Nested loops explain low irradiation-induced swelling rate

In the quest of radiation and oxidation-resistant materials for the fuel claddings of nuclear power stations, chromium (Cr) emerges as a good candidate. It shows a significantly lower irradiation-induced swelling rate of nearly an order magnitude lower than other body-centered cubic materials, such as W and Fe. This phenomenon indicates that there is an unsuspected aspect, beyond the crystal structure that affects the swelling of materials. To understand it, the structure and distribution of irradiation-induced dislocation loops in Cr were investigated after ion irradiation at 550 degrees C from 0.1 to 15 dpa. It appears that after irradiation Cr exhibits nested loops, either vacancy-type loops called 'trough' or interstitial-type loops called 'island', which are composed of a large dislocation loop containing multiple small and coplanar loops with inverse nature. Our results indicate that the nested trough and island dislocation loops stem from the 1D migration of dislocation loops. This loop formation mechanism is an alternative mechanism of recombination and annihilation of point defects, which in turn can explain the low swelling rate in Cr. This mechanism provides a new insight into the irradiation-induced swelling rate of metals.