Two plates of Alloy HT-9 and one plate of modified Alloy 9Cr-lMo were irradiated at temperatures ranging from 93 to 300°C to study alloy resistance to radiation-induced embrittlement for fusion reactor applications. The neutron fluences were ~1 × 10²⁰ neutrons (n)/cm² (E > 1.0 MeV); assessments included the effects of alloy composition (12Cr versus 9Cr) and melt processing (Electroslag Refining (ESR) versus Argon Oxygen Decarburization (AOD)) on radiation resistance. Fracture resistance change was judged from pre versus postirradiation notch ductility and dynamic fracture toughness properties determined, respectively, with standard Charpy-V (C_v) and fatigue precracked Charpy-V (PCC_v) tests.

A significant difference in radiation resistance was observed for the two alloys. The dissimilarity can be attributed to their respective compositions or microstructures or both. Comparable radiation resistance was found for ESR versus AOD plates from the same (parent) melt. Irradiation at 93°C elevated the brittle/ductile transition of the Alloy HT-9 to the simulated service temperature. In contrast, an appreciable toughness reserve against additional fluence was exhibited by both alloys after irradiation at 149°C (or higher).

The data indicate a general agreement of C_v and PCC_v methods in their independent assessment of irradiation embrittlement. However, the alloys showed a very marked difference in the relative temperature ranges of the C_v and PCC_v brittle/ductile transitions wherein modified 9Cr-lMo but not HT-9 appeared to have a marked sensitivity to test specimen notch acuity. Scanning electron microscope (SEM) examinations of the modified 9Cr-lMo specimens showed predominantly cleavage failure in PCC_v tests at temperatures where C_v specimens failed predominantly in shear.

New data for Alloy HT-9 weld heat affected zone material and AISI Type 308-16 weld deposit materials irradiated at 288°C are also presented.