Investigations into helium-ion-induced nanoblistering in single-crystal vanadium

Irradiation of single-crystal vanadium with helium ions produces "nanoblisters" on the surface with crack-like cavities. This thesis combines finite-element analysis (Abaqus 2023) with experimental setups, such as nanoindentation, AFM, and SEM, to examine whether indentation of pressurized blisters drives perimeter crack growth of cavities or primarily reshapes the blister cap geometry. An axisymmetric blister model with an internal seam (sharp crack) and depth-dependent (due to irradiation dose distribution) elastic-plastic properties is assigned via a USDFLD field variable. Contact with an equivalent conical Berkovich indenter (and separately a flat-punch) is used to evaluate J-Integral, local stresses, and plastic equivalent strains (PEEQ) under different cavity loading conditions. The cavity pressures during indentation are set as constant, increasing, or zero. Material data is extracted from micropillar compression tests at various He irradiation dose levels. Simulations reveal blister expansion resulting from gas pressure, which generates high stresses and strains concentrated around the crack-tip and, to a certain extent, on the blister cap. Simulation of Berkovich indentations into blisters (to 10% deeper than blister height) shows a decrease in stresses and strains around the crack-tip and J-Integral values (a measure for crack driving force) due to indentation. They subsequently rebound to marginally higher values due to indentation release (when the indenter retracts from the blister). Plastic strain accumulates primarily during pressure loading and indentation release. Experiments show the following trends. AFM measurements reveal no discernible increase in diameter after indentation (-4% to +5%) and mostly reduced heights after Berkovich indentation. In contrast, flat-punch tests exhibit visible blister cap fracture in SEM and post-indentation increase in blister cap height and overall blister volume in AFM. Comparison of force-displacement curves from experiments and simulations show a good agreement in overall curve shape, having very similar regimes, but with a higher reaction force reported in simulations (by up to 60%), attributed to swelling strains and material model inaccuracies among other. This means that Berkovich indentation up to the studied depths does not measurably drive perimeter crack growth due to insufficient crack driving force, supporting the concept of a rising resistance curve for plastic crack growth. Flat-punch indentation instead promotes blister cap fracture. A depth-dependent constitutive model is essential for reproducing the observed trend and provides a basis for future low-temperature studies, where brittleness may facilitate crack growth.