Effects of irradiation-induced defects and grain orientations on the indentation behaviors of a Fe9Cr ferritic model alloy

To better understand the effects of irradiation-induced defects on incipient pop-ins and subsequent dislocation activities in a Fe9Cr ferritic model alloy, the alloy was irradiated with 5 MeV Fe²⁺ ions at room temperature (RT) up to a nominal damage level of 1 dpa. Experimental indentation tests were performed perpendicular to the specimen surface using a spherical indenter (radius: 1870 nm), which revealed that the maximum shear stress (τ_max) at the onset of pop-in events was slightly increased by ion-irradiation, weak dependance of grain orientations. Dislocation loops, approximately 3.4 ± 0.5 nm in size with the number density of 3.9 × 10²¹ m^-3, were observed within the depth range of 900 – 1400 nm (∼ 2.0 dpa in average) by using transmission electron microscopy (TEM). Irradiation-induced defects, particularly dislocation loops, significantly contributed to the hardening of the Fe9Cr alloy by 0.66 GPa and the shorter dislocation mean free path, which was obtained through experimental indentation tests with a Berkovich indenter. The influence of dislocation loops on dislocation activities of Fe9Cr alloy was further explored by using molecular dynamics (MD) simulations on <1 0 0>, <1 1 0> and <1 1 1> grain orientations. Pre-existing dislocation loops also caused a slight increase in load at the first pop-in event in the crystal structures with the aforementioned grain orientations. Dislocation nucleation and propagation were observed at the sites of pre-existing dislocation loops during incipient pop-in events, possibly suggesting the mechanisms for the increase in the experimental pop-in stress.