Microstructural evolution and hardness changes in ion irradiated Ni-based superalloys

Author

Qinyun ChenFollow

Date of Award

12-2024

Degree Type

Dissertation

Major

Nuclear Engineering

Major Professor

Steven J. Zinkle

Committee Members

Steven J. Zinkle, Maik Lang, Khalid Hattar, Eric Lass, Shradha Agarwal

Abstract

Nickel-based superalloys are promising structural material candidates for molten salt reactors and other advanced nuclear reactors. However, traditional Ni alloys exhibit severe mechanical degradation after neutron irradiation, and the gamma-prime precipitates in these alloys are susceptible to dissolution during irradiation. This study focuses on the microstructure evolution and mechanical property changes of Ni-based superalloys subjected to ion irradiation at different temperatures and doses. Several high-performance commercial Ni-base superalloys (solid solution-strengthened Haynes 244, Haynes 214, and Hastelloy C-276, precipitation-strengthened Haynes 244 and Haynes 282) were irradiated with 8 MeV Ni ions to midrange doses of 1 and 10 dpa at 600 °C and 750 °C.

During 750 °C irradiation to 10 dpa, the Ni₂ (Mo, Cr, W) precipitates in aged Haynes 244 exhibited remarkable stability. In contrast, such precipitates were partially dissolved in the 600 °C samples. For solid solution treated Haynes 244, the Ni₂ (Mo, Cr) grew in both irradiated regions (500–1000 nm depth from the sample surface) and pristine regions subjected to long time high-temperature exposure, except in the 750 °C, 1 dpa sample, suggesting longer nucleation time is required at the high temperature close to the critical transformation temperature from short-range ordering to long-range ordering. Ni₃Al precipitates grew in the solution annealed Haynes 214 under irradiation and thermal exposure. A high number density of Cr₂₃C₆ carbides was exclusively observed in the irradiated region of the 750 °C, 1 dpa sample, attached to the dislocation lines originating from mechanical polishing. Ni₃(Al, Ti) precipitates in Haynes 282 showed remarkable radiation resistance and thermal stability at 600 °C and 750 °C. No statistical difference in size and number density of precipitates was observed after irradiation. Hastelloy C-276 demonstrates superior metallurgy stability since there is no precipitation of Ni₂(Mo, Cr) or Mo-enriched carbides within the matrix after irradiation. The modified NHM model and the Ostwald ripening LSEM model were employed to predict precipitate evolution under thermal exposure and irradiation conditions. Factors that might impact the accuracy of models were discussed. The post-irradiation nanoindentation analysis revealed significant hardening for both solution-annealed Haynes 214 and Haynes 244. The superposition method and dispersed barrier hardening model were applied to correlate the microstructures with the mechanical properties. Tan's and Zhu's methods are applied to derive the yield strength from nanoindentation hardness. Overall, the yield strength predictions showed acceptable agreement with experimental data.

Recommended Citation

Chen, Qinyun, "Microstructural evolution and hardness changes in ion irradiated Ni-based superalloys. " PhD diss., University of Tennessee, 2024.
https://trace.tennessee.edu/utk_graddiss/11342