Prabhakaran, Ramprashad. Mechanical characterization of neutron irradiated NF616 (T92) as a function of doses and temperatures

Principal Investigator
First Name:
Ramprashad
Last Name:
Prabhakaran
Institution:
Pacific Northwest National Laboratory
Title:
Scientist
Team Members:
Name: Institution: Expertise: Status:
Mychailo Toloczko Pacific Northwest National Laboratory Mechanical Testing; Microstructural Characterization; Radiation Effects Other
Kumar Sridharan University of Wisconsin nuclear materials, radiation effects Faculty
Experiment Details:
Experiment Title:
Mechanical characterization of neutron irradiated NF616 (T92) as a function of doses and temperatures
Describe the work that you are proposing in detail. Please include as many specifics as possible (e.g., dose, dose rate, ion energy, types of ions, beam line x-ray energy, irradiation temperature, analysis temperature, atmosphere, etc.):
* No irradiation task is included in the scope of this RTE project, since the neutron irradiation on NF616 tensile specimens was already performed at the ATR through the UW-Madison NSUF IE led by Dr. Kumar Sridharan (co-PI of this RTE proposal). * PIE: NF616 tensile specimens were neutron irradiated in the ATR at ~4 dpa at two temperatures (387 and 451°C). TEM specimens were neutron irradiated at temperatures (429, 450 and 469°C) at 3-8 dpa. Two tensile and four TEM discs are available per irradiation condition, and the proposed work would utilize only one tensile and one TEM specimen (for hardness testing) per condition. The proposed work at PNNL is to perform mechanical testing (hardness and tensile testing) on 2 control tensile specimens (present in PI’s location) and 6 irradiated specimens (present in the NSUF library; tensile and/or hardness) and follow this with fractographic exams on the tensile specimens. The fractography will provide additional information on the ductility response. PNNL is a partner NSUF facility that has well established capabilities for performing microhardness and elevated-temperature tensile testing. * Task 1: Perform tensile testing at actual irradiation temperatures and microscopy: Tensile properties are sensitive to the irradiation temperature. Four tensile specimens (2 control and 2 neutron irradiated: 387 and 451°C at ~4 dpa) will be tested using a shoulder-loaded fixture at irradiation temperature to evaluate the mechanical properties (yield strength, ductility, work hardening) and to understand the effects of radiation damage on NF616. Fractography and image analysis (for measuring ductility parameters) will be performed to further characterize the deformation response of materials. * Task 2: Perform microhardness testing (room temperature): Radiation hardening varies with the irradiation temperature and dose. Hence, Vickers microhardness testing could serve as an effective method to characterize the basic mechanical properties (hardness & estimated yield strength) of neutron-irradiated NF616. A small section (away from the gage) from the tensile shoulder will be obtained from the tested specimen. This small section (from tested tensile specimen) and TEM discs would be prepared to the necessary surface finish to perform microhardness testing. Microhardness testing would be performed on seven specimens (1 control and 6 neutron irradiated: 387, 429, 450 and 469°C at 3-8 dpa) * Task 3: Develop appropriate temperature-dose correlations: Tensile testing (room temperature) of control/unirradiated NF616 will be performed using existing project funds. The mechanical test data (microhardness at room temperature and tensile testing at ambient and actual irradiation temperatures) from the proposed work would be analyzed to understand the effects of radiation damage on NF616, and to develop appropriate property-temperature-dose correlations. If the microstructural information (by using FIB to prepare TEM specimens from tested tensile specimens’ shoulder and TEM discs) can be obtained in a follow-up RTE, it will be used to identify the source(s) of hardening and also to perform barrier hardening coefficient determinations. 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.
Technical Abstract
NF616 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. 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. Limited number of proton and neutron irradiated studies on NF616 have been recently reported. Hence, in order to get a comprehensive understanding of the mechanical behavior of NF616 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 occurs 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 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 (387-469°C; 3-8 dpa). For irradiated F-M steels, the increase in yield strength is quite steep up to around 10 dpa. Extrapolation of the information learned from the 8 dpa irradiations is best accomplished by also examining lower dose samples because having data at two doses results in a more accurate extrapolation to higher doses. The proposed project aims to perform mechanical characterization on control (unirradiated) and neutron irradiated NF616 specimens as a function of irradiation temperatures and doses. The mechanical test data would be analyzed to understand the effects of radiation damage on NF616, and to develop appropriate temperature-dose correlations. By doing this work, our team can contribute to filling the gap in the literature on understanding irradiation effects on NF616 and F-M steels in general.
Awards / Invited Presentations
Name Date Location Description
2017 TMS Structural Materials Division Young Professional Best Poster Award 3/1/2017 San Diego, CA Poster (related to NSUF work) Title: “Effect of Neutron Irradiation on Friction Stir Processed ODS Alloys (MA956 & MA754)”
Invited Lecture 2/16/2017 Corvallis, OR Materials Science and Engineering department seminar
FY18 NSUF-2 Award; CFA-18-14787; PI 8/16/2018 NSUF High-dose Ion Irradiation Testing and Relevant Post-irradiation Examination of Friction Stir Welded ODS MA956 Alloy
FY17 RTE #880; PI 4/26/2017 NSUF Mechanical characterization of neutron irradiated FSW ODS alloys
FY18 RTE #1156; PI 2/1/2018 NSUF Mechanical characterization of three heats (ORNL, LANL and EBR II) of HT-9 after side-by-side neutron irradiation at LWR and fast reactor relevant temperatures
FY19 RTE# 1687; PI 2/14/2019 NSUF Mechanical characterization of three lower dose HT-9 heats (ORNL, LANL and EBR II) after side-by-side neutron irradiation at LWR and fast reactor relevant temperatures