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Role of grain boundaries in the diffusion of deuterium in nickel base alloy 600 studied by thermal desorption mass spectroscopy

Abstract : The role grain boundary networks play in the diffusion of hydrogen in polycrystalline metals is a highly debated subject. Some researchers have found that the grain boundary network plays an accelerating short-circuit role whereas other groups have shown that this network may slow the hydrogen diffusion or have a mixed effect based upon grain orientation and size. The diffusion of deuterium (2H), an isotopic hydrogen tracer, has been studied both in a polycrystalline nickel base alloy (A600) and an A600-like single crystal material using thermal desorption mass spectroscopy (TDS). The polycrystalline alloy was first subjected to a high temperature heat treatment and quench followed by tempering to eliminate the majority of residual trap sites (i.e. chromium carbides, dislocations and vacancies) present in the alloy. This created a model material in which primarily only grains and grain boundaries were present. Both polycrystalline and single crystal samples were then charged with 2H by an electrochemical method before being subjected to TDS analysis where the deuterium (2H2) desorption flux was continuously monitored during a temperature ramp or at an isotherm. Specific in-situ aging treatments, i.e. a prolonged exposure at room temperature or at higher temperature, were used to try to identify or isolate the effect of grain boundaries on the diffusion and trapping of 2H in A600. The acquired experimental TDS spectra obtained from polycrystalline and single crystal specimens were compared. The diffusion coefficient pre-exponential constant and activation energy were derived using a developed numerical code which adjusts Fick's second law of diffusion to fit experimental spectra. The parameters were then verified though a series of validation steps which involved simulating TDS spectra for the same material having been subjected to different experimental conditions. After validation, the measured diffusion coefficient in a polycrystalline alloy was used to simulate the single crystal experimental results and was found to reproduce very well the spectra. It was therefore concluded that grain boundaries do not play a significant role in 2H transport in A600, for the grain size and experimental conditions investigated.
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C. Hurley, F. Martin, L. Marchetti, C. Blanc, E. Andrieu, et al.. Role of grain boundaries in the diffusion of deuterium in nickel base alloy 600 studied by thermal desorption mass spectroscopy. 2016 International Hydrogen Conference, Sep 2016, Moran, United States. ⟨cea-02442328⟩

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