PSI - Issue 57

A. Radi et al. / Procedia Structural Integrity 57 (2024) 642–648 Achraf radi/ Structural Integrity Procedia 00 (2019) 000 – 000

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were conducted for a plastic strain amplitude equal to 0.3% at the saturation stage. We will focus in the present paper on the inter-band spacing I B. The measurements of the inter-band spacing I B and the band thickness w exhibited a strong correlation between the AFM and SEM images. The corresponding I B for both states hydrogen-free and charged samples are documented in Table 2 . It is noteworthy in presence of hydrogen, the inter-band spacing I B decreased by about 41% for HT0 and 24% for HT4. TEM observations of both metallurgical states hydrogen charged and hydrogen free exhibited similar dislocation structures, characterized by dislocations debris without any specific arrangement, along with certain defects such as pile-ups and stacking fault. These observations allowed us additionally to measure the band thickness w and the dislocation density within the shear bands (ρ SB ), which both seems to be affected by the presence of hydrogen.

Table 2: Values of inter-band spacing I B for HT0 and HT4 hydrogen-free and charged samples.

Metallurgical states HT0 Hydrogen-free

I B (  m)

2.2

HT0 Hydrogen-charged HT4 Hydrogen-free HT4 Hydrogen-charged

1.29 2.38 1.8

4. Discussion 4.1. Cyclic behavior

The hydrogen-induced softening of the flow stress during the initial fatigue cycle is predominantly influenced by the concurrent softening of the effective stress for both precipitate states (HT0 and HT4). The softening of (σ eff ) in the presence of hydrogen exhibits a similar magnitude of approximately 60 MPa for both metallurgical states, as demonstrated in Table 1 . The work of Hachet et al. and Ghermaoui et al. on a nickel single crystal shows the same behavior of the effective stress in the presence of hydrogen [15,16]. The observed softening has been attributed to a decrease in short-range interactions, related to the elastic shielding effect [17,18]. Sofronis, Birnbaum, and more recently Girardin et al. [18,19], showed that hydrogen shielding effect significantly reduces the repulsive interaction between dislocations. Therefore, the shielding effect associated with hydrogen can support the HELP mechanism by increasing the mobility of dislocations locally. The reduction of short-range interactions between dislocation due to the shielding effect leads to a decrease in flow stress. This is consistent with our observations in this case. The fact that the effect of hydrogen on (σ eff ) is less dependent on precipitate states and hydrogen content (C H (HT0) = 10wppm and C H (HT4) = 27wppm) suggests that the impact of hydrogen is mainly due to a modification of elastic properties of γ’ phase in shear bands than any impact on the precipitation states. In other part, the effect of hydrogen on the internal stress X is different to the one observed on σ eff . X is the stress that arises due to the deformation incompatibilities between the soft phase (band) and the hard phase (matrix) [20]. The softening of the deformation band in the presence of hydrogen (decreases of σ eff with hydrogen) leads to an increase in deformation incompatibilities between the two phases. The H-induced softening of the effective stress keeps increasing for HT4 going from 60 MPa at the first cycle to reach 98 MPa at the saturation cycle, as shown in Table 1. While for HT0, the softening of the effective stress tends to decrease from cycle 1 to saturation going from 60MPa to 37MPa. This difference between both precipitate states may be due to the amount of hydrogen trapped around the precipitates. It should be noted that the γ' precipitates can act as reversible traps for hydrogen, and the amount of trapped hydrogen strongly depends on the size of the precipitates and the interfacial zone between the γ' phase and the matrix. Jia -He Ai et al. [21] found that larger γ' precipitates with more coherent interfaces (i.e., fewer defects or dislocations at the interface) are more effective in trapping hydrogen. They also found that the shape and morphology of the precipitates can influence the hydrogen