Materials at High Temperature Vol 17, Issue 4, 2000

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High temperature fatigue crack growth in

Inconel 718

F.V. Antunes1*, J.M. Ferreira2, C.M. Branco1 and J. Byrne3

1ICEMS/DEM/FCTUC, Pólo II, Pinhal de Marrocos, 3030 Coimbra, Portugal

2IST/UME, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal

3Department of Mechanical and Manufacturing Eng., University of Portsmouth, UK

The nickel base superalloys are extensively used in high temperature applications, so it is important to know their behaviour under conditions of high-temperature fatigue. This paper studies the influence of _K, loading frequency, stress ratio and temperature on the high temperature fatigue crack growth rate of nickel base superalloys. This study is based on fatigue tests carried out in corner crack specimens of Inconel 718 at 600ºC and at room temperature. Three stress ratios (R = 0.05, 0.5 and 0.8) and loading frequencies ranging from 0.0017 to 15 Hz were considered in the tests. For frequencies below 0.25 Hz, the load waveshape was trapezoidal with different dwell times at maximum load. At relatively high frequencies the propagation is cycle dependent, while for lower frequencies it is time dependent. At intermediate frequencies a mixed crack growth occurs. The transition frequencies from cycle dependent to mixed regime and from mixed to time dependent regime were obtained for each R. The increase of R increases the transition frequencies, i.e., extends the time dependent crack growth to higher frequencies. The increase of R also produces an increase of cyclic crack growth rate for all regimes of crack growth. In the time dependent regime, a higher variation is observed, that can be explained by an acceleration of oxidation damage promoted by the increase of maximum stress. An approach for modelling the high-temperature fatigue crack growth in nickel base superalloys is presented. A good agreement was observed between time dependent fatigue results and mathematical models based on static load results.

Keywords: high temperature fatigue, nickel base superalloys, loading frequency, stress ratio, modelling.

The oxidation behaviour of an Fe, Al, Si alloy containing uranium as an impurity, oxidised for short times at high temperatures

P. Fox, J. Ritherdon and A. J. Papworth*

Materials Science and Engineering, The Department of Engineering, The University of Liverpool, Liverpool L69 3BX, UK

*Present address: Materials Research Centre, Lehigh University, Whitaker Laboratory, 5 East Packer Avenue, Bethlehem, PA 18015-3195 USA

The short term oxidation behaviour of an Fe,Al,Si alloy, oxidised at 1000°C in unpurified air, has been characterised using a range of techniques including transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was observed that this alloy formed a flat adherent alumina scale, even though analysis did not detect any conventional reactive elements. The research showed that the scale developed during the early stages of oxidation was similar to that formed at longer times (up to 24 hours) and that the tendency for the scale to remain flat and adherent was due to the presence of an impurity, uranium. Upon initial oxidation, uranium was found to segregate strongly to the metal/oxide interface, but was not detected at the oxide grain boundaries until oxidation times of approximately 20 minutes. Sulphur was observed within the scale during the initial stages of oxidation but was not associated with the uranium, the oxide grain boundaries or the metal/oxide interface. Therefore, uranium is not acting as a sulphur getter, reducing sulphur activity in the scale, but instead it appears to either block the sites at the oxide/metal interface where sulphur would normally segregate or change the chemistry of the interface in such a way as to make the segregation of sulphur thermodynamically unfavourable.

Keywords: alumina, reactive element effect, segregation, high temperature oxidation

Corrosion by carbonaceous gases, carburization and metal dusting, and methods of prevention†

H. J. Grabke

Max Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany

Exposure of metallic materials to carbonaceous gases can lead to (i) carburization, i.e. internal carbide formation causing embrittlement and failure especially at high temperatures of cracking tubes for olefine production, (ii) metal dusting, a disintegration of metals to a dust of carbon and fine metal particles, occurring at intermediate temperatures, e.g. in synthesis gas for ammonia or methanol production or in direct reduction of iron ores, and (iii) coking, the carbon deposition which is an annoying phenomenon in many processes. The mechanisms of these phenomena are well understood now and they can be suppressed or effectively retarded either by the presence of some sulphur in the process gases and/or by a protective dense oxide scale. This review describes the mechanisms and kinetics of corrosion by carbonaceous gases and the fundamentals of prevention.

Keywords: carburization, metal dusting, coking, protection, sulphur, oxide scale

Contribution of solid electrochemical techniques to high temperature oxidation studies†

Yasutoshi Saito

Professor Emeritus, Tokyo Institute of Technology, Tokyo, Japan

Present Address: National Institution for Academic Degrees, 3-29-1 Otsuka, Bunkyo-ku, Tokyo 112-0012, Japan

Solid electrochemical techniques have been applied to high temperature oxidation studies in experiments and concept. The principles of the parabolic oxidation law and solid electrolyte cells have been derived from the same origin developed by Carl Wagner. Oxygen sensors and pumps using solid electrolyte cells are very useful for the study of oxidation kinetics as a function of oxygen partial pressure. A tip-type solid electrolyte CO2 sensor is very sensitive to detect the evolution of CO2 gas, and is useful for the analysis of high temperature oxidation of metal carbides. A mechanistic model proposed for the reactive element effect of chromia-forming alloys has been confirmed by the measurements and data analysis of thermodynamic properties, electrical conductivity and diffusivity of YCrO3 on the basis of solid electrochemical techniques and defect structures.

Keywords: solid electrochemical techniques, high temperature oxidation studies, defect structures

Research and development of ultra-high temperature materials in Japan

Ryohei Tanaka

Japan Ultra-high Temperature Materials Research Institute

Gas turbines for aircraft engine and power generation are typical fields of application of high temperature materials. Nickel-based superalloys are excellent and most useful materials for these applications and have been well developed especially with the outstanding progress of jet engines. In future, from a view point of global environmental problems, there will be strong demands for special materials for high temperature and high efficiency gas turbines for power generation. However, the temperature capability of the superalloys will saturate to some limit because of their melting points which are lower than 1400°C. So-called ultra-high temperature materials, such as intermetallic compounds, refractory metals and alloys, ceramics, and various composite materials are expected to surpass the temperature capability of the superalloys, although these materials have several problems such as difficulty of processing, lack of ductility and toughness or the poor resistance to oxidation and hot-corrosion. In this paper, present and future prospects of R&D of these ultra-high temperature materials have been briefly reviewed.

Keywords: jet engine, power generation, superalloys, intermetallics, refractory metals, ceramics, ceramic matrix composites,

carbon/carbon composites, ultra-high temperature materials.

Internal oxidation processes under non-ideal conditions†

D.J.Young

School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

Wagner’s classical theory of internal oxidation is concerned with a single oxidant diffusing into an alloy to precipitate particles of a single compound which do not themselves affect significantly the diffusion process. This paper is concerned with some of the complexities which arise when this ideal description is inapplicable. The simultaneous precipitation of multiple chromium compounds in heatresistant alloys exposed to two or more of oxygen, sulphur, carbon and nitrogen is shown to be in qualitative agreement with theoretical prediction: discrete precipitation zones form and widen according to parabolic kinetics. When lamellare or acicular precipitates grow parallel to the oxidant diffusion direction, the aligned precipitate-matrix interfaces can provide rapid diffusion pathways, accelerating the reaction rates. The effect is demonstrated by molybdenum sulphide needles in Ni-Mo alloys, Al2O3 rods in austenitic and ferritic alloys, lamellar and Widmanstatten Cr2N in nickel base alloys and lamellar Cr23C6 in both austenitic and ferritic alloys. In many cases, the formation of the aligned precipitates can be described as a cellular phase transformation at the reaction front.

Keywords: internal oxidation processes

Determination of creep parameters from indentation creep experiments: a parametric finite element study for single phase materials

Z.F.Yue*, M. Probst-Hein and G. Eggeler

Institut für Werkstoffe, Ruhr-Universität Bochum, 44 801 Bochum, Germany

*To whom correspondence should be addressed. On leave from: Department of Applied Mechanics, Northwestern Polytechnical University, 710072 Xian, PR China

This paper explores the possibilities of determining creep parameters for a simple Norton law material from indentation creep testing. Using creep finite element analysis the creep indentation test technique is analysed in terms of indentation rates at constant loads. Emphasis is placed on the evolving stress distribution in front of the indenter during indentation creep. Moreover the role of indenter geometry, size effects and of macroscopic constraints is explicitly considered. A simple procedure is proposed to translate indentation creep results into constitutive creep equations for cases where the dimensions of the tested material are significantly larger than the indenter. The influence of macroscopic constraints becomes important when the size of the indenter is of the same order of magnitude as the size of the testing material. As a striking example for size effects and for macroscopic constraints the indentation creep process in a thin film is analyzed. The results contribute to a better mechanical understanding of indentation creep testing.

Keywords: indentation creep testing, finite element creep stress analysis, determination of creep parameters, indentation rates, stress fields, semi-infinite solids, thin films