The effect of neutron radiation on the mechanical properties of alloy HT9 was evaluated by conducting postirradiation tension and fracture toughness tests. Samples of HT9 were irradiated in the Experimental Breeder Reactor II (EBR-II) and Fast Flux Test Facility (FFTF) as part of material experiments. Additionally, selected samples were obtained from a duct used in fuel pin tests in FFTF. The peak neutron fluence for these samples was 15.9 × 10 n/cm2 (E > 0.1 MeV), or about 80 dpa (displacements per atom).
Results of tests conducted at 205°C and at ambient temperature showed that the irradiation temperature had a larger influence on the mechanical behavior than neutron fluence. Irradiation at temperatures below 400°C produces a significant increase in the yield strength of HT9. At 360°C, an increase of more than 80% was found for tests conducted at ambient temperature. Tests on notched tension specimens that had been irradiated at low temperature exhibited a linear elastic behavior with failure occurring with no apparent yielding or elongation. Fracture toughness tests conducted on similar material showed a low level of toughness (approximately 30 MPaVm) at ambient temperature, but tests at 205°C showed a high level of toughness (> 100 MPaVm). These conclusions were also supported by fractographic analyses.
Alloy HT9 remains an excellent material for cladding and duct applications in liquid metal reactors. Caution must be taken for postirradiation handling operation since some irradiation conditions can produce a material that might fail in a brittle manner at room temperature.