Wetting by liquid sodium and fracture path analysis of sodium induced embrittlement of 304L stainless steel
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ierry Augera) Laboratoire de Mécanique des Sols, Structures et Matériaux, CentraleSupélec, UMR CNRS 8579, Université ParisSaclay, Chatenay-Malabry 92295, France; and Laboratoire PIMM, ENSAM–CNRS–CNAM, UMR CNRS 8006, Paris 75013, France
Jean-Louis Courouau Den-Service de La Corrosion et du Comportement des Matériaux dans Leur Environnement (SCCME), CEASaclay, Université Paris-Saclay, Gif-sur-Yvette F-91191, France
Julie Bourgon Institut de Chimie et des Matériaux Paris-Est, UMR 7182, CNRS/UPEC, Thiais 94320, France (Received 24 August 2017; accepted 23 October 2017)
The wettability of the 304L steel is an important parameter in Liquid Metal Embrittlement studies. Empirically, it is found to be greatly enhanced by pre-exposure to oxygenated liquid sodium. The corrosion interface formed during exposure to sodium has been analyzed at the nanoscale by transmission electron microscopy using the focused ion beam sampling. A thin layer of sodium chromite (NaxCrO2 with x # 1) is detected at the interface validating wetting on an oxide mechanism for the enhanced wetting after pre-exposure. Fracture micromechanisms and the crack path of liquid sodium-embrittled austenitic steel 304L at 573 K have been investigated down to the nanoscale. High-resolution orientation mapping analyses immediately below the fracture surface show that abundant martensitic transformations (c ! a) and twinning occur during deformation of austenite. The preferential crack path is intergranular along the newly formed c/c interfaces. It is concluded that these transformations play a major role in the fracture process.
I. INTRODUCTION
Liquid Metal Embrittlement (LME) is a special type of environmental effect where an otherwise ductile material has a brittle fracture mode when mechanically tested while in contact with a liquid metal. LME of austenitic steels by liquid sodium (Na) is of relevance for sodiumcooled nuclear power plants or solar thermal power plants using intermediate heat transfer sodium loops. It is therefore a subject of importance related to the structural safety of such structures and it is highly desirable to assess the potential sensitivity to LME. The recent outcome of LME research shows that to obtain a correct sensitivity assessment, great care must be taken to define proper testing conditions that take into account the physico-chemistry of the solid–liquid interface1 and ensure good surface’s wettability.2 By correct assessment, one implies reproducible results in mechanical properties as well as a quantification of the maximum
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.435
possible mechanical degradation. Nonobservance of these conditions, particularly the requirement of prewetting, potentially leads to a spread in the degree of measured embrittlement and at the extreme wrong null-results. The requirement of good wetting is crucial in particular to enable the supply of liquid at the crack tip during crack propagation (provided that the sy
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