DOSE DEPENDENCE OF THE MICROSTRUCTURAL EVOLUTION IN NEUTRON-IRRADIATED AUSTENITIC STAINLESS-STEEL

Microstructural data on the evolution of the dislocation loop, cavity, and precipitate populations in neutron-irradiated austenitic stainles s steels are reviewed in order to estimate the displacement damage lev els needed to achieve the ''steady state'' condition. The microstructu ral data can be conveniently divided into two temperature regimes. In the low temperature regime (below about 300-degrees-C) the microstruct ure of austenitic stainless steels is dominated by ''black spot'' defe ct clusters and faulted interstitial dislocation loops. The dose neede d to approach saturation of the loop and defect cluster densities is g enerally on the order of 1 displacement per atom (dpa) in this regime. In the high temperature regime (approximately 300 to 700-degrees-C), cavities, precipitates, loops and network dislocations are all produce d during iffadiation; doses in excess of 10 dpa are generally required to approach a ''steady state'' microstructural condition. Due to comp lex interactions between the various microstructural components that f orm during iffadiation, a secondary transient regime is typically obse rved in commercial stainless steels during irradiation at elevated tem peratures. This slowly evolving secondary transient may extend to dama ge levels in excess of 50 dpa in typical 300-series stainless steels, and to > 100 dpa in radiation-resistant developmental steels. The deta iled evolution of any given microstructural component in the high-temp erature regime is sensitive to slight variations in numerous experimen tal variables, including heat-to-heat composition changes and neutron spectrum.