MICROSTRUCTURE EVOLUTION AND MECHANICAL RESPONSE OF THERMALLY AGED AND IRRADIATED FE-CR ALLOYS

Estimations of bulk hardness from nanoindentation are frequently subject to considerable uncertainties due to indentation size effects (ISE), pileup effects, and potential influence of surface quality or test methods. This study examined materials science principles of the nanoindentation test method to enable accurate prediction of bulk hardness from nano scale, and correlated microstructure with mechanical properties. The research started by developing a scientific nanoindentation test protocol on a series of high purity Fe and Fe-(3-25 wt. %) Cr alloys. These materials were tested in as-annealed and thermally aged (100-900 hours at 475 ℃ to produce Cr-rich a^’ precipitates) conditions. Nanoindentation with a Berkovich indenter at constant strain rate (0.05 to 0.5 /s) and constant loading rate conditions provided comparable bulk equivalent hardness (H₀) extracted by Nix-Gao model, indicating a weak strain rate sensitivity at room temperature. Results from electropolished and fine mechanically polished samples gave comparable measured hardness. Pileup corrections produced a 5-14% correction to H₀ which agreed with the experimental bulk Vickers hardness within ~10% for most tested materials. The microstructural model-predicted and measured strength values agreed for aged samples. A derived analytic expression demonstrates that an ISE error, associated with inappropriate methods such as hardness ratios or changes at a reference depth, can be as large as 60% in estimated bulk hardness for the investigated Fe-Cr alloys. Subsequently, the nanoindentation protocol was applied to ion irradiated binary Fe18Cr alloys. The microstructure was characterized by APT, S/TEM and EDS to understand the irradiation effects on the mechanical properties. An accurate way to calculate the strength factor was provided based on the size of the defects. A newly refined hardening model with superposition method was applied to correlate the microstructures with the mechanical properties, which further demonstrated the consistency between model prediction and nanoindentation measurement. This study also provides insight into the fidelity of ion irradiation as a surrogate of neutron irradiation based on the dual ion irradiated and neutron irradiated T91 and HT9 ferritic/ martensitic steels. Microstructures and mechanical properties were quantified and compared according to the protocol developed in the thermally aged and ion irradiated binary alloys.