Grain boundary softening from stress assisted helium cavity coalescence in ultrafine-grained tungsten

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