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In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The F...

The transition between insulator and metal conductor states, induced by oxygen non-stoichiometry, was studied for NaNbO3 : Mn crystals. Conditions for an optimal reduction were determined on the basis of TGA tests. The temperature dependencies of the resistance measured on the macroscale showed that the transition from thermally activated to metallic features depends on the level of oxygen deficiency. The LC-AFM measurement exhibited non-homogeneous electric resistance on the nanoscale. We ascribed the local insulator–metal transition to changeover in the electronic state of the Nb ions occurring in filaments. The Mn dopant stabilised the induced oxygen non-stoichiometry and the metallic conduction down to room temperature.

The oxidation behaviour of a binary Ni–25Cr model alloy as well as ternary/quaternary NiCr-base alloys with minor additions of Mn and/or Y was studied in Ar–O 2 and Ar–H 2 –H 2 O at 1000 and 1050 °C. The oxidation mechanisms were investigated using thermogravimetry and two-stage oxidation tests with 18 O 2 - and H 2 18 O-tracers. In the case of the binary alloy it was found that chromia scales formed in Ar–H 2 –H 2 O grew faster, but showed better adherence than those formed in Ar–O 2 . This could be attributed to differences in oxide grain size and the resulting predominant transport processes in the chromia scale changing from nearly exclusive outward growth in Ar–O 2 to mainly inward growth in Ar–H 2 –H 2 O. This change in scale microstructure and growth mechanism explains why Y-addition hardly affects the scale growth and adherence during exposure in Ar–H 2 –H 2 O. Mn additions of a few tenth's of a percent enhance the growth rate in the high- p O 2 gas but decrease it in the low- p O 2 gas. This is explained by a p O 2 -dependence of Mn solubility in Cr 2 O 3 and in the Cr/Mn-spinel, in combination with segregation of Mn to the chromia grain boundaries.

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N 2 –O 2 –H 2 O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H 2 18 O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs + -SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H 2 O (g) /O 2 ratio rather than the absolute level of H 2 O (g) . The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H 2 O (g) /O 2 ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H 2 O (g) /O 2 -ratios, and the healing of any such scale is promoted.

(3YSZ) 1− x –(MgO) x form composites above x = 0.15, which consist of stabilized-zirconia and magnesia. Electrical conductivities of the two-phase composites were measured by the four-probe DC technique. Negative influences of impurities such as SiO 2 on the conductivity are discussed with the aid of microstructural investigations using SEM and TEM. In the investigated composites, the impurities do not directly affect the electrical conduction as current blockers at grain boundaries as usually observed in pure YSZ electrolytes. Microstructural investigation using HRTEM revealed that grain boundaries of the stabilized zirconia are very clean because the silicon and aluminum oxide impurities react with MgO to form discrete Mg 2 SiO 4 and MgAl 2 O 4 grains in the electrolytes, respectively. A theoretical approach taking into consideration continuous volumes of existing phases reveals that the conductivity of the two-phase composites depends on the phase continuity. The reduction of the continuity of the zirconia phase is the main reason for the decrease of conductivity in the present system.

The Ni-base superalloy Waspaloy was investigated as candidate material for rotor and disc applications in steam turbines of the next generation. Quantitative metallography has been performed using various microscopic techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been applied for a stereological evaluation of the particle size distribution, particle number density and volume fraction of the γ′ precipitates. Waspaloy was investigated after heat treatment and after ageing at relevant application temperature of 700 °C. In cooling experiments, the influence of the cooling rate on the γ′ formation and growth was studied. The existence of γ′ nano-stals with a volume fraction

Two principal methods are used to derive constitutive equations for single crystal Ni-base superalloys: development of a creep potential according the Hill’s theory and the summarization of all activated slip systems: theoretically, the octahedral and the cubic slip system must be considered. An overview of dislocation structure evolution has been performed using transmission electron microscopy (TEM) techniques on 〈001〉-orientated CMSX-4 specimens after tensile tests. The aim of the applied conventional TEM methods was to provide with more details about the influence of temperature and deformation rate on the deformation behaviour of CMSX-4 nickel-base single crystal. A correlation of the γ' phase deformation mechanisms and associated mechanical responses for different tensile testing conditions has been done. With TEM methods the dislocation dipole identification as well as the stacking fault nature has been demonstrated.

Apart from reactive element additions, commercial FeCrAl alloys contain a number of poorly controlled or unavoidable impurities, which might significantly affect the material's oxidation behaviour. In the present study it was found that magnesium, if present in a concentration of only 80 ppm, can substantially affect composition and morphology of the alumina surface scales. This was found for scales formed after short (a few minutes) as well as very long (up to 1000 h) oxidation treatments. Magnesium appeared to diffuse outward through the alumina scale and it became enriched in the outer part of the oxide scale, forming a layer of MgAl2O4 spinel. An important effect associated with the Mg enrichment and spinel formation was the development of porosity within the scale which likely affects the scale growth rate and adherence, however, did not fundamentally change the sub-parabolic character of the scale thickening rate, which has previously frequently been reported for pure α-alumina scales.

A series of uniaxial and multiaxial creep tests was carried out on Hastelloy XR and Ni-Cr-W superalloy, which were developed as the high temperature structural materials for nuclear application at the Japan Atomic Energy Research Institute, in order to investigate multiaxial creep behavior of these materials and to verify the laws for the description of deformation under multiaxial loadings. Norton's creep law and von Mises' flow rule were applied to the prediction of multiaxial creep behavior of a tube under some significant loading conditions. In most cases the multiaxial creep behavior of these materials were successfully described with the constitutive equations based on the material parameters fitting uniaxial creep test results, though a few exceptional cases were observed. The present study has revealed that the method based on Norton's creep law and von Mises' flow rule are basically applicable for the description of the multiaxial creep behavior for Hastelloy XR and Ni-Cr-W superalloy as the conv...

Quantitative metallography has been performed on the Ni-base superalloy Waspaloy using various microscopic techniques. Analytical transmission electron microscopy, X-ray diffractometry and atomic force microscopy have been applied for an evaluation of microstructure and stereological parameters for the particle size distributions, particle number density and volume fraction of primary and secondary γ' precipitates. The influence of the specimen preparation technique on the measured quantitative parameters for γ' was investigated and described.

The nickel-based superalloy Waspaloy, which is precipitation strengthened by coherent γ′ particles, was investigated using different transmission electron microscopy (TEM) imaging conditions in the as-received condition and after long time exposure at 650°C and 750°C. Scanning–transmission electron microscopy (STEM) bright-field images, conventional TEM dark-field images and convergent-beam electron diffraction (CBED) patterns have been taken for a stereological evaluation of the particle size distributions, particle density and volume fraction of primary and secondary coherent γ′ precipitates.

Dielectric and pyroelectric characteristics and changes of electric conductivity were investigated for Nb 2 O 5 -doped Pb(Zr 0.92 Ti 0.08 )O 3 ceramics. The influence of this dopant on the ceramics microstructure was also studied. Correlation between the investigated electric characteristics and grain structure was confirmed. Some progress in understanding the influence of Nb-dopant was reached in this way.

Two batches of a commercial alumina forming Fe based ODS alloy were studied in respect to their oxidation behaviour in the temperature range 1100–1300°C. In the early stages of exposure the oxidation kinetics of the two materials appeared to be virtually identical, whereas strong differences in oxide growth rate occurred after longer exposure times. Extensive microstructural studies using optical metallography, SEM, TEM and MCs+-SIMS showed that these differences are related to a larger volume fraction of Ti(C,N) precipitates in one of the studied batches. The Ti(C,N) precipitates present in this batch were found to become incorporated into the growing oxide scale. Transformation of these particles into Ti-rich oxides leads to a volume change resulting in microcracking of the scale and subsequent enhanced oxygen access. The results show that the question whether Ti alloying additions are beneficial or detrimental in respect to alumina scale properties strongly depends on the form in which Ti prevails in the alloy. Unterschiede im Oxidationsverhalten zwischen verschiedenen Chargen von aluminiumoxidbildenden Fe-Basislegierungen Das Oxidationsverhalten von zwei Chargen einer kommerziellen FeCrAl-Legierung wurde untersucht im Temperaturbereich zwischen 1100 und 1300°C. Wahrend bei kurzen Auslagerungszeiten die Oxidationskinetik beider Chargen nahezu gleich war, wurden nach langeren Zeiten starke Unterschiede in den Oxidschichtwachstumsraten beobachtet. Mikrostrukturelle Untersuchungen mittels optischer Metallographie, REM, TEM und MCs+-SIMS ergaben, das diese Unterschiede bei langen Auslagerungszeiten mit der Anwesenheit von feinen Ti(C,N)-Ausscheidungen in einer der untersuchten Chargen korreliert ist. Die Carbonitridteilchen werden in die wachsende Oxidschicht aufgenommen und anschliesend in Ti-reiche Oxide umgewandelt. Die Umwandlung ist mit einer Volumenanderung verknupft und fuhrt zur Bildung von Poren und Mikrorissen in der Deckschicht, und anschliesend zu einer beschleunigten Oxidation. Die Ergebnisse zeigen, das die Frage, ob Ti-Zusatze einen positiven oder negativen Einflus auf die Deckschichteigenschaften haben, abhangt von der Form in der das Ti in der Legierung vorliegt.

Anode substrates based on Ni/TiO 2 cermets were fabricated for planar solid oxide fuel cells (SOFC) with the aim of improving the thermoelastic behavior of Ni/8 mol.% Y 2 O 3 -stabilized zirconia (8YSZ) substrates. This offers the possibility of adjusting the thermal expansion coefficient to any electrolyte membrane considered. NiO/NiTiO 3 specimens were fabricated from Coat-Mix® powders by rate-controlled and constant heating rate sintering. The heat treatments led to a significant change in activation energy and, especially at high sintering temperatures, in the morphology of the microstructures compared with NiO/8YSZ substrates. A TEM investigation revealed droplet-shaped impurities at the grain boundaries, which are regarded as the reason for liquid phase sintering of these substrates.

Real components in operation are exposed to cyclic multiaxial loadings. For the evaluation of the allowable operation time the effect of those cyclic loadings on deformation and creep strain behaviour must be derivated. The theoretical discussion of strain-, time- and energy-hardening theory and their mathematical description by creep laws and constitutive equations (unified models) results in an approach for the strain hardening rule according to the proposals of Nadai and Timoschenko, which allows, in using the von Mises deviatoric hypothesis and the theory of invariants a multiaxial formulation of the stress-strain behaviour. Results of creep experiments at 950°C with tubular specimen of NiCr 23 Co 12 Mo (Alloy 617) loaded over a period of more then hundred hours by a combination of steady axial load and steady internal pressure superposed by cyclic torsional moments are compared with the theoretical results. By defining a scalar typ of hardening function, which depends on the effective strain, the behaviour can satisfactoraly be predicted. Using the ORNL-model, however, the strain behaviour under cyclic loading cannot fairly enough predicted.

High temperature component design is very important in different industrial fields. This paper discusses work done in Germany on the design of metallic components for the Helium Cooled High Temperature Reactor. Candidate materials for application at the different temperature regions are presented. Criteria are developed to specify the allowable strength values of the materials (S m , S t ) for use in design codes. The following typical relevant phenomena are treated and discussed: • —creep under uniaxial and multiaxial conditions up to temperatures of 1000° C; • —creep fatigue interaction, where holding times during the stress strain cycles have a great influence on the life time; • —creep buckling for pressure loaded shells with initial imperfections; • —creep ratcheting as a phenomenon, where cyclic secondary stresses produce enhanced deformations; • —fracture mechanics at high temperatures with the aspects of creep crack growth and fatigue crack growth.

Tllrce Ni-base \vrought alloys with different hardening mechanisms (Inconel 706, Waspaloy and Inconel 617) are candidates for steam turbine disc applications with tenlperatures up to 700 "C and were examined in respect to their microstructure and ~nicrostructural stability. The materials were investigated after different heat treatments and after short and long term ageing by metallography, scanning and transmission electron microscopy. Special DTA experiments has been perfo~nled to confirm the knonn TTT diagrams and to study the initial precipitation of different phases and their growth, as function of cooling rate after the heat treatment. The amount and size of precipitations uere evaluated by image analysis and stereological methods. The Nb containing alloy Inconel 706 shomrs a complex microstructure containing y', y" and q phases which are stable under long term service up to 620 "C. At higher temperatures a strong particle coarsening and phase transfornlation u a s observed. Waspaloy is hardened by y' particles with a bimodal size distribution. After ageing at 700 "C and higher a coarsening was observed by loss of the bimodal size distribution. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M13C6 carbides. After long te1-111 ageing at temperatures of 650 "C to 750 "C the carbides tended to form carbide films along the grain boundaries and at 700 OC to 750 "C ;/' precipitated to hon~ogeneously distributed particles with low coarsening under long term ser\rice. Superalloys 718. 625. 706 and Various Dcri~atives Edited b?, E.:\. Lorin 'I'RIS ('Phz h4inerals. Metals S( Materials Socict! ). 2001

Tungsten and its alloys are considered as structural materials for plasma-facing applications. The divertor design requires a material with a low ductile?brittle transition temperature and a recrystallization temperature (TR) of about 1570?K. One of the materials under consideration is W?1.7%TiC. The aim of this work was to perform a detailed microstructural characterization of the two materials, pure W and W?1.7%TiC alloy, by electron microscopy and a new technique, electron tomography. Thin foil preparation for transmission electron microscopy investigation of W-based materials is extremely difficult. After many trials by various techniques, a method for thin foil preparation was successfully elaborated and a transmission electron microscope (TEM) investigation was carried out. A pure tungsten specimen was investigated as a reference material. The application of TEM and focused ion beam?scanning electron microscopy techniques enables the identification of some creep-induced features in pure tungsten and the determination of the size and morphology of three-dimensional pores and particles in W?TiC-based alloy.

Electrochemical behavior of the (Lao 0.65 Sr 0.3 MnO 3 (LSM)/porous functional layer(FL)/8 mol % Y 2 O 3 -stabilized ZrO 2 (8YSZ) electrolyte} half cell in the presence of a Cr source was studied by dc method and impedance spectroscopy. The porous FL was made of a LSM and 8YSZ powder mixture. The degradation rate of the cell was found to depend strongly on the FL thickness. The increase in the thickness of the FL up to 13 μm leads to a significant reduction of the Cr poisoning of the cathode. The effect of high current density (up to 0.5 A/cm 2 ) on the long-term performance of half cells in the presence of chromium source was investigated. A few processes occur near the {LSM/SYSZ/gas} contact under current load in the presence of a chromium source: chemical transformations of this contact accompanied by the formation of chromium oxide under a relatively high current load, destruction of the electrolyte and penetration of chromium into the electrolyte along the grain boundaries. The chemical nature and rate of the transformations taking place near the {LSM/8YSZ/gas} contact under current load, most probably, determine the electrochemical performance of the cell for the initial operation period whereas the two other processes could contribute to the degradation during long-term performance.

The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

For extrapolation of the time-dependent stress values the following two methods are introduced: • -extrapolation with constitutive equations • -extrapolation with time-temperature parameters. The first method is a system of coupled equations with creep and damaging parts (a physical-mathematical method), the second is an extrapolation method which combines three well-known time-temperature parameters (a statistical method). On the basis of the latter method time-dependent stress intensity values for a high temperature reactor material are derived and as an example the time safety against rupture is shown for a heat exchanger tube at a temperature of 950°C.

At high temperatures, the creep deformation of metallic materials is correlated with the accumulation of creep damage. Creep crack growth leads to a decrease of bearing cross section and an increase in stationary creep. Both variables, creep strain rate and creep damage, are described by a system of coupled differential equations. The solutions of these equations are given for the boundary conditions of creep tests under constant load, for creep rupture behavior, for damage accumulation in the creep region, and for creep-regimed low-cycle fatigue. A general correlation between applied stress, cumulative strain, and cumulative damage is given.

At temperatures above 800°C the material behaviour under mechanical load is determined by creep. The service of heat exchanging components leads to multiaxial loading conditions. For design and inelastic analysis of the component behaviour time dependent design values and suitable constitutive equations are necessary. The present report gives a survey of the approaches to describing creep under multiaxial loading. Norton's law and v. Mises' theory are applied. The load combinations of internal pressure, tensile and torsional stress are studied more closely, cyclic stress superposition in the tensile-pulsating range is discussed and cases of partial relaxation are examined. Experimental results are presented for the loading conditions discussed, and satisfactory agreement between theory and experiment has been found up to now for these results. Regarding lifetime determination under multiaxial creep load, a more precise analysis of creep damage is presented suggesting a suitable deviatoric stress for evaluation in the long-time range.

The work concerned with design codes for high temperature reactor (HTR) components operating at temperatures above 800°C is summarized. Using the experimental results from the German HTR materials development programmes, in particular the time dependent properties, the structural design analysis for an intermediate heat exchanger is discussed, with reference to creep, fatigue, creep buckling and creep ratchetting. The analysis provides the basis for a critical consideration of ASME Code, Case N 47, and the applicability of the code case rules for service temperatures above 800°C.

The transferability of materials data obtained in uniaxial tests to multiaxial load conditions depends on the nature of the multiaxial load and the type of semi-finished product. Whereas in the stationary creep range there is good agreement in the deformation behaviour of rod and tube material, tubes with slight wall thicknesses display considerable strengthening in the primary creep range. Lifetime prediction on the basis of the deviatoric stress alone is not sufficient. The time course of the largest elongation component must rather be considered in determining the specimen life.

The multiaxial creep of Alloy 617 tubes at temperatures above 900 "C is evaluated by theoretical calculations and by experimental tests with the main emphasis to torsion loading. The stress-strain-time behaviour can be discribed satisfactorally by the v. Mises theory and the use of Norton's typ creep law.