Phosphorus-containing austenitic alloys in the solution annealed condition were irradiated at 745 to 760 K (470 to 485°C) in the Fast Flux Test Facility (FFTF) reactor to doses ranging from 0.1 to 10 dpa. The alloys were variations on Fe-13Cr-15Ni-0.05P with respective additions of 0.8 silicon, 0.2 titanium, or 0.8 silicon plus 0.2 titanium; also included were low (0.01) and zero phosphorus compositions (all values in wt %). This alloy set was studied previously using heavy ion irradiations.
The reference ternary and the two phosphorus-only variations contained little precipitation and numerous voids and swelled rapidly, while the three variants containing phosphorus with silicon or titanium or both showed little or no void formation and profuse phosphide precipitation. These results indicate that phosphorus in solution alone does not have a major influence on void swelling, whereas fine-scale phosphide precipitation is quite effective at suppressing void formation. The possible contributing role of impurity oxygen to void formation is considered, but this role is found to be secondary to the principal mechanism restricting swelling, which is the effect of the dense precipitate microstructure. These precipitates foster profuse cavity nucleation that in turn dilutes the helium accumulation per site. The cavities together thereby require more total helium atoms (and more time) in order for individual cavities to surpass their critical size and number of gas atoms necessary for subsequent growth as voids.