Effect of He/dpa ratio on bubble characteristics in Fe9Cr1.5W0.4Si F/M steel during irradiation and annealing.

• The higher the He/dpa ratio, the smaller the size and higher the density of the irradiated bubbles. However, He/dpa didn't affect the final saturation size of irradiation bubbles. • After annealing, high He/dpa caused large, immobile, dense polyhedral bubbles, while low He/dpa caused small, low-mobility, low-density spherical bubbles. • The higher the He/dpa ratio, the greater the swelling during irradiation and annealing. • Tunnel structure was first found in F/M steel during in-situ irradiation. The He production rate (i.e., He/dpa) in nuclear reactors strongly affects the degradation of material properties. This is an important but not yet fully understood issue. Here, the effect of He/dpa on bubble characteristics in Fe9Cr1.5W0.4Si ferrite-martensitic (F/M) steel was in situ studied during 400 keV Fe⁺ and 30 keV He⁺ dual-beam irradiation at 723 K with three ratios of 100, 500, and 2500 appm He/dpa and subsequent stepwise annealing using transmission electron microscopy (TEM). He/dpa strongly affected the bubble characteristics. During irradiation, the higher the He/dpa, the smaller the size of irradiated bubbles, but the higher their density. However, He/dpa didn't affect the final saturation size of irradiated bubbles for all three cases, which was ∼2.2 nm. During annealing, high He/dpa caused large, immobile, dense polyhedral bubbles with a wider bubble size distribution, while low He/dpa caused small, low-mobility, and relatively low-density spherical bubbles. It was found that the higher the He/dpa ratio, the greater the swelling during irradiation and annealing, and annealing further enhanced the swelling. Moreover, the tunnel structure was first found in body-centered cubic (BCC) F/M steel during in-situ irradiation. The current work provides valuable and potential insights for further understanding the He/dpa effects in materials serving in different nuclear reactors. [Display omitted] [ABSTRACT FROM AUTHOR]