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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(a)
(b)
ND
//RD
RD
FCC
30 µm
30 µm
Fig. 1. EBSD measurement on the initial material (a) image quality map; (b) inverse pole figure map with indication of high angle grain boundaries in black and twin boundaries in white.
O
ND
10 µm
RD
Cr
20 µm
10 µm
Mn
10 µm
S
10 µm
Fig. 2. EDX analysis on inclusions found within the HEA.
The TDS and isothermal TDS data are presented in Fig. 3. For comparison, TDS was also performed on a generic 304L austenitic stainless steel. The TDS data showed a typical asymmetrical one peak spectrum as a result of a diffusion-controlled desorption process. Similarly, isothermal TDS data are governed by hydrogen diffusion. The ex perimental data were fitted with a numerical model based on Fick’s second law for diffusion (Claeys, et al., 2020). The fit is illustrated for the TDS data in Fig. 3a as well. The hydrogen content at saturation of the HEA was 142 ± 69 wppm. For the 304L ASS, similar charging conditions led to a hydrogen content of 75 wppm. The effective hydrogen diffusion coefficient of the HEA obtained from the fit of both the TDS and isothermal TDS measurements at 80°C was 4.2E-14 m²/s. The results of the fit of the 304L ASS data gave a hydrogen diffusion coefficient of 0.7E-14 m²/s at 80°C. Both the hydrogen solubility and diffusivity are thus higher than the FCC stainless steel. It has been shown in literature that chromium and manganese presence increase the hydrogen concentration because of local trapping (Claeys, Depover, Verbeken, 2022 and Correa Marques, et al., 2021). With respect to the hydrogen diffusivity, research on TWIP steels have shown that a critical manganese content exist above which channels are created through which hydrogen can diffuse at a higher rate (Ismer, Hickel and Neugebauer, 2010). This limit is clearly exceeded in the present alloy. It could thus be hypothesized that the chemistry of the HEA determines to a large extent the value of the hydrogen properties. The results of the slow strain rate tensile tests are presented in Fig. 4. Due to the higher thickness of the specimens which is required for reliable mechanical testing, no homogeneous hydrogen concentration was obtained. It is clear from the simulation in Fig. 4a that the center of the specimen was only just reached, resulting in a significant hydrogen gradient through the thickness. The simulation was performed based on the fitted parameters of the TDS and isothermal TDS measurements. The relatively high hydrogen concentration within the specimen resulted in a high embrittlement index of about 60% evaluated based on the elongation at the ultimate tensile strength. (Pu, et al., 2017)
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