NSUF 23-4651: Investigating nitrogen effects on the mechanical properties and microstructure evolution in neutron irradiated HT-9 steel
Ferritic/martensitic (F/M) steels are being considered as potential structural materials for next generation nuclear reactors, and variants of the alloy HT9 are some of the most promising candidates. In this proposed research, neutron irradiated low and high N variant HT9 will be investigated using post-irradiation examination facility at Oak Ridge National Laboratory (ORNL). The objective is to investigate N effects in HT9 steels, especially on the microstructure evolution and mechanical properties after neutron irradiation at different temperatures and doses. This will also provide information about temperature and dose dependent behaviors on each of the HT9 heats with variation in nitrogen content. Microstructure characterization will be conducted using a focused ion beam (FIB) to produce transmission electron microscopy (TEM) specimens. Then, the bulk tensile testing will be conducted. TEM sample preparation on 4 specimens will require 4 days of FIB time. TEM characterization will take 4 days and bright field, scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) will be used. Tensile testing can be performed within two day at RT and this can be conducted concurrently during the TEM data collection. PI and team have already published results on the same N varied HT9 steels irradiated using an ion beam at high dose [1]. This study will provide valuable neutron data to compare against the previous ion beam results and help predict how this material will perform in a reactor environment. In the previous study, we observed that low and high N HT9 showed different void swelling and G-phase formation behaviors due to altered diffusion rate of vacancies and Ni-vacancy complexes [1]. The high N HT9 showed significantly lower void swelling and higher number density and smaller size G-phase precipitates compared to the low N HT9. The reduced void swelling was explained by defect-N atom complex formation which increases defect recombination, and the G-phase feature was explained by reduced diffusion rate of vacancy-solute atom (Ni) complex due to high N contents. In this study, we anticipate similar G-phase formation behaviors in the neutron irradiated HT9 and expect to see a higher density and smaller size G-phase precipitates in the high N HT9 specimens. However, we do not know at this stage how the irradiation temperature or lower dose rate will affect the G-phase formation on each variant. Void swelling is not expected as doses were low, 8 and 16 dpa. Specimens irradiated at the lower temperature (300 ℃) are expected to form α’ precipitates, and since Cr also diffuses by a vacancy mechanism, α’ formation will be affected by N level. Therefore, due to higher density of G-phases and α’ precipitates, the high N HT-9 is hypothesized to will experience increased hardening compared to the low N HT-9. Loop formation will be characterized and studied for each variant irradiated at different conditions which was not coved by the previous ion irradiation study. [1] H. Kim et. al., JNM 560 (2022) 153492. https://doi.org/10.1016/j.jnucmat.2021.153492
Additional Info
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| Abstract | Ferritic/martensitic (F/M) steels are being considered as potential structural materials for next generation nuclear reactors, and variants of the alloy HT9 are some of the most promising candidates. In this proposed research, neutron irradiated low and high N variant HT9 will be investigated using post-irradiation examination facility at Oak Ridge National Laboratory (ORNL). The objective is to investigate N effects in HT9 steels, especially on the microstructure evolution and mechanical properties after neutron irradiation at different temperatures and doses. This will also provide information about temperature and dose dependent behaviors on each of the HT9 heats with variation in nitrogen content. Microstructure characterization will be conducted using a focused ion beam (FIB) to produce transmission electron microscopy (TEM) specimens. Then, the bulk tensile testing will be conducted. TEM sample preparation on 4 specimens will require 4 days of FIB time. TEM characterization will take 4 days and bright field, scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) will be used. Tensile testing can be performed within two day at RT and this can be conducted concurrently during the TEM data collection. PI and team have already published results on the same N varied HT9 steels irradiated using an ion beam at high dose [1]. This study will provide valuable neutron data to compare against the previous ion beam results and help predict how this material will perform in a reactor environment. In the previous study, we observed that low and high N HT9 showed different void swelling and G-phase formation behaviors due to altered diffusion rate of vacancies and Ni-vacancy complexes [1]. The high N HT9 showed significantly lower void swelling and higher number density and smaller size G-phase precipitates compared to the low N HT9. The reduced void swelling was explained by defect-N atom complex formation which increases defect recombination, and the G-phase feature was explained by reduced diffusion rate of vacancy-solute atom (Ni) complex due to high N contents. In this study, we anticipate similar G-phase formation behaviors in the neutron irradiated HT9 and expect to see a higher density and smaller size G-phase precipitates in the high N HT9 specimens. However, we do not know at this stage how the irradiation temperature or lower dose rate will affect the G-phase formation on each variant. Void swelling is not expected as doses were low, 8 and 16 dpa. Specimens irradiated at the lower temperature (300 ℃) are expected to form α’ precipitates, and since Cr also diffuses by a vacancy mechanism, α’ formation will be affected by N level. Therefore, due to higher density of G-phases and α’ precipitates, the high N HT-9 is hypothesized to will experience increased hardening compared to the low N HT-9. Loop formation will be characterized and studied for each variant irradiated at different conditions which was not coved by the previous ion irradiation study. [1] H. Kim et. al., JNM 560 (2022) 153492. https://doi.org/10.1016/j.jnucmat.2021.153492 |
| Award Announced Date | 2023-06-01T09:04:06.913 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Kory Linton |
| Irradiation Facility | None |
| PI | Hyosim Kim |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | None |