Hydrogen-induced pinning of dislocations in tungsten probed by spherical nanoindentation

Li Yu Ryelandt, Sophie [UCL] Favache, Audrey [UCL] et al.

Intense hydrogen plasma interactions with tungsten occur in the divertor region of ITER and the effect of hydrogen plasma on the mechanical behavior of tungsten is an important open question [1]. Such a study is unfortunately not feasible with most conventional mechanical testing techniques since low energy hydrogen plasma initiates modifications at shallow depths (typically less than a few µm). Nanoindentation probes the right length scale, but the resulting information is generally limited if a conventional analysis methodology is used [2]. In this study, a dedicated spherical nanoindentation data protocol is applied to extract more meaningful indentation stress-strain curves other than what the conventional technique provides [3]. For clarity, a recrystallized tungsten sample was used, which was exposed to deuterium plasma at a surface temperature of ~ 800 oC and ion energy of ~ 5 eV for 100 seconds using Magnum-PSI [4]. Spherical nanoindentation has been performed on reference and plasma exposed samples, while also exploring areas with a similar local environment (grain orientation, distance to grain boundaries, assured with the help of electron backscatter diffraction). We found that plasma-induced damage yields an increase of the local flow stress by more than 40%. This effect may be attributed to the obstruction of dislocation movement by plasma-induced defects. Besides the intensive sub-surface plastic deformation generated by high flux plasma loading reported earlier [5], it is hypothesized that plasma-originated hydrogen segregates at dislocation cores (of edge type dislocations). This renders the hardening of tungsten to be controlled by immobilized edge dislocations, which entails an increase of the flow stress as measured by nanoindentation. In addition, focused ion beam cross section examination and thermal desorption spectroscopy are carried out to relate the observed mechanical modifications to the underlying microstructures. The results reported here may have an impact on the fundamental understanding of the role of hydrogen plasma on the mechanical behavior of tungsten, which would be invaluable in defining the ITER divertor operational lifetime.