IRRADIATION HARDENING OF PRESSURE VESSEL STEELS AT 60°C: THE ROLE OF THERMAL NEUTRONS AND BORON

摘要

Six split melt A533B-type model alloys, with different combinations of copper, nickel and boron contents, were irradiated at 60°C in a neutron spectrum with a thermal-to-fast (E > 1 MeV) ratio of about 370. The resulting yield stress increases were compared to hardening produced by irradiations at a low thermal-to-fast flux ratio. The latter data were obtained in a larger experiment on 60°C hardening that showed: i) a negligible effect of flux; ii) hardening increases with the square root of fluence above a low threshold; and iii) an individually weak, but collectively significant, influence of copper, nickel, manganese, phosphorous and molybdenum. These and other observations have been interpreted to suggest that low temperature hardening is caused by features formed in displacement cascades. However, this interpretation is not consistent with the results of this study which show that yield stress increases are even larger for conditions dominated by low energy PKAs created by thermal neutrons. The thermal-to-fast dpa hardening efficiency factor was estimated to be about 1.7±0.5, which is comparable to the corresponding theoretical ratio of residual defects. The data also suggest that transmutant products from the ~(10)B(n,α) reaction may contribute a small increment to hardening.