PSI - Issue 54

Lisa Claeys et al. / Procedia Structural Integrity 54 (2024) 250–255 Claeys/ Structural Integrity Procedia 00 (2023) 000 – 000

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compared to different iron-based alloys showing a significant reduction in ductility (Li, et al., 2022). A conclusive explanation of this observation has not been found yet, although similar hydrogen embrittlement mechanisms might operate as reported for conventional alloys. More specifically, hydrogen-enhanced nano-twin formation was put forward to explain the limited reduction in ductility (Luo, et al., 2018) as well as the intrinsically low dislocation mobility in HEAs due to the severe lattice distortion (Varvenne, et al., 2016). Current literature, however, contains many different experimental approaches to introduce hydrogen to the HEA leading to varying hydrogen concentrations and distributions through the thickness. This work aims to correlate a through thickness hydrogen concentration analysis with the fracture behavior of a near-equiatomic CoCrFeMnNi HEA as a method to define critical hydrogen concentrations for certain hydrogen-influenced fracture modes. 2. Materials and methods A HEA consisting of five main elements was cast followed by hot rolling at 1200°C, quenching and cold rolling to a final thickness of about 2 mm. The material was subsequently annealed at 800°C for 1h. The final chemical composition is given in Table 1.

Table 1. Chemical composition of the high entropy alloy. Element Atom % Cobalt 17.0 ± 0.8 Chromium 24.4 ± 2.1 Iron 19.1 ± 0.9 Manganese 21.3 ± 0.6 Nickel 17.2 ± 1.2

Hydrogen charging was performed at an elevated temperature of 80°C in a 0.1M NaOH electrolyte. Galvanostatic hydrogen charging was applied with a constant current density of 5 mA/cm² for 7 days. Platinum foils at both sides of the specimen served as counter electrodes. To evaluate the hydrogen properties, the specimen thickness was reduced to 0.2 mm since this allowed to charge a homogeneous hydrogen concentration through the thickness (Claeys, Depover, Verbeken, 2022). The specimens were circular with a diameter of 20 mm. Thermal desorption spectroscopy (TDS) was performed at 900 K/h. Moreover, isothermal TDS measurements were performed at constant temperatures of 300°C, 400°C and 500°C. The mechanical performance of the alloy was evaluated on dogbone-shaped specimens with a gauge of 10x4x1.3 mm. Charging conditions were kept constant meaning that these specimens experienced a hydrogen concentration gradient through the thickness. The tensile tests were performed ex-situ and compared to reference tests where no hydrogen charging was applied. A constant strain rate of 5E-5 s -1 was used. Evaluation of the microstructure and fracture surfaces was done with scanning electron microscopy (SEM) utilising a FEI Quanta 450 FEG SEM with an accelerating voltage of 20 kV and a spot size of 5. Energy dispersive X-ray spectroscopy was used to make elemental mappings of specific locations on the SEM images. Electron backscatter diffraction (EBSD) was performed within the SEM as well. The specimen was tilted to 70° with respect to the horizontal. A hexagonal grid was scanned with a step size of 70 nm. 3. Results and discussion The initial microstructure after heat treatment was characterized. EBSD analysis revealed a fully FCC microstructure with equiaxed grains, random texture and the presence of annealing twins as illustrated in Fig. 1. Several inclusions were detected in the microstructure as well. Fig. 2 indicates the presence of micrometer-sized chromium oxides and manganese sulphides, distributed throughout the full thickness of the material.