Crack path in liquid metal embrittlement: experiments with steels and modeling

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 Focussed on Crack Paths Crack path in liquid metal embrittlement: experiments with steels and modeling T. Auger CentraleSupelec/MSSMAT, UMR CNRS 8579, Grande voie des vignes, Chatenay-Malabry, France S. Hémery ISAE–ENSMA, Institut Pprime, UPR CNRS 3346, département “Physique et mécanique des matériaux”, ENSMA, Téléport 2, 1, avenue Clément-Ader, BP 40109, 86961 Futuroscope Chasseneuil-du-Poitou cedex, France M. Bourcier CentraleSupelec/MSSMAT, UMR CNRS 8579, Grande voie des vignes, Chatenay-Malabry, France C. Berdin University of Paris Sud/ICMMO, CNRS UMR 8182, Orsay, France M. Martin Institute für Materialphysik, Georg-August Universität Göttingen, Germany I. Robertson University of Wisconsin-Madison, Wisconsin, United States of America ABSTRACT. We review the recent experimental clarification of the fracture path in Liquid Metal Embrittlement with austenitic and martensitic steels. Using state of the art characterization tools (Focused Ion Beam and Transmission Electron Microscopy) a clear understanding of crack path is emerging for these systems where a classical fractographic analysis fails to provide useful information. The main finding is that most of the cracking process takes place at grain boundaries, lath or mechanical twin boundaries while cleavage or plastic flow localization is rarely the observed fracture mode. Based on these experimental insights, we sketch an on-going modeling strategy for LME crack initiation and propagation at mesoscopic scale. At the microstructural scale, crystal plasticity constitutive equations are used to model the plastic deformation in metals and alloys. The microstructure used is either extracted from experimental measurements by 3D-EBSD (Electron Back Scattering Diffraction) or simulated starting from a Voronoï approach. The presence of a crack 250 T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 within the polycrystalline aggregate is taken into account in order to study the surrounding plastic dissipation and the crack path. One key piece of information that can be extracted is the typical order of magnitude of the stress-strain state at GB in order to constrain crack initiation models. The challenges of building predictive LME cracking models are outlined. KEYWORDS. Crack path; Liquid Metal Embrittlement; Steels; Multi-scale crack propagation modelling. INTRODUCTION L iquid metal embrittlement (LME) is the transition from ductile to brittle fracture when a metallic material is stressed in contact with a liquid metal. In spite of decades of studies, mostly by macro-scale experiments, but also down to the atomic scale (in particular by high resolution transmission electron microscopy on grain boundaries), this phenomenon is still fundamentally not well understood. While many investigations have focused on “model-systems” such as Cu/Bi, Ni/Hg or Al/Ga, LME is also observed with steels where this phenomenon has received much less attention despite its considerable practical importance (fission or fusion cooling systems). One reason is that the phenomenology with steel’s LME is often obscured by the specific physico-chemistry details of each system or by the technical difficulties due to the requirements of high quality testing at high temperature. Nevertheless, among today’s critical topics in LME, the assessment of potential crack paths at the microstructural scale is a highly important piece of missing information. The insight based on synthesis of literature data is unclear, especially regarding steels. Whereas intergranular cracking is the most common case in LME [1] [4], cleavage and quasi-cleavage have been identified in steels as well [2][3] Up to recent time, the investigations of the fracture modes were limited to scanning electron microscopy or TEM of surface replicas [5]. However, complex microstructures of industrial alloys, such as martensitic steels, make the crack path identification difficult using conventional fractography tools, thereby leading to misidentification of the crack path as discussed in [6]. One key finding in this later work was the explanation of the unusual fracture surface as resulting from interlath cracking in plane stress loading condition. It was at the same time found that interlath cracks can be arrested by carbide precipitation at interlaths. Meanwhile, it was shown that interlath cracking is clearly enhanced, and correspondingly so is LME susceptibility, when the carbide distribution is modified such that carbides are depleted at the lath boundaries [7]. This implies that not only knowledge about the atomic scale mechanism is required but also a detailed mesoscopic understanding is crucially important for a correct understanding of LME’s occurrence and the definition of sound strategies for LME’s mitigation. The present study mainly focuses on steels to highlight that there is a common fracture mode induced by liquid metals. To do so, fracture of martensitic, ferritic and austenitic steels in liquid sodium have been studied to assess the common denominator in the crack path for these cases. Various embrittling liquid metals are used to show the universality involved in steel’s LME. Once one has clearly established LME’s main crack path characteristics, one can proceed with the modeling of LME crack propagation. We sketch an on-going modeling strategy for LME crack initiation and propagation at mesoscopic scale. At the microstructural scale, crystal plasticity constitutive equations are used to model the plastic deformation in metals and alloys. The microstructure used is either extracted from experimental measurements by 3D-EBSD (Electron Back Scattering Diffraction) or simulated starting from a Voronoï generated aggregate. The presence of an intergranular crack within a polycrystalline aggregate can be taken into account in order to study the surrounding plastic dissipation and the crack path. One key piece of information that can be extracted is the typical order of magnitude of the stress-strain state at the GB in order to constrain crack initiation models. Different fracture criteria can then be discussed. The challenges of building predictive LME cracking models are outlined. CRACK PATH IN LME Experimental T 91 martensitic steel, AISI 1010 ferritic-pearlitic steel and AISI 304L austenitic steel are sensitive to LME as shown in [3, 6, 8-10]. As a consequence, those steels have been selected for study of the LME crack path as a function of the microstructure. The compositions of the steels are shown in Tab. 1. Standard heat treatment of T91 steel 251 T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 (austenitization then tempering at 750°C) results in a tempered martensite microstructure with prior austenitic grain size of 20µm. A much finer lath structure (...truncated)