The effects of loop size on the unfaulting of Frank loops in heavy ion irradiation

Abstract When stainless steels are exposed to high irradiation dose, a fraction of Frank loops will unfault and transform into perfect loops. After unfaulting, perfect loops can glide and impinge into other loops to form dislocation networks, and dislocation networks will act as sinks for point defects and solute elements. Besides, unfaulting will influence the extent of irradiation hardening as well. Based on previous theories, the size of Frank loops was considered to be a key factor in unfaulting. To better understand the unfaulting process, 316L stainless steel model alloy was irradiated to 5dpa at 350 °C, 400 °C and 450 °C by 3 MeV Ni ions. Method was developed to quantitatively compare the size and density between Frank loops and perfect loops through relrod and weak-beam dark field techniques. Results showed that unfaulting barely occurred at 350 °C but was distinct at 400 °C and 450 °C. The majority of perfect loops were in the size range of 8–16 nm at 400 °C and were in the size range of 12–20 nm at 450 °C. The existence of a critical loop size for unfaulting was not supported by our results. Moreover, by our results, the energy of Frank loops does not play a decisive role in unfaulting, and the simple approximation of geometric constraint in the rate theory to calculate unfaulting diameter needs to be revisited. Irradiation hardening was also analyzed by nano-indention. It was found that loop unfaulting played an important role in the hardening decrease from 350 °C to 450 °C.