Paula Freyer

Profile Information
Paula Freyer
Fellow Engineer
NSUF Partnering Lab
"Application of backscatter electrons for large area imaging of cavities produced by neutron irradiation" V.I. Pastukhov, S.A. Averin, V.L. Panchenko, I.A. Portnykh, Paula Freyer, L.A. Giannuzzi, Frank Garner, Journal of Nuclear Materials Vol. 480 2016 289-300 Link
It is shown that with proper optimization, backscattered electrons in a scanning electron microscope can produce images of cavity distribution in austenitic steels over a large specimen surface for a depth of ∼500?700 nm, eliminating the need for electropolishing or multiple specimen production. This technique is especially useful for quantifying cavity structures when the specimen is known or suspected to contain very heterogeneous distributions of cavities. Examples are shown for cold-worked EK-164, a very heterogeneously-swelling Russian fast reactor fuel cladding steel and also for AISI 304, a homogeneously-swelling Western steel used for major structural components of light water cooled reactors. This non-destructive overview method of quantifying cavity distribution can be used to direct the location and number of required focused ion beam prepared transmission electron microscopy specimens for examination of either neutron or ion-irradiated specimens. This technique can also be applied in stereo mode to quantify the depth dependence of cavity distributions.
"Development of a nondestructive inspection method for irradiation-induced microstructural evolution of thick 304 stainless steel blocks" J. Etoh, M. Sagisaka, T. Matsunaga, I. Isobe, Frank Garner, Paula Freyer, J.M.K. Wiezorek, T. Okita, Journal of Nuclear Materials Vol. 440 2013 500-507 Link
Ultrasonic testing was conducted on two long, Type 304 stainless steel blocks with a hexagonal cross-section that were removed from the reflector region of the decommissioned EBR-II reactor. One block had a dose range of 17?33 displacements per atom (dpa) and based on dimensional measurements exhibited a maximum of ∼2% average density decrease across its thickness. The second block had a dose range of ∼0.3?4 dpa, and exhibited smaller but positive range of density changes. Comparison of the ultrasonic measurements and the spatial variations in density change, as well as local swelling arising from voids and precipitates as determined by electron microscopy illustrate excellent agreement. Furthermore, this study clearly revealed that radiation-induced microstructural features produce measurable changes in elastic modulus and ultrasonic velocity. These results clearly demonstrate that ultrasonic techniques can be used to nondestructively measure the average swelling across a thick component.
"Effect of laser welding on deformation mechanisms in irradiated austenitic stainless steel" Janelle Wharry, Keyou Mao, Cheng Sun, Xiang Liu, Haozheng Qu, Aaron French, Paula Freyer, Frank Garner, Lin Shao, Journal of Nuclear Materials Vol. 528 2020 151878 Link
Deformation mechanism of a laser weld on neutron irradiated AISI 304L stainless steel was studied by in-situ microcompression test at room temperature. The deformation-induced austenite-to-martensite phase transformation occurs in {101}-oriented grains in the irradiated base metal, while deformation twinning prevails in {101}-oriented grains in the weld heat affected zone (HAZ). A high number density of irradiation-induced voids in the base metal provides sufficient nucleation sites for the austenite-to-martensite phase transformation under compression at room temperature. A deformation map is established to predict critical twinning stress for face-centered-cubic (fcc) metals and alloys. Our results show that irradiation-induced voids can tailor the deformation mechanisms of austenitic stainless steel.
"Grain orientation dependence of nanoindentation and deformation-induced martensitic phase transformation in neutron irradiated AISI 304L stainless steel" Keyou Mao, Cheng Sun, Yina Huang, Ching-Heng Shiau, Frank Garner, Paula Freyer, Janelle Wharry, Materialia Vol. 5 2019 100208 Link
"Mechanical behavior of AISI 304SS determined by miniature test methods after neutron irradiation to 28 dpa" Ellen Rabenberg, Brian Jaques, Bulent Sencer, Frank Garner, Paula Freyer, T. Okita, Darryl Butt, Journal of Nuclear Materials Vol. 448 2014 315-324 Link
The mechanical properties of AISI 304 stainless steel irradiated for over a decade in the Experimental Breeder Reactor (EBR-II) were measured using miniature mechanical testing methods. The shear punch method was used to evaluate the shear strengths of the neutron-irradiated steel and a correlation factor was empirically determined to predict its tensile strength. The strength of the stainless steel slightly decreased with increasing irradiation temperature, and significantly increased with increasing dose until it saturated above approximately 5 dpa. An effective tensile strain hardening exponent was also obtained from the data which shows a relative decrease in ductility of steel with increased irradiation damage. Ferromagnetic measurements were used to observe and deduce the effects of the stress-induced austenite to martensite transformation as a result of shear punch testing.
"Microstructural characterization and density change of 304 stainless steel reflector blocks after long-term irradiation in EBR-II" Yina Huang, J.M.K. Wiezorek, Frank Garner, Paula Freyer, T. Okita, M. Sagisaka, Y. Isobe, Todd Allen, Journal of Nuclear Materials Vol. 465 2015 516-530 Link
While thin reactor structural components such as cladding and ducts do not experience significant gradients in dpa rate, gamma heating rate, temperature or stress, thick components can develop strong local variations in void swelling and irradiation creep in response to gradients in these variables. In this study we conducted microstructural investigations by transmission electron microscopy of two 52 mm thick 304-type stainless steel hex-blocks irradiated for 12 years in the EBR-II reactor with accumulated doses ranging from ∼0.4 to 33 dpa. Spatial variations in the populations of voids, precipitates, Frank loops and dislocation lines have been determined for 304 stainless steel sections exposed to different temperatures, different dpa levels and at different dpa rates, demonstrating the existence of spatial gradients in the resulting void swelling. The microstructural measurements compare very well with complementary density change measurements regarding void swelling gradients in the 304 stainless steel hex-block components. The TEM studies revealed that the original cold-worked-state microstructure of the unirradiated blocks was completely erased by irradiation, replaced by high densities of interstitial Frank loops, voids and carbide precipitates at both the lowest and highest doses. At large dose levels the amount of volumetric void swelling correlated directly with the gamma heating gradient-related temperature increase (e.g. for 28 dpa, ∼2% swelling at 418 °C and ∼2.9% swelling at 448 °C). Under approximately iso-thermal local conditions, volumetric void swelling was found to increase with dose level (e.g. ∼0.2% swelling at 0.4 dpa, ∼0.5% swelling at 4 dpa and ∼2% swelling at 28 dpa). Carbide precipitate formation levels were found to be relatively independent of both dpa level and temperature and induced a measurable densification. Void swelling was dominant at the higher dose levels and caused measurable decreases in density. Void swelling at the lowest doses was larger than might be expected based on the dpa level, an observation in agreement with earlier studies showing that the onset of void swelling is accelerated by decreasing dpa rates.
"Microstructure analysis of laser beam weldments performed on neutron-irradiated 304L steel containing 3 and 8 appm helium" Maxim Gussev, Weicheng Zhong, Frank Garner, Paula Freyer, Jonathan Tatman, Jesse Werden, Journal of Nuclear Materials Vol. 563 2022 Link
"Microstructure and microchemistry of laser welds of irradiated austenitic steels" Keyou Mao, Aaron French, Xiang Liu, Lucille Giannuzzi, Cheng Sun, Megha Dubey, Paula Freyer, Jonathan Tatman, Frank Garner, Lin Shao, Janelle Wharry, Materials and Design Vol. 206 2021 Link
"Role of cavities on deformation-induced martensitic transformation pathways in a laser-welded, neutron irradiated austenitic stainless steel" Janelle Wharry, Keyou Mao, Cheng Sun, Ching-Heng Shiau, Kayla Yano, Paula Freyer, Anter EL-AZAB, Frank Garner, Aaron French, Lin Shao, Scripta Materialia Vol. 178 2020 1-6 Link
The role of cavities on deformation-induced martensitic phase transformations is studied in a laser-welded and neutron irradiated austenitic stainless steel. Orientation dependent nanoindentation experiments are performed in the base metal and the weld heat affected zone (HAZ) at room temperature. Transmission electron microscopy study of deformed microstructures indicates indentation-induced α’-martensite forms in the base metal, whereas α’- and ε-martensite arise in the HAZ. The different pathway of martensite phase transformation is attributed to the laser weld-induced annealing of cavities. Our results suggest that deformation-induced martensitic phase transformation of austenitic stainless steel is correlated to neutron irradiated cavity structures.
"Using UT to assess neutron-induced damage" Y. Isobe, J. Etoh, M. Sagisaka, T. Matsunaga, Paula Freyer, Frank Garner, Nuclear Engineering International Vol. 2014 Link
It has recently been shown that the behavior of ultrasonic waves in metals changes significantly in response to microstructural components formed during neutron irradiation, especially when the microstructure contains a high density of vacancy voids. We have developed a predictive model of ultrasonic wave behaviour caused by such changes in neutron-irradiated 304 stainless steel. Calculations performed for unirradiated archive and irradiated metal yielded results reasonably consistent with experimental data
"Validation of ultrasonic velocity measurements in first wall structural materials" Frank Garner, T. Okita, J. Etoh, M. Sagisaka, T. Matsunaga, Y. Isobe, Paula Freyer, J.M.K. Wiezorek, Fusion Science and Technology Vol. 66 2014 Link
Time-of-flight ultrasonic measurements were conducted on a thick hexagonal block of 304 stainless steel irradiated to ∽33 dpa in EBR-II, and the results of ultrasonic-implied void swelling and carbide-induced densification were compared with those obtained by immersion density measurements and TEM observation. The three types of measurement were found to agree rather well with each other. This study confirmed that ultrasonic velocity measurement is a powerful non-destructive technique to measure the through-thickness-average volumetric changes induced by neutrons in thick structural materials.
"Hot Cell Pulsed Laser Welding of Neutron Irradiated Type 304 Stainless Steel With a Maximum Damage Dose of 28 DPA" Paula Freyer, Jonathan Tatman, Frank Garner, Benjamin Sutton, Pressure Vessels and Piping Conference July 14-19, (2019) Link
"Measurement of depth-dependent swelling in thick non-uniform irradiated 304 stainless steel blocks using nondestructive ultrasonic techniques" Frank Garner, Paula Freyer, T. Okita, Y. Isobe, J. Etoh, M. Sagisaka, T. Matsunaga, Yina Huang, J.M.K. Wiezorek, Douglas Porter, Fontevraud 8 - Contributions of Materials Investigations and Operating Experience to LWR's Safety, Performance and Reliability September 3-6, (2014) Link
"Microstructural and mechanical integrity of laser weldment of neutron irradiated AISI 304 SS" Keyou Mao, Paula Freyer, Frank Garner, Janelle Wharry, TMS 2018 March 11-15, (2018)
"Microstructural and micromechanical characterization of laser weld repairs on neutron irradiated 304 stainless steel" Keyou Mao, Paula Freyer, Cheng Sun, Frank Garner, Janelle Wharry, NUMAT 2018 October 15-18, (2018)
"Microstructure evolution of laser weld repairs of 304SS at high dose ion irradiation" Keyou Mao, Paula Freyer, Jonathan Tatman, Frank Gift, Frank Garner, Janelle Wharry, Poster - FONTEVRAUD 9 September 17-20, (2018)
"Transmission Electron Microscopy of 304-type Stainless Steel after Exposure to Neutron Flux and Irradiation Temperature Gradients" J.M.K. Wiezorek, Yina Huang, Frank Garner, Paula Freyer, M. Sagisaka, T. Isobe, Microscopy and Microanalysis 2014 August 3-7, (2014) Link
"Void swelling and resultant strains in thick 304 stainless steel components in response to spatial gradients in neutron flux-spectra and irradiation temperature" Frank Garner, Paula Freyer, Douglas Porter, Yina Huang, Proceedings of 16th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors [unknown] Link