NSUF

Principal Investigator

Name:: Caleb Schenck
Email:: nsuf@inl.gov
Phone:: (208) 526-6918

Team Members:

Name:	Institution:	Expertise:	Status:
Ayman Hawari	North Carolina State University	Dosimetry, Neutron Beam, Neutron Imaging, Neutron Physics, Neutron Scattering, Neutron Thermalization, Positron Annihilation Spectroscopy, Radiation Damage, Simulation, Spectrometry	Faculty
Kaustubh Bawane	Idaho National Laboratory	Additive Manufacturing, Electron Microscopy, Ion-Irradiation, Materials Processing, Radiation Damage, Sintering, Steels	Other
Bharat Gwalani	North Carolina State University	Corrosion, Microscopy	Faculty

Experiment Details:

Experiment Title:: Investigating ion irradiation at high temperatures on high entropy carbide ceramics using correlative positron annihilation spectroscopy and transmission electron microscopy)
Hypothesis:: Previous ion irradiation studies have shown very high resistance of HECs to form voids, it is hypothesized that the chemically complex energy landscape of the HEC lattice impacts the formation and migration of vacancies and interstitials. Furthermore, the complex energy landscape in the lattice acts as a strong barrier to the formation of vacancy clusters and void swelling.
Work Description:: The high entropy carbide (HEC) samples with target compositions: [(Cr0.2Hf0.2Ta0.2Ti0.2Zr0.2)C - HEC-Cr] and [(Hf0.2Ta0.2Ti0.2W0.2Zr0.2)C - HEC-W] were synthesized by collaborators at Missouri University of Science and Technology using a highly optimized carbothermal reduction method, followed by grinding, sieving, and an optimized direct current sintering. The HEC samples located at NC State University have been sectioned, polished, and transmission electron microscopy (TEM) lamella lifted out via the focused ion beam (FIB) technique using a Helios Hydra G5 dual beam plasma FIB. FIB damage was avoided by a final 2 keV cleaning in Ar. Extensive TEM imaging of the samples was carried out using both a ThermoFisher Talos F200X G2 and an aberration-corrected ThermoFisher Titan 80-300. This analysis consisted of scanning TEM (STEM) with energy dispersive x-ray spectroscopy (EDS) to investigate the compositional homogeneity of the samples and the grain boundary character, as well as to act as a reference. Commercial Inconel 718 (IN718) reference samples are to be sectioned, polished, TEM lamella created, and TEM analysis will follow the method described above. Positron annihilation lifetime spectroscopy experiments prior to irradiation of the polished HEC and IN718 samples will be carried out at NC State's PULSTAR facility using the sample-source-sample geometry at room temperature where a Na22 source is placed between two identical samples and evaporated. The PALS system records detector events with a high temporal resolution to deconvolute the bulk annihilation rate and the lifetime of material defects. These measurements will act as the reference for the pre-irradiation material defect density. The HEC and IN718 samples will then be packaged and clearly labeled as HEC-W, HEC-Cr, and IN718. This package will be sent to the Accelerator Lab at Texas A&M or TAMU. The 3 MV NEC Tandem accelerator equipped with a hot stage located in TAMU will be used to irradiate the samples using 5 MeV Zr+2 ions at three different temperatures of 500, 800, and 1,000 °C. The ion flux used will be 5.94e11 ions/cm2-s to a total fluence of 5.94e16ions/cm2. Using these parameters in SRIM simulations resulted in implantation and damage profiles showing a damage rate of 1e-3 dpa/s for a peak damage of 100 dpa after 27.8 hours. Since the samples are roughly 5x5 mm, the HEC-Cr, HEC-W, and IN718 can all be irradiated at the same time at the same temperature, thus reducing the time required on the beam. After the irradiation is completed at each temperature, the samples will be sent back to NC State for TEM and PALS experiments. TEM lamella will again be lifted out via the FIB technique described above for TEM investigation on each of the samples. TEM investigation will include characterization of ion irradiation defects such as dislocation loops - loop size, loop density, loop character (perfect vs. faulted Frank), voids, and radiation-induced chemical segregation. PALS experiments, using the method previously described, will measure the change after ion irradiation in defect density. These defects are in the range of single vacancies to ~30 nm, and thus have the potential to give unprecedented insight into the role point defects play in ion irradiation of HECs.

Project Summary

High entropy carbides (HECs) are an emerging class of ultra-high temperature ceramics with high hardness, increased oxidation resistance, complex thermal behavior, and robust irradiation tolerance. This project seeks to reveal the high dose and high temperature ion irradiation behavior of HECs concerning phase stability, vacancy formation/clustering, and void swelling using 5 MeV Zr ions. This will be investigated using correlative positron annihilation lifetime spectroscopy (PALS) and transmission electron microscopy (TEM). It is hypothesized that the chemically complex energy landscape of the HEC lattice impacts the formation and migration of vacancies and interstitials in the material. Previous irradiation studies of HECs primarily observed lattice expansion and dislocation loop formation. By irradiating the material to higher damage levels (100 dpa) and at higher temperatures (500-1,000 °C) than previously reported, the resistance to void swelling and irradiation-induced vacancy formation can be studied under advanced reactor-relevant conditions (GFR, VHTR). These conditions have yet to be reported and have the potential to significantly further the understanding of these materials. PALS is a powerful technique that is unique in its ability to measure defects from single vacancies to nanovoids that has yet to be used on HECs. PALS combined with TEM can be used to reveal fundamental details about the development of ion irradiation induced defects in HECs as a function of temperature. The outcome of this project is a significant increase in the knowledge of ion irradiation induced voids, along with the first reported irradiation experiments done above ~20 dpa and 800 °C in HECs.

Relevance

The proposed research aligns with the Department of Energy Office of Nuclear Energy's strategic goal to enable the deployment of innovative nuclear energy systems (Goal 2, Objective 2). Specifically, this project supports the development of advanced reactor designs that can operate at very high temperatures (750°C and above). These high temperature reactors, such as gas-cooled fast reactors (GFRs) and very high temperature gas reactors (VHTRs), have the potential to dramatically improve power generation efficiency and provide process heat for industrial applications. The proposed research will evaluate the irradiation behavior of high entropy carbide ceramics, an emerging class of materials that shows promise for use in the highest temperature sections of advanced reactors. By irradiating these materials at higher temperatures and damage levels than previously studied, this project will generate critical data to determine if high entropy ceramics can withstand the demanding environment within advanced reactors. Filling these knowledge gaps will significantly advance the understanding of these materials and their potential to enable the development of advanced nuclear energy systems aligned with DOE-NE goals. In summary, by evaluating high entropy ceramics for advanced reactor applications, this project directly supports DOE-NE's mission to develop innovative nuclear technologies, including reactors capable of very high temperature operation that can expand energy markets. The knowledge generated will provide crucial insights into materials performance to inform the development of advanced reactor concepts for next generation nuclear energy systems.

Investigating ion irradiation at high temperatures on high entropy carbide ceramics using correlative positron annihilation spectroscopy and transmission electron microscopy