Stability of £¤ precipitates in nickel based oxide dispersion-strengthened superalloys under high-temperature and heavy irradiation conditions

The stability of cuboidal £ A precipitates under heavy irradiation was studied for newly developed Ni-based Oxide Dispersion-Strengthened (ODS) superalloys, to explore the suitability of these as core structural materials in Very High-temperature Reactors (VHTR) or Gas-cooled Fast Reactors (GFR). Ion irradiation was applied at a dose level of 100 dpa at 873 K, 1073 K and 1273 K. Under these conditions, £ A precipitates retained their cuboidal shape at 873 K, were deformed somewhat at 1073 K (as predicted by the Nelson-Hudson-Mazey (NHM) model), and were massively deformed and agglomerated at 1273 K. This deformation-agglomeration process is attributed to cascade collision, whereby the increased Gibbs free energy of the disordered phase induces a change in element distribution inside the irradiated area. Ordered, cuboidal £ A precipitates were reproduced from the disordered state after cooling and the cessation of ion irradiation.