NSUF 16-705: Atom Probe Characterization of Two Different Precipitation Regimes in a U-6wt.%Nb alloy
U-Nb alloys are of current interest in defense applications and could potentially be used for metallic reactor fuels. Alloys near 6 wt.% (=14 at.%) have remarkable corrosion resistance and ductility. However, long term aging and other thermal excursions have an impact on corrosion resistance and mechanical properties. These concerns have motivated a wide-ranging study of U-Nb aging over the last decade at LANL. The aging pathways in U-Nb are complex, and comprise at least 5 decomposition mechanisms, including nonlamellar (NL) with varying Nb redistribution, discontinuous precipitation (DP), and discontinuous coarsening (DC) regimes. Chemistry and precipitation morphologies representative of most of these decomposition mechanisms exist, but no data exist concerning the fine-scale structure and chemistry for the NL3 mechanism (further described in the project narrative), a fine nanoscale precipitation phase. It is our hypothesis that the NL3 reaction is kinetically competitive with the DP reaction, competing for chemical driving force and storing or releasing energy at the interfaces. To this end, it is also interesting to this study to investigate the interface between the NL3 and DP precipitation regimes. Samples aged at 600°C for 10, 30, and 100 minutes that exhibit structures formed by the NL3 and DP mechanisms will be investigated by local electrode atom probe (LEAP). The LEAP instrument will provide characterization of precipitation associated with the NL3 reaction at the atomic scale, and provide site specific chemistry and structure data of the DP region at or near the NL3/DP interface. Improved understanding of what microstructural features are responsible for the notable mechanical property change from a soft ductile material at time=0 during aging to a hard, reduced-ductility material at the peak aged condition. The proposed work will augment LANL’s body of knowledge about age hardening in this system, especially in concentrated alloys where precipitate volume fractions are high. Recent LEAP investigations at the CAES facility have already begun to elucidate microstructural features and chemistry within the NL3 regime at 500 and 600°C up to an aging time of 10000 minutes. Results indicate that the NL3 precipitates have strong Nb partitioning (~0 at.% Nb within the precipitates and ~40 at.% Nb within matrix) and have coalesced for the aging conditions already investigated by APT during our previous visits to CAES. The proposed experiments will be performed over 1.5 weeks in 2017, with data analysis and publication of the results expected in 2017-2018.
Additional Info
| Field | Value |
|---|---|
| Abstract | U-Nb alloys are of current interest in defense applications and could potentially be used for metallic reactor fuels. Alloys near 6 wt.% (=14 at.%) have remarkable corrosion resistance and ductility. However, long term aging and other thermal excursions have an impact on corrosion resistance and mechanical properties. These concerns have motivated a wide-ranging study of U-Nb aging over the last decade at LANL. The aging pathways in U-Nb are complex, and comprise at least 5 decomposition mechanisms, including nonlamellar (NL) with varying Nb redistribution, discontinuous precipitation (DP), and discontinuous coarsening (DC) regimes. Chemistry and precipitation morphologies representative of most of these decomposition mechanisms exist, but no data exist concerning the fine-scale structure and chemistry for the NL3 mechanism (further described in the project narrative), a fine nanoscale precipitation phase. It is our hypothesis that the NL3 reaction is kinetically competitive with the DP reaction, competing for chemical driving force and storing or releasing energy at the interfaces. To this end, it is also interesting to this study to investigate the interface between the NL3 and DP precipitation regimes. Samples aged at 600°C for 10, 30, and 100 minutes that exhibit structures formed by the NL3 and DP mechanisms will be investigated by local electrode atom probe (LEAP). The LEAP instrument will provide characterization of precipitation associated with the NL3 reaction at the atomic scale, and provide site specific chemistry and structure data of the DP region at or near the NL3/DP interface. Improved understanding of what microstructural features are responsible for the notable mechanical property change from a soft ductile material at time=0 during aging to a hard, reduced-ductility material at the peak aged condition. The proposed work will augment LANL’s body of knowledge about age hardening in this system, especially in concentrated alloys where precipitate volume fractions are high. Recent LEAP investigations at the CAES facility have already begun to elucidate microstructural features and chemistry within the NL3 regime at 500 and 600°C up to an aging time of 10000 minutes. Results indicate that the NL3 precipitates have strong Nb partitioning (~0 at.% Nb within the precipitates and ~40 at.% Nb within matrix) and have coalesced for the aging conditions already investigated by APT during our previous visits to CAES. The proposed experiments will be performed over 1.5 weeks in 2017, with data analysis and publication of the results expected in 2017-2018. |
| Award Announced Date | 2016-08-16T13:00:53.383 |
| Awarded Institution | University of Michigan |
| Facility | Michigan Ion Beam Laboratory |
| Facility Tech Lead | Kevin Field, Yaqiao Wu |
| Irradiation Facility | None |
| PI | Clarissa Yablinsky |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 705 |