Reactor conditioning of Nimonic PE16 in the solution-treated and aged (STA) condition has been shown to enhance the swelling rate over that of the unirradiated condition under nickel-ion bombardment by a factor of 4 to 5. The peak swelling rate for nickel-irradiated virgin (originally unirradiated) and reactor-conditioned specimens and for high fluence neutron-irradiated material was shown to be directly related to the volume fraction of γ′ which reflected the chemical composition of the matrix. The virgin material had a γ′ volume fraction of 1.5 percent and a swelling rate of 0.008 percent/displacements per atom (dpa) nickel; the reactor-conditioned material an intermediate γ′ volume fraction of ∼5.5 percent and an intermediate swelling rate of 0.046; and the neutron-irradiated material the highest γ′ volume fraction of ∼7 percent and the highest steady-state swelling rate, 0.114. A one-to-one dose equivalency between ion- and neutron-generated displacements (50 dpa nickel = 10²³ n/cm²) was consistent with the results of these experiments. The temperature shift between the peaks of the neutron and ion swelling rate curves was about 100 deg C. In the material reactor conditioned near the neutron peak swelling temperature, the shift tended to decrease with increasing reactor conditioning temperature.

Observations on PE16 STA showed that the γ′ volume fraction of ion-irradiated virgin and reactor-conditioned specimens was essentially unchanged during the short time at temperature to attain simulated high doses, whereas the γ′ volume fraction increased continuously with fluence during the long time at temperature for the neutron-irradiated material. Thus, because of the sluggish evolution of the matrix chemistry, the swelling rates measured for ion-irradiated material were characteristic of intrinsically lower swelling conditions of the alloy.