Effect of neutron irradiation on the microstructure of NF616 (Grade 92) as a function of dose and temperature
Principal Investigator
- Name:
- Indrajit Charit
- Email:
- [email protected]
- Phone:
- (208) 526-6918
Team Members:
| Name: | Institution: | Expertise: | Status: |
|---|---|---|---|
| Assel Aitkaliyeva | |||
| Brandon Miller | |||
| Ramprashad Prabhakaran | Pacific Northwest National Laboratory | Mechanical Properties, friction stir welding, Small-Scale Mechanical Testing, Advanced Fuels, Irradiation Damage Behavior, Fuel Cladding | Other |
| Danny Edwards | Pacific Northwest National Laboratory | Metallurgy, Radiation Damage | Other |
| Kumar Sridharan | University of Wisconsin | Microstructure-Property Correlations; Physical Metallurgy; Radiation Effects | Faculty |
Experiment Details:
- Experiment Title:
- Effect of neutron irradiation on the microstructure of NF616 (Grade 92) as a function of dose and temperature)
- Hypothesis:
- Neutron irradiation can affect NF616 microstructure (prior austenite grain size, carbide sizes and volume fraction, martensite lath structure and martensite packet) and failure mode that are critical for superior mechanical behavior and radiation damage resistance. Hence, the objective of this study is to perform microstructural characterization (SEM-EBSD) and then utilize the existing mechanical test data to develop appropriate processing-property-structure-temperature-dose correlations.
- Work Description:
- * Materials: Tensile and TEM specimens of NF616 were previously neutron irradiated in the ATR at various temperatures (388-469C) and doses (3-8 dpa). PNNL recently completed tensile testing (room and irradiation temperatures) under a funded RTE project (#2879). Microhardness testing at room temperature was also performed on polished TEM discs and small sections (obtained from shoulders of tested tensile specimens). Failed/tested tensile (fractured gage portions and polished sections from shoulders) and polished TEM specimens of NF616 (control and irradiated) are available at PNNL for the proposed study. * PIE: The proposed work to be performed at PNNL NSUF facility will involve microstructural characterization (SEM and EBSD) on NF616 control and neutron irradiated samples. * Task 1: Perform SEM fractography on tensile tested tensile NF616 samples: Efforts will be made to section small pieces from seven tested unirradiated and irradiated (dose: 4 dpa; irradiation temperatures: 388C and 452C; two specimens were available per condition; one was tensile tested at room temperature and the other was tested at the irradiation temperature) specimen gage sections using a low-speed saw. SEM fractography will be performed to the determine the mode of failure on the following seven fractographic samples: (a) Three unirradiated; tensile tested at RT, 388C and 452C (b) Four irradiated/4 dpa; tensile tested at RT, 388C and 452C. * Task 2: Preparation of SEM/EBSD specimens from tested NF616 tensile samples and TEM discs: A low-speed saw was earlier utilized to section small samples from the shoulders of tested tensile specimens. These sections along with TEM discs were polished to a 1-micron finish and microhardness testing was performed earlier under a funded RTE project (#2879). If funded, vibratory polishing (colloidal silica) will be performed on the following seven samples for SEM and EBSD studies: (a) one unirradiated (b) six irradiated (3 dpa/450C; 4 dpa/388C; 4 dpa/452C; 4 dpa/469C; 5 dpa/448C; 8 dpa/430C). * Task 3: Perform SEM/EBSD studies on NF616 samples (control and neutron irradiated): SEM/EBSD will be used to evaluate seven samples (control and irradiated) to determine the general grain structure, prior austenite grain size, martensite packet, lath structure and primary carbide structure (inter and intra lath precipitation of carbides). EDS will be utilized to map the carbide distributions and observe local microchemical variations. Image analysis techniques will be employed to evaluate the sizes and fraction of the carbides within the microstructure. SEM-EBSD will be used to provide a quantitative measurement of the prior austenite grain size and high angle grain boundaries. * Task 4: Develop appropriate processing-property-structure-temperature-dose correlations: The microstructural information obtained from the proposed work will aid in understanding the effect of neutron irradiation on NF616 as a function of dose and irradiation temperature on its fracture mode, prior austenite grain size, carbide sizes and volume fraction, martensite lath structure and martensite packet, which are critical for superior mechanical behavior and radiation damage resistance. Microhardness and tensile testing were recently performed under a funded RTE project (#2879). Efforts are currently ongoing under a funded RTE project (#4259) to perform TEM characterization of few NF616 irradiated samples. The results obtained from the proposed RTE project (fractography and microstructural characterization using SEM and EBSD) would be extremely beneficial to get a comprehensive understanding on the effect of neutron irradiation on NF616 and thus enable us to obtain properties-microstructure-temperature-dose correlations that would facilitate their deployment in present and future reactors.
Project Summary
NF616 (Grade 92) is being considered as a candidate structural material for advanced reactors, due to their superior resistance to radiation induced void swelling, microstructural stability, and thermal properties. Based upon elevated temperature creep-rupture strength and impact toughness, HT-9 has significant weakness when compared with NF616. Although elevated-temperature mechanical properties favor NF616, neutron irradiation data is very limited. Hence, to get a comprehensive understanding of the mechanical behavior under neutron irradiation, it is essential to study neutron irradiated samples at various doses and temperatures. The progressive change in the microstructure with irradiation dose and temperature includes void formation, increases in dislocation density, second phase formation, and other changes that can lead to swelling, hardening, and embrittlement. The strongest hardening and embrittlement occur at temperatures below ~425°C in F-M steels due to strong increases in dislocation density and the formation of several different populations of second phases that all act to reduce dislocation mobility. The extreme hardening and low fracture toughness that occur for irradiation temperatures below 425°C is a serious issue for the use of NF616 because many reactor concepts call for core components to see temperatures as low as 320°C. To address the issue of low-temperature neutron irradiation hardening and embrittlement, systematic investigations on the mechanical behavior and microstructure of NF616 are needed over a range of doses and temperatures. As a part of the UW-Madison Irradiation Experiment, NF616 was neutron irradiated in the ATR (387-469C; 3-8 dpa).
Our team recently won a RTE (#2879) award and performed mechanical characterization of irradiated NF616 as a function of doses and temperatures at PNNL to evaluate the degree of low-temperature (~425°C) neutron irradiation hardening. Our team also won a RTE (#4259) award and efforts are currently ongoing at PNNL to perform TEM characterization of few irradiated NF616 samples. Maximum impact of this work will be obtained by performing fractographic (failed tensile specimens) and microstructural characterization (SEM and EBSD) of these neutron irradiated NF616 samples present at PNNL. Hence, the goal of the proposed RTE project is to utilize SEM and EBSD to evaluate the control and irradiated samples and determine the failure mode (fractography), general grain structure, prior austenite grain size, martensite packet, lath structure and primary carbide structure as a function of irradiation doses and temperatures, since these parameters can significantly affect the mechanical behavior and radiation resistance of NF616. The results obtained from the proposed RTE project (fractography and microstructural characterization using SEM and EBSD) would be extremely beneficial to get a complete understanding on the effect of neutron irradiation on NF616 and thus enable us to obtain properties-microstructure-temperature-dose correlations. The results of the proposed work could be extended beyond NF616, and it would be relevant to many F-M steels. Thus, the proposed work will have substantial implications for the deployment of next-generation advanced reactors.
The project performance (sample preparation, imaging, and analysis) is expected to take place during July-September 2022 and will result in one conference presentation and one journal article publication.
Our team recently won a RTE (#2879) award and performed mechanical characterization of irradiated NF616 as a function of doses and temperatures at PNNL to evaluate the degree of low-temperature (~425°C) neutron irradiation hardening. Our team also won a RTE (#4259) award and efforts are currently ongoing at PNNL to perform TEM characterization of few irradiated NF616 samples. Maximum impact of this work will be obtained by performing fractographic (failed tensile specimens) and microstructural characterization (SEM and EBSD) of these neutron irradiated NF616 samples present at PNNL. Hence, the goal of the proposed RTE project is to utilize SEM and EBSD to evaluate the control and irradiated samples and determine the failure mode (fractography), general grain structure, prior austenite grain size, martensite packet, lath structure and primary carbide structure as a function of irradiation doses and temperatures, since these parameters can significantly affect the mechanical behavior and radiation resistance of NF616. The results obtained from the proposed RTE project (fractography and microstructural characterization using SEM and EBSD) would be extremely beneficial to get a complete understanding on the effect of neutron irradiation on NF616 and thus enable us to obtain properties-microstructure-temperature-dose correlations. The results of the proposed work could be extended beyond NF616, and it would be relevant to many F-M steels. Thus, the proposed work will have substantial implications for the deployment of next-generation advanced reactors.
The project performance (sample preparation, imaging, and analysis) is expected to take place during July-September 2022 and will result in one conference presentation and one journal article publication.
Relevance
To satisfy the growing electricity demand, climate change, safety and waste concerns, efforts are ongoing to design advanced nuclear reactors with greater thermal efficiency, flexibility, safety and economics than the current generation. These advanced reactors require high performance materials that can operate under more aggressive service conditions (such as higher operating temperatures, higher radiation doses and corrosive environment). One of the goals of the NE R&D programs is to develop advanced structural materials for the next generation of reactors that have good neutronics, dimensional stability, corrosion resistance, mechanical and thermal properties under irradiation. Based upon elevated temperature creep-rupture strength and impact toughness, HT-9 has significant weakness when compared with NF616. Hence, NF616 is being considered for nuclear applications due to its greater strength that tends to provide greater safety margins, design flexibility and lower cost of reactor components. Although elevated-temperature mechanical properties favor NF616, neutron irradiation data is very limited.
The extreme hardening and low fracture toughness that occur at irradiation temperatures below 425°C is a serious issue for the use of F-M steels because many reactor concepts call for core components to see temperatures as low as 290-330°C. This includes most sodium cooled fast reactor concepts, advanced LWR concepts, and fusion reactor concepts. To address the issue of low-temperature (~425°C) neutron irradiation hardening and embrittlement, it is necessary to conduct systematic investigations on the mechanical behavior and microstructure of NF616 over a wide range of doses and temperatures. As a part of the UW-Madison Irradiation Experiment, NF616 was neutron irradiated in the ATR (387-469C; 3-8 dpa). Our team recently won a RTE award and performed mechanical characterization of irradiated NF616 as a function of doses and temperatures to evaluate the degree of low-temperature (~425°C) neutron irradiation hardening. Our team also won a RTE award and efforts are currently ongoing to perform TEM characterization of few irradiated NF616 samples. Maximum impact of this work will be obtained by performing fractographic (failed tensile specimens) and microstructural characterization (SEM and EBSD) of these neutron irradiated NF616 samples present.
Hence, the goal of the proposed RTE project is to utilize SEM and EBSD to evaluate the control and irradiated samples and determine the failure mode (fractography), general grain structure, prior austenite grain size, martensite packet, lath structure and primary carbide structure as a function of irradiation doses and temperatures, since these parameters can significantly affect the mechanical behavior and radiation resistance of NF616. The results obtained from the proposed RTE project (fractography and microstructural characterization using SEM and EBSD) would be extremely beneficial to get a complete understanding on the effect of neutron irradiation on NF616 and thus enable us to obtain properties-microstructure-temperature-dose correlations. The results of the proposed work could be extended beyond NF616, and it would be relevant to many F-M steels. Thus, the proposed work will have substantial implications for the deployment of next-generation advanced reactors.
The extreme hardening and low fracture toughness that occur at irradiation temperatures below 425°C is a serious issue for the use of F-M steels because many reactor concepts call for core components to see temperatures as low as 290-330°C. This includes most sodium cooled fast reactor concepts, advanced LWR concepts, and fusion reactor concepts. To address the issue of low-temperature (~425°C) neutron irradiation hardening and embrittlement, it is necessary to conduct systematic investigations on the mechanical behavior and microstructure of NF616 over a wide range of doses and temperatures. As a part of the UW-Madison Irradiation Experiment, NF616 was neutron irradiated in the ATR (387-469C; 3-8 dpa). Our team recently won a RTE award and performed mechanical characterization of irradiated NF616 as a function of doses and temperatures to evaluate the degree of low-temperature (~425°C) neutron irradiation hardening. Our team also won a RTE award and efforts are currently ongoing to perform TEM characterization of few irradiated NF616 samples. Maximum impact of this work will be obtained by performing fractographic (failed tensile specimens) and microstructural characterization (SEM and EBSD) of these neutron irradiated NF616 samples present.
Hence, the goal of the proposed RTE project is to utilize SEM and EBSD to evaluate the control and irradiated samples and determine the failure mode (fractography), general grain structure, prior austenite grain size, martensite packet, lath structure and primary carbide structure as a function of irradiation doses and temperatures, since these parameters can significantly affect the mechanical behavior and radiation resistance of NF616. The results obtained from the proposed RTE project (fractography and microstructural characterization using SEM and EBSD) would be extremely beneficial to get a complete understanding on the effect of neutron irradiation on NF616 and thus enable us to obtain properties-microstructure-temperature-dose correlations. The results of the proposed work could be extended beyond NF616, and it would be relevant to many F-M steels. Thus, the proposed work will have substantial implications for the deployment of next-generation advanced reactors.
Research Report
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The Nuclear Science User Facilities (NSUF) is the U.S. Department of Energy Office of Nuclear Energy's only designated nuclear energy user facility. Through peer-reviewed proposal processes, the NSUF provides researchers access to neutron, ion, and gamma irradiations, post-irradiation examination and beamline capabilities at Idaho National Laboratory and a diverse mix of university, national laboratory and industry partner institutions.
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