  A literature review of studies investigating the topography of nano-indents
in ion-implanted materials reveals seemingly inconsistent observations, with
report 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 (AFM) of indents in
unimplanted tungsten shows little orientation dependence. However, in the
implanted material a much larger, more localised pile-up is observed for <001>
grains than for <110> and <111> orientations. Based on the observations for
<001> grains, we hypothesise that a large initial hardening due to
helium-induced defects is followed by localised 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 indent morphology. The
results suggest that the mechanism governing the interaction of helium-induced
defects with glide dislocations is orientation independent. Rather, differences
in 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
behaviour of ion-implanted materials using nano-indentation.
