Temperature and dose effects on dislocation loops in self-ion irradiated high-purity iron

Body-centered cubic (BCC) Fe-based alloys are promising candidate materials for advanced nuclear reactors. However, a detailed understanding of irradiation induced dislocation loop microstructure development remains unresolved. It is a widespread belief that <001> loops become increasingly favorable over ½ <111> loops as irradiation temperature rises above ∼300 C. Unfortunately, the temperature effects on <001> loop have been primarily examined in in-situ irradiation on TEM thin foils but poorly explored on bulk Fe due to exceedingly limited experimental studies on bulk specimens, raising concerns about the potential influence of TEM thin foil artifacts on observed results. In this study, we conducted experiments on ultra-high purity BCC Fe specimens irradiated with 6.7-8 MeV Fe ions over a wide temperature range on bulk samples. We investigated the effects of temperature (Tirr = 250-500°C), dose rate (10⁻⁵ to 10⁻³ dpa/s), and dose (0.35 to 3.5 dpa) on the formation and evolution of <001> and ½ <111> loops as well as cavity (void) formation. Post-irradiation Burgers vector analysis via g•b method on dislocation segments and loops revealed that <001> loop fraction does not show a monotonic positive correlation with irradiation temperature. Combined with previous and current theoretical as well as experimental findings, we explore the temperature effects on all existing models of <001> loop formation. We conclude that the prevailing reports regarding the dominance of <001> loops in Fe at elevated temperatures are mainly attributable to the loss of glissile ½ <111> clusters in TEM thin foil experiments.