Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 252 (austenitization then tempering at 750°C) results in a tempered martensite microstructure with prior austenitic grain size of 20µm. A much finer lath structure is present inside those grains with typical lath width of about 500nm. AISI 304L steel have been hyperquenched to obtain a 30 µm austenitic grained microstructure. Finally, AISI 1010 steel was tested in a near equilibrium ferrite-pearlitic state. The ferrite grain size was about 20 µm. All steels were tested in high purity liquid sodium to study the influence of the microstructure on the crack path. To assess the LME crack path as a function of the liquid metal species, indium, sodium, and lead bismuth eutectic (LBE) have been used on T91 steel. This steel is known to be sensitive to LME in various liquid metals [3, 6, 11]. Steel C Cr Mo V Nb S Mn P Ni Si T91 0.08 – 0.12 8.00 – 9.50 0.85 – 1.05 0.18 – 0.25 0.06 – 0.10 0.01 0.3 – 0.6 <0.032 <0.4 0.2 – 0.5 1010 0.08 – 0.12 - - - - <0.05 0.3 – 0.6 <0.04 - - 304L <0.03 18 - 20 - - - <0.03 <2.0 <0.045 8 - 12 <0.75 Table 1: Specification in composition of T91, AISI 1010 and 304L steels (wt %) Round notched tensile specimens were used in sodium. The specimen diameter was 4 mm with a gage length of 15 mm. The notch was 500 µm deep with an angle of 60°. In indium and LBE, center cracked tensile (CCT) specimens were used. The specimen thickness was 1.5 mm (plane stress condition). The total width of the CCT specimens was 50 mm for a notch length of 10 mm. Specimens were heated by Joule effect in an inert gas atmosphere. The temperature was maintained for a 10 min period prior to testing to ensure a homogeneous temperature along the gauge length. A pyrometer ensures a control of the temperature with an accuracy of +/- 5K. The embrittling liquid, brought in small quantity at the notch before testing, melts by conduction and follows the crack by capillarity during the test due to good wetting. The temperature set during the tensile test was selected to ensure high embrittlement intensity above the melting temperature of the embrittling metal. In the case of sodium, crosshead displacement rate was chosen to limit the vaporization of the liquid metal in order to ensure its presence at the crack tip during the whole test duration. Temperatures and displacement rates are mentioned for every case studied. Due to its high reactivity, tests in sodium are performed under a high purity argon atmosphere (oxygen content <5ppm and moisture content <20ppm). Further details about the test procedure are given in [9] for sodium tests and in [6] for indium and LBE tests. Since wetting is known to be a critical requirement for LME, specific preparation procedures are required before testing to promote wetting at the notch. Sodium wetting was improved by introducing pre-exposure steps in sodium at temperatures in the range 723 K – 823 K for 50h to 200h. Limited corrosion influence is expected in these conditions and further details about the sodium pre-exposure can be found in [8][9]. For indium and LBE, the notch of the CCT specimens is wetted with the help of a soft soldering flux mainly composed of zinc chloride, ammonium chloride after mechanical and electro polishing. Given the differences in microstructural states, different investigation scales and tools had to be used to properly identify the crack path. Therefore, although scanning electron microscope (SEM) was used for conventional fractography to extract the crack path from fracture surfaces, Electron BackScattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) were also used to investigate crack propagation in finer low angle misorientation microstructures. Before characterization of the fracture surfaces, the liquid metal needs to be removed. Sodium is eliminated in ethanol; LBE is dissolved in a mixture of acetic acid, hydrogen peroxide, and ethanol known to not affect the fracture surface; indium is removed by amalgamation with mercury at room temperature followed by ultrasonic cleaning in distilled water. Scanning Electron Microscopy observations were carried out using a LEO 1530. To assess the crack path next to the surface, some fractured samples were plated with gold followed by a 100 µm thick nickel electrodeposit allowing preparing cross sections by mechanical polishing. An un-plated sample cross section was also prepared using mechanical grinding followed by polishing using a JEOL Cross-Polisher. EBSD orientation maps were then acquired with a TSL Digiview4 camera using the OIM acquisition software. Some TEM samples containing arrested cracks were also extracted from broken T91 specimens using a Focused Ion Beam (FIB). The samples tested in sodium have been characterized using STEM imaging and transmission EBSD at 30kV in a Helios 660 FEI FIB. TEM analysis was performed on samples tested in liquid LBE and indium using an automated indexing of the diffraction pattern (ASTAR ® analysis system) using a JEOL 3010-LaB6 operated at 300 kV.