Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 254 cracks to characterize the surrounding microstructure. A representative orientation map is shown in Fig. 1b. Despite a small misorientation expected from the opening of the crack, fracture is obviously intercrystalline. Misorientations profile measurements confirm this conclusion and show large step across the crack area typical of lath packet misorientation. Additional data was obtained from transmission EBSD of TEM samples containing an arrested crack. The bright field STEM image of the microstructure surrounding the crack is shown in Fig. 1c. The corresponding crystalline orientation map of the microstructure surrounding the arrested crack obtained by transmission EBSD is shown in Fig. 1d. It confirms the intergranular crack path for liquid sodium embrittlement of T91 steel. AISI 304L specimens were tested at 573 K for a crosshead displacement rate of 0.025 mm/min. According to SEM observations at low magnification of the fracture surface, a flat ring extends starting from the notch towards the center of the specimen over several hundreds of microns. These features were absent from the reference specimen tested under argon without sodium, hinting at the occurrence of LME for this steel. Given the mean grain size (30µm), SEM combined with the EBSD technique was used to assess the crack path. The SEM fractography revealed a complicated fracture surface where the crack path is difficult to link with microstructural features as shown in Fig. 2a. A typical orientation map on a cross-section is shown in Fig. 2b. The observed cracks, corresponding to the deep trenches on the fracture surface, are clearly intergranular. The conclusion for the remaining areas is less clear; due to the large amount of plastic strain visibly experienced next to the fracture surface, martensite and mechanical twins are observed but, while not excluded, we have so far no indication that a preferential crack path is constituted by the austenite ferrite interface or by mechanical twins. Figure 2: a. SEM micrograph of the brittle fracture surface near the notch tip of a AISI 304L specimen tested in sodium, b. Orientation map of the microstructure surrounding an arrested crack AISI 1010 specimens were tested in sodium at a crosshead displacement rate of 0.015 mm/min and a temperature of 573 K. Conventional SEM here was sufficient to reveal flat area of brittle aspect extending from the notch tip over hundreds of microns. Several areas on the fracture surfaces clearly show intergranular cracking. One of them is shown in Fig. 3. Elsewhere, pearlite, with its numerous interfaces, and plastic strain makes the identification of the crack path more difficult. Overall, it reveals the occurrence of LME as already found by Hilditch et al. on a ferritic-pearlitic steel [10]. Figure 3: SEM micrograph of the brittle fracture surface of an AISI 1010 specimen tested in sodium