Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 253 Influence of the microstructure To assess the role of the microstructure on the crack path, austenitic, ferritic-pearlitic and martensitic steels have been tested in liquid sodium on axi-symetrical tensile specimens at a temperature of 573 K under argon atmosphere. The crosshead displacement rate has been adjusted in relation to the total elongation during testing to avoid complete vaporization of sodium before the end of the test. The ductility of the materials being different and LME sensitivity being higher at low strain rates, 1010 and T91 steels were tested using a crosshead displacement rate of 0.015 mm/min whereas 304L steel was tested at 0.025 mm/min. Each of the selected test condition resulted in a brittle fracture of the specimen initiating from the notch and propagating over a total length of several hundred microns. To test the innocuity of the environmental exposure only, some specimens were pre-exposed to liquid sodium for long durations equivalent to those of the mechanical tests to be carried out in sodium. The remaining sodium was then removed in ethanol in order to carry out reference tests under argon atmosphere. Conventional fractography showed a ductile fracture behavior of this pre- exposed material when failed in the absence of liquid metal, showing the requirement of the presence of the liquid metal to cause embrittlement. Figure 1: a. SEM micrograph of the brittle fracture surface near the notch tip of a T91 specimen tested in sodium, b. Crystalline orientation map of the microstructure surrounding an arrested crack obtained by conventional EBSD, c. Bright field STEM micrograph of a FIB machined sample, d. Orientation map of the microstructure surrounding arrested crack obtained by transmission EBSD. SEM observations of the fracture surface of the T91 specimen tested in sodium at a crosshead displacement rate of 0.015 mm/min show an ubiquitous flat fracture surface. A SEM micrograph of the fracture surface is shown in Fig. 1a. The flatness confirms the brittle behavior of the material as observed in [9]. According to the observations and since the mean prior austenitic grain size is 20µm, prior austenitic grain boundaries do not seem to constitute a preferential crack path. In the conventional language of fractography, it is often referred as quasi-cleavage fracture surface. To properly identify the fracture mode either as cleavage or interlath cracking, finer crystallographic investigations were carried out. EBSD mapping on transverse cut sample prepared using a JEOL Cross-Polisher was performed in areas containing arrested