The effect of stress on the microstructure of neutron irradiated cold worked type 316 stainless steel

Abstract

20% cold worked Type 316 stainless steel in the form of uniaxially stressed solid rod and pressurized tubes was irradiated at 450°C in the EBR-11. Five irradiated and three unirradiated specimens were examined by transmission electron microscopy to determine the effect of stress on microstructure and possible mechanisms related to creep occurring during fast reactor irradiation. The dislocation density markedly decreased during the interval of transient irradiation creep. The in-reactor creep of the steel¹ occurred independent of swelling since cold work suppressed void formation in both the stressed and the unstressed irradiated material. For the conditions investigated, stress had only a minor effect on the final microstructure compared to that due to irradiation. Although uniaxial stress significantly affected the size of the irradiation induced Frank loops, irradiation creep does not appear to be controlled by the strain from stress biased Frank loop formation. Within a 90% certainty level the mean Frank loop size increases with the increase in the stress component normal to the {111} plane of the loop. The corresponding maximum diameter of these sessile loops was also stress dependent. While the loop size increased with an increase in the stress component normal to the loop plane, the loops which formed on planes having the uniaxial stress nearly parallel to the { 111} planes were smaller than loops produced in the unstressed irradiated sample. These results suggest that uniaxial stress influences the growth rate of Frank loops. A similar effect of stress on loop size was not observed in the sample biaxially stressed during irradiation. No significant effect of stress on loop nucleation could be determined. Effects of stress on loop number density might be masked by the large variations in the numbers of loops formed on the different { 111} planes.