Abstract
Hydrogen-deformation interactions and their role in plasticity are well accepted as key features in understanding hydrogen embrittlement. In order to understand the nature of the hydrogen-induced softening process in f.c.c. metals, a substantial effort was made in this study to determine the effect of hydrogen on the tensile stress-strain behavior of nickel single crystal oriented for multiple-slips. It was clearly established that the hydrogen softening process was the result of a shielding of the elastic interactions at different scales. Hydrogen-induced softening was then formalized by a screening factor S of 0.8 ± 0.05 for 7 wppm of hydrogen, which can be incorporated into standard dislocation theory processes. The amplitude of softening suggests that the shielding process is mainly responsible for the stress softening through the formation of vacancy clusters, rather than a direct impact of hydrogen. This effect is expected to be of major importance when revisiting the impact of hydrogen on the processes causing damage to the structural alloys used in engineering.
Highlights
The effect of hydrogen on the mechanical behavior of metallic materials has been widely studied[1,2,3,4,5]
By extending the linear-elastic theory of the equilibrium between interstitial solid solution atoms and host metal lattices used in several studies[6,7,8,9,10], we have recently determined the degradation of elastic properties as a function of hydrogen alone, monovacancies and vacancy cluster concentrations using an analytical approach verified by DFT calculations[11]
The present results suggest that clusters of superabundant vacancies (SAV) are probably the main defect, which explains the softening at different microstructural scales: forest hardening and dislocation patterning
Summary
The effect of hydrogen on the mechanical behavior of metallic materials has been widely studied[1,2,3,4,5]. Some studies showed an impact of hydrogen on the localization of strains observed on the surface as a form of deformation bands[24,25] The link between this localization and the deformation structures developed during tensile loading has only recently been studied in single-slips and the impact on hardening clearly demonstrated[22]. In this orientation, the formation of geometric necessary boundaries (GNBs, defined as a type I pattern26) both screen the long-range internal stress fields and decrease the mean free path of mobile dislocations. The decrease in GNB spacing directly impacts strain hardening by decreasing the length scale[22]
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