Effect of liquid metal environment on nucleation and propagation of fatigue cracks

    Research output

    Abstract

    The new Generation IV of nuclear reactors is being designed with the goal to provide clean, efficient, and safe energy that is economically competitive. Liquid metals are being proposed as reactor coolants for these advanced nuclear systems due to the several advantages that they provide to efficiency and safety. However, one of the main challenges for heavy liquid metal coolants is establishing their compatibility with the candidate materials of the reactor's structural components. This thesis examines the effect of a liquid metal coolant, lead-bismuth eutectic (LBE), on the fatigue resistance of a candidate structural material, austenitic stainless steel 316L. The loading and environmental conditions chosen for this investigation are based on the expected operational conditions of the Multipurpose hYbrid Research Reactor for High-tech Applications (MYRRHA), being developed at the Belgian Nuclear Research Centre (SCK CEN). In this thesis, the effect of the LBE environment is assessed on three aspects of the fatigue behaviour of 316L: 1) the number of cycles to failure, also known as fatigue life; 2) the initiation of fatigue cracks on the surface of the material; and 3) the propagation of fatigue cracks through the material. The effect of LBE on these processes is compared to the effect of two reference environments: vacuum and air. The effect of LBE on the overall fatigue life of 316L is quantified through experimental fatigue life tests, adapting the environmental factor methodology currently accepted to evaluate the effect of water environments on reactor materials. The experimental database allows to evaluate the effect of the 316L-LBE interaction parameters, namely, oxygen concentration, temperature, and strain rate, on the fatigue life, and is large enough to perform a statistical significance analysis. The results show that the fatigue life of 316L in LBE is around 3 times shorter than in vacuum, but comparable to the fatigue life in air. However, the nature of the phenomenon is different for the three environments. In addition, no significant effect of temperature, oxygen concentration, or strain rate is evidenced for the conditions tested. The fatigue crack initiation in LBE is evaluated through the analysis of the depth and number of microcracks that nucleate on the surfaces of fatigue samples. The results show that the nucleation of fatigue microcracks is enhanced by LBE, but the majority of these cracks do not propagate beyond the grain size of the steel (50 μm). An analysis with finite element methods shows that the large number of small (< / 10 μm) microcracks nucleated in LBE environments has a negligible impact on the sample's stiffness, as opposed to the fewer but deeper (100-500 μm) microcracks that initiate in air, which produce an effect of apparent softening on the mechanical stress response of the fatigue sample. The effect of LBE on the propagation of fatigue cracks is estimated using experimental measurements of the fatigue crack growth rates (FCGRs) and a fractographic analysis of the crack's path. The results show that there is a transition point below which FCGRs are slower in LBE than in air. This behaviour is confirmed by the ductile striation spacing in fracture surfaces. Additionally, the fracture surface features in LBE show a more crystallographic dependence when compared to air and vacuum, but the predominantly transgranular fracture and ductile fatigue striations confirm that the metal retained its ductility under all the conditions tested. The interpretation of the results of this work leads to the proposal of a mechanism of fatigue in a liquid metal environment. In this mechanism, the nucleation and propagation of cracks is enhanced due to the adsorption of LBE along the slip bands that form in the metal's surface and ahead of crack tips. One of the main implications of this work is the possibility to use fatigue life curves in air to reasonably predict fatigue lives in LBE within the parameters examined in this thesis. In addition, the practical insights of this work contribute to the collective experience in the assessment of environmental effects on mechanical properties. Finally, this work assists in the understanding of the deformation of solid metals in contact with liquid metals, and the role that interaction mechanisms such as corrosion and adsorption have on the behaviour of austenitic stainless steels.e cracks, focusing on this particular solid metal-liquid metal couple.
    Original languageEnglish
    QualificationDoctor of Science
    Awarding Institution
    • KU Leuven
    Supervisors/Advisors
    • Seefeldt, Marc, Supervisor, External person
    • Wevers, Martine, Supervisor, External person
    • Gavrilov, Serguei, SCK CEN Mentor
    Date of Award7 Feb 2024
    Publisher
    StatePublished - 7 Feb 2024

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