Effect of Damage, Temperature, and Helium on Irradiated Nanoprecipitation Stability and Helium Sequestration Ability in an Advanced Ferritic/Martensitic Fe-9Cr Steel
Creators: name: Green, Theresa Mary
Publisher: name: My University
Year: 2023
Subjects: subject: strengthening nanoprecipitates
subject: radiation resistance
subject: fusion structural materials
subject: phase stability
subject: ion irradiation
subject: ferritic/martensitic steel
subject: Nuclear Engineering and Radiological Sciences
subject: Engineering

description: Fusion first-wall and blanket materials need to withstand the synergistic effects of atomic displacements, transmutation products such as helium, and elevated temperatures. Particularly, helium can have significant effects on the microstructural evolution of materials under irradiation, including increased cavity swelling, changes in solute segregation, altered secondary phase evolution, and/or altered point defect transport. Methods for mitigating helium effects, which can in turn improve swelling resistance, include the introduction of microstructural sinks to disperse and sequester helium homogeneously. This delays the localized accumulation of helium to a single or few sites within the microstructure, such as in each cavity, and lengthens the onset to steady-state swelling. One such microstructural sink is secondary phases and precipitates. Previous studies have shown the interfaces of Ti-rich MX precipitates in Ti-modified 316 austenitic stainless steel were able to sequester most or all of helium generated during neutron irradiation and extend the onset of steady-state swelling. In addition, helium has also been shown to bind to solute atoms. Hence, helium is predicted to have strong interactions with solutes and precipitate-matrix interfaces and to have significant consequences for the swelling behavior of steels. However, there exists no conclusive experimental evidence in literature that semi-coherent nanoprecipitates in current fusion candidate materials will act as sites for helium sequestration. The objective of this work centers on filling the knowledge gap on the evolution of such precipitates and their interfaces under simultaneous atomic displacements and helium transmutation. This research aims to understand the co-evolution of helium and MX-TiC semicoherent precipitates using dual-beam ion irradiations on a reduced activation ferritic/martensitic (RAFM) Fe-9Cr alloy that is a candidate fusion first-wall and blanket material. This work examines the precipitate behavior under ion irradiation without helium and then with helium to assess the helium effects on precipitate stability as well as on the precipitate interface. This work found that co-injected helium suppressed the radiation-assisted coarsening response of precipitates. This is hypothesized to occur due to helium atoms binding to solutes in the matrix and rendering those helium-solute clusters immobile. MX-TiC precipitates were also found to sequester helium atoms in the form of nano-scale bubbles on the precipitate-matrix interface as a function of temperature prior to precipitate dissolution. Such helium sequestration is desired for greater swelling resistance, but it was found that the MX-TiC precipitates were not present at high enough densities or stable to high enough damage levels to affect the onset of steady-state swelling or the steady-state swelling rate, as compared with prior generations of RAFM steels. This work provides insights into the single and combined effects of damage rate, damage, temperature, and helium co-injection on the stability of nanoprecipitates and their effect on swelling resistance in an advanced RAFM Fe-9Cr steel alloy. The consequences of this research are to assess the current state of advanced fusion structural materials and to inform future alloy designers on the relationship of precipitation and swelling. This work will pave a roadmap for understanding the irradiation stability and function of secondary phases that have been specifically designed to provide radiation resistance during fusion operation.