Thermal stability of nanoscale helium bubbles in a 14YWT nanostructured ferritic alloy

•14YWT NFA irradiated with 335keV He+ to 6.75x1020 He m-2 at 400 °C then thermally treated at 750 °C for up to 100 hr.•As-irradiated material displayed poly-disperse bubble size distribution that became more inhomogeneous with thermal treatment.•The bubbles were observed to coarsen following thermal treatment.•Coarsening behaviour exhibits characteristics of self-interstitial and dislocation loop punching, Brownian motion and coalescence and Ostwald ripening. A 14YWT nanostructured ferritic alloy has been irradiated with 335keV He+ to a total fluence of 6.75×1020 He m−2 at a temperature of 400°C and subsequently thermally treated at 750°C for up to 100h. Transmission electron microscopy has been used to characterize the size and distribution of the resultant helium bubbles. The results indicate that the bubbles generally increase in size and the distribution becomes more inhomogeneous during the thermal treatment. The results are discussed in terms of the helium supply and vacancy supersaturation, Brownian motion and coalescence, and Ostwald ripening mechanisms.