Hardening and microstructure evolution in proton-irradiated model and commercial pressure-vessel steels

In an effort to understand the mechanisms of irradiation embrittlement of reactor pressure-vessel steels, hardening and microstructure evolution in a number of simple ferritic model alloys and complex bainitic steels irradiated with 3.2 MeV protons over a range of doses, dose rates and temperatures were characterized. Irradiations were conducted on selected model alloys to 1 dpa, which is a much higher dose than has been explored for neutron irradiations of these materials. Irradiation hardening was determined from Vickers hardness measurements, and the microstructures were characterized using small angle X-ray scattering (SAXS) in selected cases. At low-to-intermediate dose, the hardening trends in the proton-irradiated ferritic alloys without nickel were similar to those under neutron irradiation. Hardening also decreased with the proton irradiation temperature in this case, broadly consistent with neutron irradiation trends, and was generally relatively insensitive to dose rate. Quantitative differences were observed between the proton and neutron irradiations of model alloys and, to a lesser extent, complex steels, containing both copper and nickel. These differences can be rationalized by shifts in the hardening curves to higher dose, due to proton dose rates that are 700 or more times higher than for neutrons. Precipitate sizes in the proton-irradiated alloys generally increase with dose and are qualitatively similar to those observed in neutron-irradiated alloys. However, much larger scattering features were also detected at 1 dpa. All the alloys irradiated to this high dose were remarkably hardened by amounts from 490 to 740 MPa.