Comparison of scanned and defocussed beam ion irradiation hardening of UHP Fe and Fe-Cr alloys

• Beam scanning found to not affect irradiation hardening in UHP Fe-Cr binary alloys • RIS/clustering suggested to have low sensitivity to pulsed flux effects • There may be an engineering relevant regime where scanning effects are negligible Charged particle irradiation is often used as a surrogate for neutron irradiation, and some facilities use a beam focussed to spot which is then scanned over the target surface to ensure a uniform fluence and allow fine adjustment of the flux. This results in intermittent irradiation of an area segment at a higher instantaneous flux than the time-averaged value, in contrast to the quasi steady state exposure in a reactor environment. Recent experiments have shown this pulsed irradiation to influence damage microstructure in nuclear relevant structural materials, raising questions about its use to emulate in-service conditions. Here we compare hardening of ultra high purity Fe and Fe-Cr alloys irradiated with scanned and defocussed beams to investigate whether difference in instantaneous fluxes produces a dose rate effect. No significant difference in irradiation hardening was observed. This is discussed in terms of the mechanisms leading to irradiation induced microstructure, and suggests a regime exists where beam scanning may have a negligible effect on irradiation hardening.