Swelling resistance of advanced austenitic alloy A709 and its comparison with 316 stainless steel at high damage levels

Alloy A709 is an austenitic alloy developed for power boiler applications in thermal power plants and is being considered as a candidate structural material for Generation IV reactors and fusion reactors, based especially on its increased creep strength over Type 316 stainless steel. However, improved thermal creep properties would not necessarily imply improved radiation resistance, especially with respect to void swelling. Since there are currently no neutron irradiation data on A709 to high fluences, A709 and cold-worked 316 were irradiated in the present study under identical conditions to doses between 100 and 400 peak dpa using irradiation by 3.5 MeV Fe+2 ions. The swelling behavior of 316 is well-known for both neutron and ion irradiation, thus the relative swelling behavior of A709 and 316 under ion irradiation might provide an indication of the swelling behavior of A709 in a neutron irradiation environment. Swelling of A709 under ion irradiation was observed over the range of 500–600 °C, peaking at 575 °C. Both A709 and 316 eventually swelled at a rate of ∼1%/dpa under ion irradiation at 575 °C, consistent with the swelling of 316 observed during neutron irradiation. But A709 had a significantly longer transient regime than 316 at any given dpa rate, demonstrating enhanced swelling resistance of A709 over 316 under self-ion irradiation. The swelling levels reached at 400 peak dpa were 55% and 90% for A709 and 316, respectively. The assigned local dpa levels were adjusted both temporally and spatially at each dose for swelling-induced increases in the ion range and concomitant decreases in density. The duration of the transient regime in each alloy was also observed to increase as the local dpa rate increased, but with a different flux sensitivity that is speculated to arise from the very different composition of the two alloys.