  The formation of helium cavities in coarse-grained materials produces
hardening proportional to the number density and size of the cavities and due
to the interaction of dislocations with intragranular helium defects. In
nanostructured metals containing a high density of interfacial sinks,
preferential cavity formation on the grain boundaries instead produces
softening and often attributed to enhanced interfacial plasticity. Employing
two grades of ultrafine-grained tungsten, we explore this effect using targeted
implantation studies to map cavity evolution as a function of the irradiation
conditions and quantify its impact on the mechanical response through
nanoindentation. Softening is reported at implantation temperatures above the
threshold for preferential grain boundary cavity formation but at a
sufficiently low fluence prior to the growth of intragranular cavities.
Collective changes in the mean cavity size, density, and morphology beneath a
residual impression on an implanted surface indicate that cavity coalescence
accompanied the reduction in hardness. Complementary atomistic simulations
demonstrate that, in tungsten grain structures exhibiting softening, grain
boundary bubble coalescence is driven by stress concentrations that further act
to localize strain in the grain boundaries through cooperative deformation
processes involving local atomic shuffling and sliding, dislocation emission,
and even the nucleation of unstable twinning events.
