Two precipitation hardened Fe-Ni-Cr alloys were irradiated with nickel ions to simulate neutron irradiation and evaluate the in-reactor swelling and phase stability of these materials. Based upon the results of these tests, minor chemical adjustments were made to the base chemistry which will improve in-reactor phase stability and swelling resistance.

The alloys were preinjected with 5 appm He and Ni-ion irradiated in an aged condition to a maximum dose of ∼250 dpa at temperatures between 500°C and 750°C. Surface profilometry measurements on sheet samples showed a small swelling peak at 550°C for one alloy and at 500°C for the other. Very low swelling (less than 1%) was measured by TEM in both alloys irradiated in disc form at 550°C to a dose of ∼220 dpa. The steady-state swelling rates at the peak temperatures were 0.015%/dpa with incubation doses of 145 dpa.

The pre-existing spherical γ' precipitates in the alloys assumed irregular shapes under Ni-ion bombardment and their size distribution broadened without significant change in the average size (∼12 nm) or volume fraction (1–2%).

Substantial radiation-induced precipitation (other than γ') was observed in both alloys. Dark field microscopy and electron diffraction analysis were used to identify the precipitates. It was established that a deleterious G-phase precipitation could be eliminated by reducing the silicon content from 1.0% to 0.1%, without affecting the swelling resistance of the alloys. A reduction in the Mo content was also recommended based on phase stability considerations.