Observations of stacking-fault tetrahedra in an austenitic alloy after high-temperature neutron irradiation

In a recent experiment, a model austenitic alloy with a composition of 0·17Cr-0·167Ni-0·025Mo was irradiated to about five displacements per atom in the Oak Ridge Research Reactor at 550 and 600°C. Some of the specimens had been implanted with 10–50 at.p.p.m. helium and subjected to various post-implantation anneals prior to irradiation. At this low dose, relatively little void formation was observed in the specimens. However, a surprising observation was that a large number of stacking-fault tetrahedra were seen. These defects were observed in specimens irradiated at 550 and 600°C, with and without helium pre-injection, and with and without post-helium-implantation heat treatment. The average edge length was 15 to 20nm and the density varied between 8 × 1018 and 4 × 1020 m−3. The highest density was observed in a solution-annealed specimen irradiated at 600°C with no helium pre-injection. The results of this experiment support theoretical calculations which indicate that stacking-fault tetrahedra should be stable in austenitic stainless steels. The lack of previous evidence for these defects appearance can be ascribed to void formation at the expense of stacking-fault tetrahedra at higher doses.