf einer Baustelle in einem Tunnel, in dem eine Gas-Pipeline montiert wurde, ereignete sich beim Absenken eines gerade geschweißten Rohrabschnittes mittels einer Seilwinde ein Seilriss. Dies führte zum Absturz der Rohrkomposition um mehr als 600 Meter und zum Tod von mehreren Arbeitern. Da die Bruchlast des Drahtseiles theoretisch bei weitem ausgereicht hätte, wurde eine detaillierte Schadenuntersuchung durchgeführt. Diese ergab, dass das Seil durch Gewaltbruch an einer Umlenkrolle gerissen war. Zum Seilriss haben eine hohe statische Zugbelastung des Seiles, zusätzliche dynamische Seilkräfte, hohe Zusatzbeanspruchung des Seiles an der Umlenkrolle, gestörter Seilaufbau und sicherheitstechnische Besonderheiten der verwendeten Seilwinde beigetragen.

B2 structured single-phase (Ru,Ir)Al is synthesized directly in the ternary Ru– Al– Ir system by mechanical alloying. Complete solubility between pseudo-binary RuAl – IrAl is achieved in a composition range 10–25 at.% Ir, while the range for Al remains at 50 at.%. B2 structured (Ru, Ir) Al is formed by an abrupt reaction at the beginning of milling of all three compositions and a certain quantity of Ir was found to remain after the reaction. Further alloying and a reaction between Ir and sticking Al on the surfaces of milling tools take a much longer time and finally result in single phase B2 structured (Ru, Ir) Al. The ternary alloying element Ir has been found to ameliorate the sluggish reactivity of Ru by promoting the abrupt reaction. No decomposition process has been observed for the as-milled single-phase (Ru, Ir) Al by means of high-temperature X-ray diffraction, demonstrating the complete mutual solubility between pseudo-binary RuAl and IrAl in the present composition range. The structural evolution of as-milled materials includes reordering, strain relaxation and grain growth. The grain size of the as-milled (Ru, Ir) Al after exposure to elevated temperatures (up to 1273 K), turns out to be less than 30 nm and reflects its strong thermal stability.

Starting from a powder mixture of α -Si 3 N 4 , A1N and Ca 3 N 2 , calcium α -sialon ceramics with extensive compositions on the line of Si 3 N 4 – l/2Ca 3 N 2 : 3A1N (with a nominal composition of Ca m /2 Si 12 – m Al m N 16 ) ranged from m = 0.5 up to m = 6.0 were hot pressed at 1700 °C for 1 h under 20 MPa. The sintering behavior of the sample was found to be significantly composition-dependent, and full densification was achieved for all compositions. The solubility of Ca 2+ in the present α -sialon was defined in the range of 0.5 ≤ x ≤ 1.7, which was wider than that in the Ca- α- sialon prepared from CaO. The single-phase samples revealed a similar microstructure comprising the matrix α -sialon grains and a grain boundary phase as the usual α -sialons. The energy dispersive X-ray spectroscopy analysis showed that 70 – 85 % Ca 2+ ions entered the α -sialon structure accompanying the Si substitution by Al.

Paper deals with the low cost route of the preparation of SiC/Si 3 N 4 nanocomposite. It is shown that SiC nanoinclusions distributed within the Si 3 N 4 matrix can be produced by carbothermal reduction of fine SiO 2 particles added into a starting mixture. The amount of SiC nanoinclusions can be adjusted by the appropriate amount of SiO 2 + C additives. Almost full density of the final composite was achieved by hot-pressing at 1750 °C using a “special” heating regime allowing the outgasing of CO as a product of carbothermal reduction.

Silicon nitride-based compositions containing zirconia and ceria additives were prepared by hot pressing in argon atmosphere. The compositions studied can be divided into two groups: (a) with a constant total amount of ZrO 2 + CeO 2 additive (32 mol.%) within which the CeO 2 content was varied from 0 to 6.2 mol.%, and (b) with various contents (10 to 42 mol.%) of ZrO 2 + CeO 2 additive, the ratio of ZrO 2 to CeO 2 staying constant at a level of 7.35:1. Phase compositions were determined by X-ray powder diffraction (XRD), while energy-dispersive X-ray microanalysis (EDX) was used for compositional analysis of the ZrO 2 grains. The results showed that after sintering the Ce 4+ ions were reduced to Ce 3+ . At low initial ceria concentrations, Si 2 N 2 O was formed beside β-Si 3 N 4 and ZrO 2 (the majority of which was monoclinic). At higher ceria concentrations, however, Si 2 N 2 O disappeared by reaction with ceria and silica to form Ce – N apatite. Nitrogen-containing zirconia phases were not found in the studied compositions, up to 1750 °C.

The binary aluminum-calcium phase equilibria were investigated using X-ray diffraction methods, metallography, energy-dispersive X-ray and differential thermal analyses, and the diffusion couple technique. The complete phase diagram has been determined. Two new binary compounds AlCa and Al 3 Ca 8 were found in addition to the established Al 4 Ca and Al 2 Ca phases. The crystal structure of Al 3 Ca 8 compound was determined by X-ray powder diffraction methods.

Drop solution calorimetry was performed to determine the enthalpies of formation of the binary Al 2 Ca and Al 3 Ca 8 phases and the partial enthalpy of mixing at infinite dilution of calcium in liquid aluminum. A consistent thermodynamic model of the Al–Ca system was constructed for the four stoichiometric compounds Al 4 Ca, Al 2 Ca, AlCa and Al 3 Ca 8 and for the liquid phase with a sub-subregular solution model. Optimization of the thermodynamic parameters using the Calphad method was done by incorporating all available experimental data on phase equilibria, enthalpies of formation and mixing and the activity coefficients in the liquid. A consistent description of the phase diagram and thermodynamic properties is presented which is well supported by the experimental data. The Ca-rich part of the phase diagram differs substantially from previous reports, mainly because of the two newly discovered phases AlCa and Al 3 Ca 8 .

The Gd-Zn system has been studied by performing differential thermal analysis (DTA) in closed crucibles, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray examination (XRD). Eight intermetallic phases were found: GdZn, cubic, cP2-CsCl type, congruent melting at 1020 °C, GdZn 2 , orthorhombic, oI12-CeCu 2 type, congruent melting at 1010 °C, GdZn 3 , orthorhombic, oP16-YZn 3 type, peritectic decomposition at 905 °C, Gd 3 Zn 11 , orthorhombic, oI28-La 3 Al 11 type, peritectic decomposition at 880 °C, Gd 13 Zn 58 , hexagonal, hP142-Gd 13 Zn 58 type, melting point at 920 °C, Gd 3 Zn 22 , tetragonal, tI100-Pu 3 Zn 22 type, peritectic decomposition at about 910 °C, Gd 2 Zn 17 , high temperature (H.T.) form, rhombohedral, hR57-Th 2 Zn 17 type and low temperature (L.T.) form, hexagonal, hP38-Th 2 Ni 17 type, congruent melting at 935 °C and GdZn 12 tetragonal, tI26-ThMn 12 type, peritectic decomposition at 660 °C. Five eutectic reactions occur at 780 °C and 27 at% Zn (( α Gd) + GdZn), at 945 °C and 59 at% Zn (GdZn + GdZn 2 ), at 875 °C and 77 at% Zn (GdZn 3 + Gd 3 Zn 11 ), at 885 °C and 84 at% Zn (Gd 13 Zn 58 + Gd 3 Zn 22 ) and finally at 400 °C and 99.5 at% Zn (GdZn 12 + (Zn)). A catatectic equilibrium was found in the Gd-rich end. The results are discussed and compared with those for the other rare earthzinc (R-Zn) systems. A brief outline of the general alloying behaviour of the 12 th group elements (Zn, Cd and Hg) with the rare earth metals is also presented.

Gussgefüge ternärer Legierungen werden beschrieben, bei denen die Schmelze während der Erstarrung über folgende Vierphasenreaktionen kristallisiert: Ternäres Eutektikum, Übergangsebene und ternäres Peritektikum. Während bei einer ternär eutektischen Kristallisation die Kristallisation im eutektischen Punkt endet, ist unterhalb einer Übergangsebene ein monovarianter Dreiphasenraum mit Schmelze vorhanden und unterhalb einer ternär peritektischen Vierphasenebene existieren zwei derartige Räume. Die Ausbildung des Gussgefüges wird entscheidend dadurch bestimmt, welche Arten von monovarianten Reaktionen (eutektisch oder peritektisch) auf eine Vierphasenebene zuführen und welche von der Vierphasenebene weggehen. Anhand zwei- und dreidimensionaler Darstellungen werden die Kombinationsmöglichkeiten gezeigt und beschrieben. Insbesondere kann dadurch zwischen Gussgefüge von Übergangsreaktionen und ternären Peritektika unterschieden werden. Den schematischen Darstellungen werden Gussgefüge von Legierungen aus den ternären Systemen Pb–Cd–Sn, Ag–Cu–Cd, Fe–Cr–C, Al–Pd–Mn und Al–Mg–Si als experimenteller Beleg gegenübergestellt und mit Gefügeformeln und Differential-Thermoanalysen (DTA) ergänzt. Durch Entnahme von Quarzglas-Ampullen und rasches Abschrecken der Proben aus einer von aussen zugänglichen DTA-Apparatur können Gefügeveränderungen während der Abkühlung direkt mit den aus der DTA ermittelten Effekten verglichen werden. Dadurch ist es möglich, den Kristallisationspfad bei jeder Temperatur zu unterbrechen.

Mit der quantitativen Gefügeanalyse lassen sich aus dem Gefüge quantitativ Rückschlüsse auf die mechanischen Eigenschaften ziehen. Bei feinen und kontrastarmen Gefügen, z.B. von gehärteten Stählen, muss für die Quantifizierung auf die Mustererkennung zurückgegriffen werden. Mit Hilfe von Texturparametern nach Haralick wird durch ein speziell trainiertes neuronales Netzwerk ein Zusammenhang zwischen dem Gefüge und der Härte der Stähle C45 und 100Cr6 hergestellt.

The thermal behavior of the nanocrystalline W–Cu and Cu–Pb powders was investigated in order to gain thermal stability information for further application and also to elucidate the grain growth kinetics in an immiscible alloy system. The nanocrystalline alloy powders were prepared by mechanical alloying (MA) and characterized. Thermal behavior of this nanocrystalline alloy was analysed by DSC, XRD, SEM, TEM and EXAFS. The time exponent n and the activation energy for grain growth were determined from the grain size data. The nanocrystalline W, Cu and Pb have revealed lower n values than the polycrystalline ones. The activation energy of the nanocrystalline W, Cu, and Pb grain growth in MAW–Cu and Cu–Pb powders is 186.5 ± 1.0, 55.6 ±10.5 and 42.0 ± 3.9 kJ/mol, respectively. The melting point suppression of Pb crystalline embedded in Cu matrix in MA Cu–Pb powders can be estimated. On the basis of the grain growth parameter obtained in this study and available data in the literature, the grain growth features and kinetics in the nonequilibrium state of the immiscible alloy system are discussed.

Nickel aluminide (Ni 3 Al) was used as a binder phase to fabricate dense TiC cermets with Ni 3 Al contents of up to 60 vol. % and TiC grain sizes of ~ 5 to ~ 0.8 μm by either sintering of mixed powders or a melt-infiltration sintering process. These TiC–Ni 3 Al cermets exhibit high fracture toughness together with exceptional flexure strengths (> 1 GPa), which are retained at temperatures of ~ 1000 °C. While the flexure strength exhibited a modest increase as the Ni 3 Al content was raised, both the Weibull modulus (up to ~20) and the toughness ~ 18 MPa m$\sqrt m $ exhibited substantial increases. The increased toughness was found to result from a combination of interface debonding and grain cleavage that combined to enhance plastic deformation of the Ni 3 Al ligaments in the crack wake. Crack opening displacement measurements reveal a blunted crack profile and suggest a bridging zone length in excess of 100 microns.

Recent investigations have demonstrated that the interface spacing in fully lamellar microstructures has a major influence on the creep behavior of γ-TiAl-based alloys. In this paper, the dependence of the creep properties on interface spacing in fully lamellar Ti-46.5 at.% Al-4 at.% (Cr, Nb,Ta, B) sheet material was studied. Creep tests were conducted over a temperature range of 700 to 800 °C and at stresses between 100 and 260 MPa. The results indicate that the primary creep strain as well as the minimum creep rate decrease with decreasing interface spacing. A model which considers the limitation of the free dislocation path by stored dislocations as well as by geometrical obstacles was applied to explain the role of the interface spacing on both the primary creep strain and secondary creep rate. The applicability of the model has been verified by comparing calculated creep curves with experimentally obtained ones.

Paper deals with the low cost route of the preparation of SiC/Si 3 N 4 nanocomposite. It is shown that SiC nanoinclusions distributed within the Si 3 N 4 matrix can be produced by carbothermal reduction of fine SiO 2 particles added into a starting mixture. The amount of SiC nanoinclusions can be adjusted by the appropriate amount of SiO 2 + C additives. Almost full density of the final composite was achieved by hot-pressing at 1750 °C using a “special” heating regime allowing the outgasing of CO as a product of carbothermal reduction.

Cast products of two-phase titanium aluminides suffer from microsegregation of the alloying elements, which arises from the peritectic solidification pathway. The resulting variation in composition and microstructure occurs on a length scale in the range of 1 mm and, thus, cannot be tolerated for the anticipated applications of the materials. A significant structural and chemical consolidation can be achieved by homogenization and hot-working procedures, however, the processing experience is limited. This problem is addressed in the present paper in that the evolution of the microstructure of two-phase alloys after different stages of wrought processing is characterized by structural and chemical microanalysis involving scanning electron microscopy and energy dispersive X-ray analysis. The investigations involve different two-phase alloys in order to assess the effects of ternary and higher alloying elements on the finally obtained microstructure.

Morphology evolution of β-Si 3 N 4 grains on the end caps has shown significant dynamical features. It has been observed the (0001) growth front experiencing a shape transition on the caps from semisphere to negative-curved configuration. After our previous simplified solution, in this paper, the finite element analysis method has been used to further simulate the tip evolution for the hexagonal β-Si 3 N 4 grains.

ATi-46.5 Al-4 (Cr, Nb, Ta, B) alloy (at.%) with different microstructures (fine-grained primary annealed or coarsegrained fully lamellar) was examined both by mechanical-loss (internal-friction) and creep experiments. The mechanical-loss spectra (0.01– 10 Hz), ranging from 300 up to 1300 K, show two phenomena: (i) A loss peak at about 900 K (0.01 Hz) which occurs only in samples with a lamellar microstructure. The activation enthalpy, determined from the frequency shift, is 3.0 eV. The Debye-type peak is related to thermally activated reversible local movement of dislocation segments which are pinned at the lamellae interfaces and within gamma lamellae, (ii) A high-temperature damping background above 1000 K which is controlled by an activation enthalpy of 3.8 to 3.9 eV. This value agrees well with the activation enthalpy determined by creep tests and is in the range of values reported for self-diffusion of Al. These results indicate that both properties (high-temperature background and creep) are controlled by volume diffusion-assisted climb of dislocations.