A maximum entropy/SANS analysis of neutron irradiation processes in model pressure vessel steels

A database of small angle neutron scattering (SANS) spectra obtained on a s eries of model alloys based on the A533B pressure vessel steel specificatio n but containing systematic copper, phosphorus or nickel additions, before and after 290 degrees C neutron irradiation to 3.1 x 10(23) nm(-2) (E > 1 M eV), is analysed in detail using a maximum configurational entropy computin g procedure. Volume fraction-particle size and number density data are prod uced for each alloy to characterize the irradiation-induced microstructural changes in terms of microvoids and/or precipitates of different sizes. The derived particle size distributions exhibit narrow sharp peaks at similar to 2 nm diameter together with broader peaks in the range 10-40 nm diameter . The analysis suggests that under irradiation the total volume fraction of precipitates increases with phosphorus content, and that there is an incre ase in volume fraction with increase in both copper and nickel content at c onstant phosphorus level. Most of the volume fraction increase in low-phosp horus steels arises from 1-4 nm diameter precipitates (viz. a Cu-rich phase ), while for high-phosphorus steels there is evidence of a significant incr ease in 4-50 nm diameter precipitates (viz. a P-rich phase). Finally, calcu lations have been mane of neutron irradiation strengthening in the alloys b y input of the SANS data into detailed hardening models. The estimated hard ness increases are broadly in accord with the measured values.