Increasing Stability of Nanocrystalline Ferritic Alloys at High Temperatures and Long Times for Advanced Nuclear Energy Applications

The complex process-structure-property relationship is the basic theme of materials science and engineering. Hence, through engineering the microstructure, desired properties can be achieved. Nanostructured materials exhibit superior properties such as high strength due to their small grain size (<100 nm). Oxide dispersion strengthened (ODS) steels show good irradiation-resistance owing to the presence of numerous nanoclusters. The objective of this research is to develop new nanostructured ferritic alloys with a dispersion of nanosized oxide particles in the nanoscale matrix. The interfacial area provided by both nanometer oxides and grain boundaries could improve the irradiation resistance while maintaining high strength for structural applications in advanced nuclear energy systems. However, nanostructured alloys are highly unstable at elevated temperatures. This thesis focuses on studying the thermal stability of nanocrystalline ferritic alloys through solute additions.;Oversized solute atoms tend to segregate to the grain boundaries or form precipitates, which can reduce grain growth thermodynamically or kinetically. The addition of Hf was compared with that of Zr, where Hf showed better stabilizing effect on Fe-14Cr base alloys. The nanoscale matrix was maintained up to 1000°C with 4 at.% Hf addition. Zener pinning, one of the kinetic stabilization approaches, was considered as the major contribution to high temperature stability of nanocrystalline ferritic alloys. Based on the thermodynamic stabilization model for ternary alloys, Hf solute was predicted to be strong segregator, which contributes to the grain size stabilization as well. A deviation of predicted grain size from the actual grain size in Fe-14Cr-4Hf was found, suggesting the presence of grain boundary segregation. Further study of Fe-14Cr-4Hf/Zr using HRTEM, directly observed Hf and Cr co-segregation on the grain boundaries.;The grain size and nano-oxide effects on the evolution of irradiation-induced hardening were investigated through nanoindentation tests.;In addition to Hf/Zr solutes, Sc was found to be a good stabilizer. Only 1 at.% Sc addition can retain grain size of ODS steel in the nanometer range and microhardness of 5.6 GPa at temperature of 1000°C. Large second phases such as titanium oxides were not observed compared to conventional ODS steels. This is likely due to the consumption of Ti atoms by Sc and O, forming a more complex nanofeature [SiTiO]. The presence of extremely small nanoclusters and the disappearance of large second phases stabilized the microstructure at high temperatures. Mechanical properties were also investigated.;Long-term thermal stability of ODS steel with Sc addition has been further studied at 1000°C up to 60 hours. Grain growth was observed as a function of time through characterization techniques such as XRD, FIB and TEM and the growth rate could be obtained. More complex nano oxides of [SiTiYO] were detected from EDS elemental mapping in long-term annealed samples compared to ternary nanofeatures of [SiTiO] in short time annealed samples. The mechanisms of nanoscale precipitates formation were also investigated.