Towards better understanding the question of helium-driven swelling in RAFM-ODS alloys

We irradiated three 9Cr alloys: Fe9Cr model alloy, reduced-activation ferritic/martensitic (RAFM), and advanced oxide dispersion strengthened ferritic (RAFM-ODS) alloys, with 100keV He ions at 723K to the peaked concentration of 6500 appm (low helium level) and 65000 appm (high helium level), respectively. The question of helium-driven swelling in Fe9Cr, RAFM, and RAFM-ODS alloys was investigated by multiple characterizations. Homogeneous helium bubbles, main in the form of VmHen (n < m), nucleated on different sinks, producing swelling behavior and dominating the microstructures of irradiated Fe9Cr, RAFM, and RAFM-ODS steels. The consequent swelling rate values in Fe9Cr, RAFM, and RAFM-ODS were ∼ 0.15% (1.33%), ∼ 0.058% (0.83%), and ∼ 0.048% (0.73%) in the low (high) levels, respectively. The RAFM-ODS steel exhibits better tolerance to helium bubble swelling and superior grain size stability than Fe9Cr and RAFM, which is manifested by the smaller average sizes and the lower number densities of bubbles. It can be attributed that the oxides in RAFM-ODS steel could provide more effective helium trapping sites that sequester the helium into smaller bubbles and away from the precipitate-matrix interfaces and grain boundaries. In addition, the existence of high density of oxide particles could also delay desorption peaks of helium atoms from RAFM-ODS steel compared to RAFM and Fe9Cr.