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Quantification du glissement intergranulaire par microscopie à force atomique : contribution à l'analyse de l'endommagement intergranulaire à haute température.

Abstract : We developed an original experimental method for measuring grain boundary sliding by atomic force microscopy. The analysed alloys were previously tested at high temperatures and low strain rates, for short time tests. We measured grain boundary sliding along his perpendicular component to the sample surface. The detection limit is of 10 nm. Thin and flat austenitic stainless steels and superalloys samples were tested at high temperature (360°C to 750°C), under ultra high vacuum atmosphere, with slow rate tensile tests and constant load tests. After short times tests, AFM characterization showed that grain boundary sliding could be activated at the loading, within amplitudes of several tens of nm. Furthermore, after short time tests, grain boundary sliding amplitudes are independent of the geometrical orientation of the boundary trace compared with loading direction. On the other hand crystallographic misorientation has a strong influence on boundaries propensity to slide. Intergranular segregation was also analyzed by AES for two alloys (304H austenitic stainless steel and X- 750 nickel-base alloy), previously submitted to high temperature slow strain rate tensile tests. AES analysis showed preferential segregation of S and P near 304H triple junctions. Whereas, AES analysis on X-750 alloy showed P segregation at the bottom of microvoids distributed on a microductile grain facet and a strong segregation of S at the location of grain boundary precipitate. Our experimental methods allows the study of the correlation between two main phenomena contributing to high temperature embrittlement: grain boundary sliding and intergranular segregation embrittlement. Determining grain boundary sliding kinetics should confirm our method of step measure by AFM as a way to indicate high temperature embrittlement sensitivity of alloys in service conditions and also give information about the contribution of grain boundary sliding to stress corrosion cracking initiation and propagation.
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Submitted on : Monday, October 8, 2012 - 8:41:13 AM
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Matthieu Lenci. Quantification du glissement intergranulaire par microscopie à force atomique : contribution à l'analyse de l'endommagement intergranulaire à haute température.. Matériaux. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2009. Français. ⟨tel-00739345⟩

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