Hongbing Yu

A literature review of studies investigating the topography of nano-indents in ion-implanted materials reveals seemingly inconsistent observations, with reports of both pile-up and sink-in. This may be due to the crystallographic orientation of the measured sample point, which is often not considered when evaluating implantation-induced changes in the deformation response. Here, we explore the orientation dependence of spherical nano-indentation in pure and helium-implanted tungsten, considering grains with ⟨001⟩, ⟨110⟩, and ⟨111⟩ out-of-plane orientations. Atomic force microscopy of indents in unimplanted tungsten shows little orientation dependence. However, in the implanted material, a much larger, more localized pile-up is observed for ⟨001⟩ grains than for ⟨110⟩ and ⟨111⟩ orientations. Based on the observations for ⟨001⟩ grains, we hypothesize that a large initial hardening due to helium-induced defects is followed by localized defect removal and subsequent strain softening. A crystal plasticity finite element model of the indentation process, formulated based on this hypothesis, accurately reproduces the experimentally observed orientation-dependence of the indent morphology. The results suggest that the mechanism governing the interaction of helium-induced defects with glide dislocations is orientation independent. Rather, differences in the pile-up morphology are due to the relative orientations of the crystal slip systems, sample surface, and spherical indenter. This highlights the importance of accounting for crystallographic orientation when probing the deformation behavior of ion-implanted materials using nano-indentation.