The effects of neutron irradiation on the mechanical properties of annealed and 20 percent cold-worked AISI 316 irradiated in the experimental breeder reactor II were determined for the temperature regime of 370 to 760°C for fluences up to 8.4 × 10²² neutrons (n)/cm² (E > 0.1 MeV). At irradiation temperatures below about 500°C, both annealed and cold-worked material exhibit a substantial increase in the flow stress with increasing fluence. Furthermore, both materials eventually exhibit the same flow stress, which is independent of fluence. At temperatures in the range of 538 to 650°C, the cold-worked material exhibits a softening with eventual saturation of the flow stress with increasing fluence. Annealed AISI 316 in this temperature regime exhibits hardening and, at a fluence of 2 to 3 × 10²² n/cm² (E > 0.1 MeV), reaches the same value of flow stress as the cold-worked material.

These observations are explained in terms of the fluence and temperature dependence of the irradiation-induced microstructure. It has been shown that AISI 316 proceeds toward an equilibrium dislocation and Frank-loop microstructure that is independent of the initial microstructure. There is a significant hardening that arises from the precipitation of small γ′ and G-phase particles, both of which are formed only during irradiation and develop at temperatures less than about 550°C. Voids and Frank loops also produce hardening and exhibit a strong temperature dependence. Very good agreement between the observed data and calculated strengths was obtained for both solution-annealed and cold-worked AISI 316 through the use of simple hardening expressions for each type of microstructural defect. Extrapolation of these results to other reactor systems will lead to slightly different results because of flux and spectral effects on microstructural development.