Micromechanical testing of oxidized grain boundaries in Nickel alloys from nuclear reactors
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Micromechanical testing of oxidized grain boundaries in Nickel alloys from nuclear reactors Sergio Lozano-Perez1, Helen Dugdale1, David E J Armstrong1, Takumi Terachi2, Takuyo Yamada2, Edmund Tarleton1 and Steve G Roberts1 1
Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom.
2
INSS, 64 Sata, Mihama-cho, Mikata-gun, Fukui 919-1205, Japan.
ABSTRACT The fracture behaviour of individual grain boundaries has been studied in order to understand the mechanisms controlling stress corrosion cracking in nuclear reactors. In particular, the role of oxidation in facilitating crack initiation and propagation has been reviewed. Nickel alloys from pressurized water reactors (PWRs) have been tested in simulated primary water conditions to induce grain boundary oxidation. Microcantilevers containing an oxidized grain boundary plane have been prepared and tested for fracture. The brittle nature of the oxide was demonstrated and the required stress to fracture measured. INTRODUCTION Stress corrosion cracking (SCC) has been a concern for the nuclear industry (amongst others) for decades. At present, we lack a clear understanding of the underlying mechanisms and several have been proposed. Some of the most accepted mechanisms rely on the fracture of a brittle phase (e.g. oxide in a grain boundary). The Selective Internal Oxidation (SIO) was proposed by Scott and LeCalvar [1] after analyzing a number of thermodynamic and kinetic features of the cracking and expressing ‘the difficulties in reconciling those trends with the anodic dissolution or hydrogen embrittlement mechanistic hypothesis for cracking’. The model is based on two relatively simple observations: that Alloy 600 is known to oxidize internally along grain boundaries in the presence of primary water and that the failure of the alloy is intergranular. The model supposes that the Cr-rich oxide present along the grain boundary is brittle, and therefore causes embrittlement of the grain boundary. This then provides an easy crack pathway through the alloy, causing it to fail along the grain boundaries. The fracture occurs in stages, with oxygen diffusing down a section of grain boundary, thus embrittling it before a crack grows down that particular section. Once the crack has opened up, oxygen diffuses further into the alloy, down a deeper section of grain boundary, which then also cracks. Although the idea is plausible and some evidence of oxides ahead of crack tips [2] has been reported, there is no quantitative data on the brittleness of these oxides or what exact role they play in the crack propagation. The aim of the paper is to investigate the SIO mechanism and determine the viability of its explanation for the behavior of Alloy 600 in PWR primary water. In order to do this, a novel technique to investigate the mechanical properties of the grain boundary oxide was developed in order to determine whether the oxide did indeed have an embrittling
effect on the grain boundary. The technique relies on the controlled fracture of cantilevers contain
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