Author: Woodford, D. A.

Source: Energy Materials: Materials Science and Engineering for Energy Systems, Volume 1, Number 1, March 2006 , pp. 59-79(21)

Publisher: Maney Publishing

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• Volume 1, Number 1

Abstract:

Nickel based superalloys are critical to the safe operation of many energy conversion systems operating at high temperatures. Time dependent intergranular cracking of these alloys, under both sustained and cyclic loads, is dominated by environmental interactions at the crack tip. This review is concerned mainly with the interaction of oxygen in alloys used for combustion turbine discs, although interactions with other more aggressive species are considered. The phenomenology of this cracking is shown to be consistent with the same mechanism as that associated with oxygen embrittlement resulting from pre-exposure of uncracked material, and also with environmentally induced reduction in creep rupture life. Gas phase embrittlement (GPE), resulting from intergranular oxygen penetration, is shown to be responsible for all four streams of experimental observations. Three distinct processes of intergranular embrittlement involving oxidation reactions have been confirmed experimentally. One of these, the oxidation of intergranular sulphides, results in elemental sulphur embrittlement and subsequent local decohesion under stress. The other two, oxidation of carbon or carbides to form carbon dioxide gas bubbles and oxidation of strong oxide formers to form intergranular internal oxides, result in a reduction of the local ability to accommodate stress concentrations associated with sliding grain boundaries in an intermediate temperature range. This in turn leads to a temperature dependent minimum in ductility and maximum in crack propagation rate. Attempts to reduce the sensitivity to time dependent cracking based on chemistry (chromium level or trace element addition), microstructure control (using thermal?mechanical treatment or controlled cooling), or reversal of environmental embrittlement, are all considered. The conclusions form a basis for the development of life prediction methods for energy materials operating in diverse environments.

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Keywords: SUPERALLOYS; CREEP CAVITATION; CREEP RUPTURE; GAS PHASE EMBRITTLEMENT; OXIDATION; INTERGRANULAR FRACTURE; DUCTILITY MINIMA; HIGH TEMPERATURE ALLOYS; FATIGUE CRACKING

Document Type: Review Article

DOI: http://dx.doi.org/10.1179/174892306X99679

Publication date: 2006-03-01

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• From volume 5 onwards, Energy Materials is published as a virtual journal covering current research on materials for energy generation and storage drawn from the journals of the Institute of Materials, Minerals and Mining. The content reflects the broad range of materials contributions within this increasingly important sector and serves as a focus for the growing community with an interest in energy materials research and applications.
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