TEM Study of Helium Porosity Formation in ODS Ferritic Alloys during Post-Irradiation Annealing

Abstract

Transmission electron microscopy has been applied to study the formation of helium porosity both in conventional ferritic steel AISI410S and in two oxide dispersion strengthened (ODS) ferritic alloys: experimental ODS Cr16 steel and advanced ODS EP450 steel considered as promising structural materials for fuel cladding. The samples were implanted with low-energy helium ions at room temperature and annealed at temperature of 1073 K (0.6T m ) for 30 minutes. TEM investigation after such heat treatment has revealed a bimodal size distribution of helium bubbles in AISI410S and ODS Cr16 steels: large-sized bubbles with a large total volume were found in vicinity to grain boundaries (GBs), dislocations or outer surface, while small bubbles were observed inside of grain interiors (GIs). At the same time, helium bubbles in EP450 steel were found to be uniformly distributed throughout the grain body. It is suggested that helium bubble migration and coalescence in ODS EP450 is significantly accelerated due to its permanent cold-hardened state in comparison with two other steels. It is shown that presence of dispersed nano-oxides in ODS alloys does not have a noticeable effect on gas swelling in studied ODS alloys.