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

A model austenitic alloy with a composition of 0.17 Cr- 0.167 Ni-0.025 Mo was irradiated to about five displacements per atom in the Oak Ridge Research Reactor at 550 and 6000C. 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. The average edge length was 15 to 20 nm and the density varied between 8 x 1018 and 4 x 1020 m-3. The highest density was observed in a solution-annealed specimen irradiated at 6000C with no helium pre-injection. These results 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. (author)