Microstructural changes of proton irradiated Hastelloy-N and in situ micropillar compression testing of one single grain at different local damage levels

In situ micropillar compression was used to study the deformation of proton-irradiated Hastelloy-N at different damage levels. Multiple pillars were prepared from a single grain along the cross-section of 2.5 MeV proton-irradiated Hastelloy-N. Depending on the location of micropillars, the critical resolved shear stress was obtained as a function of local damage levels. Such an approach eliminates the variation of yield stress due to the difference in the Schmid factor. Microstructural characterization showed complicated defect structures, including (a) dislocation loops with many in corduroy-like alignments, (2) dislocations pile up, (3) element segregation, and (4) twin boundaries. Silicon atoms are found to segregate at dislocation lines, loops, and twin boundaries and form complicated patterns at nanometer scales. These complexities make it difficult to conclude which hardening mechanism contributes the most to the hardness changes. The critical resolved shear stress, τ_crss, and hardening exponents were both extracted as a function of displacements per atom values up to 2.3. There was a 60% increase in τ_crss at the highest damage level.