Fast-Neutron Effects on Type-304 Stainless Steel

Abstract

Tensile properties of irradiated Type-304 stainless steel have been measured. These results have been correlated with microstructural observations obtained by optical and transmission electron microscopy techniques. The material studied was irradiated in a fast-neutron environment to a peak exposure of 4.8 × 10²² n/cm² at temperatures ranging from 371 to 463°C in the EBR-II reactor. True yield stresses were observed to increase, and true uniform strains to decrease with both increasing neutron exposure and decreasing irradiation temperature for test temperatures <750°C. At 750°C no increases in true yield stresses over control values were noted while sharp decreases in true uniform strains were observed. It is suggested that some annealing of the microstructure occurs at this elevated temperature, allowing helium to be accumulated at grain boundaries. Microstructural examination by transmission electron microscopy revealed homogeneous distributions of polyhedral voids and Frank dislocation loops. Neither deject was observed to form on grain boundaries. It is suggested that the dislocation loop formation is primarily responsible for the increased strength of the irradiated material. Immersion density measurements are included. These results indicate that the peak void formation did not occur at the maximum flux position, thus indicating the importance of temperature to the phenomenon.