Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 255 Influence of the liquid metal species The role of the microstructure on LME crack path has been described with results in sodium liquid metal. A common potential intergranular or interlath crack path is found. One would like to know whether the crack path is similar with different liquid metals as well. Results of LME of T91 by sodium, indium and lead-bismuth eutectic (LBE) are gathered here. This steel was selected instead of AISI 1010 or AISI 304L steels because of the good knowledge about some practical aspects related to LME such as wetting by liquid metals. Despite differences in the wetting procedures and testing triaxiality (axisymetrical notched tensile geometry versus center cracked tensile geometry for indium and LBE), the conclusions are believed not to depend upon these minor details. The brittle crack path in T91 induced by liquid sodium has been already described in the previous section. Therefore, only indium and LBE induced cracking will be discussed in the following. Centre cracked tensile specimens were tested in contact with indium at 433 K and with LBE at 473 K. The tests were performed at constant crosshead speed (equivalent to 10 -4 s -1 strain rate ahead of the crack tip). SEM observations of the fracture surface reveal a brittle fracture mode, indicating the occurrence of LME. Similarly to the case of liquid sodium embrittlement, prior austenitic grain cannot be easily distinguished. The characteristic length scale seems one order of magnitude smaller. To perform investigations at the lath scale, TEM samples were extracted from areas with arrested cracks using FIB [6]. TEM observation of the samples reveals the microstructure surrounding the arrested crack as shown on the bright field micrograph presented in Fig. 4a (sample fractured in LBE). Automatic index of the diffraction pattern was performed on the microstructure neighboring the crack using the ASTAR ® analysis system. The corresponding orientation map given in Fig. 4b shows an interlath fracture mode. A similar procedure was applied to the sample fractured in indium. The result is shown in Fig. 4c. Here too, interlath fracture path interpretation is supported by misorientation profile measurements performed across the crack throughout the entire cracked area. Figure 4: a. Bright field TEM micrograph of the microstructure surrounding an arrested LBE filled brittle crack, b. orientation map corresponding to the area shown in Fig. 4a, c. Transmission EBSD orientation map of an indium filled arrested crack We do not exclude that dynamic recovery may occur in AISI 1010 even at the temperature used (≈0.3 Tf). One of the remaining questions would be whether or not in-situ formed subgrain boundaries may experience LME cracking. AISI 304L steel is also known to be in a metastable state. Indeed, martensite and mechanical twins have been observed next to the fracture surface. Nor is it excluded that newly created (in-situ) interfaces could constitute a preferential crack path, strengthening our point developed here. Indeed, LME is known to be sensitive to the structure of the grain boundary [12]. As a conclusion for this part, every LME case should be investigated using an extensive fractography work extending down to the nanoscale to properly identify the crack path. Intergranular or interlath cracking has been identified without doubt in every case studied. There remain unidentified features on the fracture surfaces (AISI 304L and AISI 1010 in liquid sodium notably) and we adopt a conservative point of view that it comes from fast linking between larger preexisting intergranular cracks through some lowest energy paths or dissipation processes. It can thus be considered that there is a common crack path encountered in LME of steels. As a consequence, numerical modeling of steel’s LME has to involve liquid metal grain boundary decohesion rather than transgranular cracking as a starting point.