In-situ TEM observation of the evolution of dislocation loops and helium bubbles in a pre helium irradiated FeCrAl alloy during annealing

Abstract

FeCrAl alloys are regarded as excellent potential candidates of fuel claddings in nuclear reactors. To further understand the evolution of helium bubbles and dislocation loops during the annealing process, He+-irradiated TEM samples with a normal chemical composition of Fe-13.2Cr-5.6Al-2Mo-2ZrC (wt.%) were annealed and observed by in-situ transmission electron microscopy. The dislocation density decreased with the increase of annealing temperature, but the loop size was opposite. No obvious bubbles were observed at the annealing temperature below 772 K, even if the annealing time was prolonged. The average diameters of helium bubbles in the sample annealed at 772, 872 and 972 K for 60 min were 1.4, 2.1 and 3.1 nm, respectively, which indicated that bubble sizes increased with the rise of annealing temperature. When the temperature was increased to 1072 K, the bubbles grew rapidly while their density decreased significantly. The dynamic process, during which two or more bubbles merged into one large-sized bubble, was observed. The migration and coalescence of small bubbles should be attributed to the control of surface diffusion mechanism.