Yongho Sohn

Profile Information

Name

Dr. Yongho Sohn

Institution

University of Central Florida

Position

Professor

h-Index

ORCID

0000-0003-3723-4743

Biography

Dr. Yongho Sohn is a Pegasus Professor and Lockheed Martin Professor of Engineering in the Department of Materials Science and Engineering, and Associate Director for Materials Characterization Facility (MCF) at University of Central Florida. MCF is a FL-state user facility for academics and industry with over $20M in analytical instrumentation and 3 full-time staff engineers.

He received his B.S. with honors and M.S. from Worcester Polytechnic Institute, Worcester, MA in mechanical and materials engineering, respectively. He graduated in 1999 with a Ph.D. in materials science and engineering from Purdue University and spent two years as a post-doctoral research scholar at the University of Connecticut. He joined University of Central Florida in 2001 as an assistant professor.

His research and teaching interests includes microstructural analysis and control, multicomponent intrinsic and interdiffusion in multiphase alloys, powder processing and additive manufacturing, thermal barrier coatings and other protective metallic/ceramic coatings, and light-weight metallic alloys and metal-matrix composites.

He has published 8 book chapters, over 175 journal papers and 60 proceedings papers. He gave over 525 presentations including 115 invited lectures at conferences around the globe. He is a Fellow of ASM International (FASM), recipient of NSF CAREER Award (2003), Outstanding Materials Engineer Award from Purdue University (2016), UCF’s 2017, 2012 and 2006 research incentive awards, UCF’s 2007 and 2013 teaching incentive award.

He is an associate editor for Journal of Phase Equilibria and Diffusion and a member of editorial board for Metallurgical and Materials Transactions. He has supervised to completion, 15 Ph.D. students, 30 M.S. students and 9 post-doctoral scholars, and currently supervises 2 post-docs, 4 Ph.D. students, 4 M.S. students, and 3 undergraduate research assistants.

Details on his research and teaching activities can be found at http://mse.ucf.edu/sohn.

Publications:

		"Characterization of Interaction Layer in U-Mo-X (X = Nb, Zr) and U-Nb-Zr vs. Al Diffusion Couples Annealed at 600 degrees C for 10 Hours" Emmanuel Perez, Ashley Paz y Puente, Dennis Keiser, Yongho Sohn, Defects and Diffusion Forum Vol. 312-315 2011 1055-1062 Link U-Mo has thus far proven to be one of the most feasible metallic fuel alloys for use in research and test reactors due to its high density and stability during irradiation. However, an adverse diffusional interaction can occur between the fuel alloy and the Al based matrix. This forms an interaction layer (IL) that has undesirable thermal properties and irradiation behavior leading to accelerated swelling and reduced fuel efficiency. This study focused on the effects of ternary alloying additions on the formation of IL between U based alloys and Al. Diffusion couples of U-8Mo-3Nb, U-7Mo-6Zr, and U-10Nb-4Zr (wt.%) vs. pure Al were assembled and annealed at 600 Degrees C for 10 hours. Both thickness and phase constituent analyses were performed via electron microscopy. The major phase constituent of the IL was determined to be the UAl3 intermetallic compound. The Nb and Zr alloying additions did not reduce growth rate of IL (1.3-1.4 m/sec1/2) as compared to couples made between binary U-Mo and Al (0.9-1.8 m/sec1/2).
		"Diffusion Barrier Selection from Refractory Metals (Zr, Mo and Nb) via Interdiffusion Investigation for U-Mo RERTR Fuel Alloy" Ke Huang, Dennis Keiser, Catherine Kammerer, Yongho Sohn, Journal of Phase Equilibria and Diffusion Vol. 35 2014 146-156 Link U-Mo alloys are being developed as low enrichment monolithic fuel under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Diffusional interactions between the U-Mo fuel alloy and Al-alloy cladding within the monolithic fuel plate construct necessitate incorporation of a barrier layer. Fundamentally, a diffusion barrier candidate must have good thermal conductivity, high melting point, minimal metallurgical interaction, and good irradiation performance. Refractory metals, Zr, Mo, and Nb are considered based on their physical properties, and the diffusion behavior must be carefully examined first with U-Mo fuel alloy. Solid-to-solid U-10 wt.%Mo versus Mo, Zr, or Nb diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for various times. The interdiffusion microstructures and chemical composition were examined via scanning electron microscopy and electron probe microanalysis, respectively. For all three systems, the growth rate of interdiffusion zone were calculated at 1000, 900 and 800 °C under the assumption of parabolic growth, and calculated for lower temperature of 700, 600 and 500 °C according to Arrhenius relationship. The growth rate was determined to be about 103 times slower for Zr, 105 times slower for Mo and 106 times slower for Nb, than the growth rates reported for the interaction between the U-Mo fuel alloy and pure Al or Al-Si cladding alloys. Zr, however was selected as the barrier metal due to a concern for thermo-mechanical behavior of UMo/Nb interface observed from diffusion couples, and for ductile-to-brittle transition of Mo near room temperature.
		"Diffusion under temperature gradient: A phase-field model study" Rashmi Mohanty, Jonathan Guyer, Yongho Sohn, Journal of Applied Physics Vol. 106 2009 Link A diffuse interface model was devised and employed to investigate the effect of thermotransport (a.k.a., thermomigration) process in single-phase and multi-phase alloys of a binary system. Simulation results show that an applied temperature gradient can cause significant redistribution of constituent elements and phases in the alloy. The magnitude and the direction of the redistribution depend on the initial composition, the atomic mobility and the heat of transport of the respective elements. In multi-phase alloys, the thermomigration effect can cause the formation of single-element rich phases at the cold and hot ends of the alloy (i.e., demixing).
		"Diffusional Interaction between U-10wt.%Zr and Fe at 903 K, 923 K, and 953 K (630 C, 650 C, and 680 C)" Keqin Huang, Young Joo Park, Ashley Paz y Puente, H. S. Lee, Bulent Sencer, Rory Kennedy, Yongho Sohn, Metallurgical and Materials Transactions A Vol. 46 2014 Link U-Zr metallic fuels cladded in Fe-alloys are being considered for application in an advanced sodium-cooled fast reactor that can recycle the U-Zr fuels and minimize the long-lived actinide waste. To understand the complex fuel-cladding chemical interaction between the U-Zr metallic fuels with Fe-alloys, a systematic multicomponent diffusion study was carried out using solid-to-solid diffusion couples. The U-10 wt pct Zr vs pure Fe diffusion couples were assembled and annealed at temperatures, 903 K, 923 K, and 953 K (630 °C, 650 °C, and 680 °C) for 96 hours. Development of microstructure, phase constituents, and compositions developed during the thermal anneals were examined by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. Complex microstructure consisting of several layers that include phases such as U6Fe, UFe2, ZrFe2, a-U, ß-U, Zr-precipitates, ?, ?, and ? were observed. Multi-phase layers were grouped based on phase constituents and microstructure, and the layer thicknesses were measured to calculate the growth constant and activation energy. The local average compositions through the interaction layer were systematically determined, and employed to construct semi-quantitative diffusion paths on isothermal U-Zr-Fe ternary phase diagrams at respective temperatures. The diffusion paths were examined to qualitatively estimate the diffusional behavior of individual components and their interactions. Furthermore, selected area electron diffraction analyses were carried out to determine, for the first time, the exact crystal structure and composition of ?, ?, and ?-phases. The ?, ?, and ?-phases were identified as Pnma(62) Fe(Zr,U), I4/mcm(140) Fe(Zr,U)2, and P42/mnm(136) U3(Zr,Fe), respectively.
		"Effects of Cr and Ni on Interdiffusion and Reaction between U and Fe-Cr-Ni Alloys" Ke Huang, Young Joo Park, Le Zhou, Yongho Sohn, Kevin Coffey, Bulent Sencer, Rory Kennedy, Journal of Nuclear Materials Vol. 451 2014 372-378 Link Metallic U-alloy fuel cladded in steel has been examined for high temperature fast reactor technology wherein the fuel cladding chemical interaction is a challenge that requires a fundamental and quantitative understanding. In order to study the fundamental diffusional interactions between U with Fe and the alloying effect of Cr and Ni, solid-to-solid diffusion couples were assembled between pure U and Fe, Fe–15 wt.%Cr or Fe–15 wt.%Cr–15 wt.%Ni alloy, and annealed at high temperature ranging from 580 to 700 °C. The microstructures and concentration profiles that developed from the diffusion anneal were examined by scanning electron microscopy, and X-ray energy dispersive spectroscopy (XEDS), respectively. Thick U6Fe and thin UFe2 phases were observed to develop with solubilities: up to 2.5 at.% Ni in U6(Fe,Ni), up to 20 at.%Cr in U(Fe, Cr)2, and up to 7 at.%Cr and 14 at.% Ni in U(Fe, Cr, Ni)2. The interdiffusion and reactions in the U vs. Fe and U vs. Fe–Cr–Ni exhibited a similar temperature dependence, while the U vs. Fe–Cr diffusion couples, without the presence of Ni, yielded greater activation energy for the growth of intermetallic phases – lower growth rate at lower temperature but higher growth rate at higher temperature.
		"Fuel-Matrix Chemical Interaction Between U-7wt.%Mo Alloy and Mg" Ke Huang, Dennis Keiser, Yongho Sohn, H. Heinrich, Defects and Diffusion Forum Vol. 333 2013 199-206 Link A solid-to-solid, U-7wt.%Mo vs. Mg diffusion couple was assembled and annealed at 550°C for 96 hours. Themicrostructurein the interdiffusion zone and the development of concentration profiles were examined via scanning electron microscopy, transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy. A TEM specimen was prepared at the interface between U-7wt.%Mo andMgusing focused ion beam in-situ lift-out. The U-7wt.%Mo alloy was bonded well tothe Mg at the atomic scale, without any evidence of oxidation, cracks or pores.Despite the good bonding, very little or negligible interdiffusion was observed.This is consistent with the expectation based on negligible solubilities according to the equilibrium phase diagrams. Along with other desirableproperties, Mgis a potential inert matrix or barrier materialfor U-Mo fuel alloy systembeing developed forthe Reduced Enrichment for Research and Test Reactor (RERTR) program.
		"Growth Kinetics and Microstructural Evolution during Hot Isostatic Pressing of U-10wt.%Mo Monolithic Fuel Plate in AA6061 Cladding with Zr Diffusion Barrier" Young Joo Park, Ke Huang, Dennis Keiser, Jan-Fong Jue, Barry Rabin, G. Moore, Yongho Sohn, Journal of Nuclear Materials Vol. 447 2014 215-224 Link Phase constituents and microstructure changes in RERTR fuel plate assemblies as functions of temperature and duration of hot-isostatic pressing (HIP) during fabrication were examined. The HIP process was carried out as functions of temperature (520, 540, 560 and 580 °C for 90 min) and time (45–345 min at 560 °C) to bond 6061 Al-alloy to the Zr diffusion barrier that had been co-rolled with U-10 wt.% Mo (U10Mo) fuel monolith prior to the HIP process. Scanning and transmission electron microscopies were employed to examine the phase constituents, microstructure and layer thickness of interaction products from interdiffusion. At the interface between the U10Mo and Zr, following the co-rolling, the UZr2 phase was observed to develop adjacent to Zr, and the α-U phase was found between the UZr2 and U10Mo, while the Mo2Zr was found as precipitates mostly within the α-U phase. The phase constituents and thickness of the interaction layer at the U10Mo-Zr interface remained unchanged regardless of HIP processing variation. Observable growth due to HIP was only observed for the (Al,Si)3Zr phase found at the Zr/AA6061 interface, however, with a large activation energy of 457 ± 28 kJ/mole. Thus, HIP can be carried to improve the adhesion quality of fuel plate without concern for the excessive growth of the interaction layer, particularly at the U10Mo-Zr interface with the α-U, Mo2Zr, and UZr2 phases.
		"Interdiffusion and Reaction Between Uranium and Iron" Ke Huang, Young Joo Park, Ashley Ewh, Bulent Sencer, Rory Kennedy, Kevin Coffey, Yongho Sohn, Journal of Nuclear Materials Vol. 424 2012 82-88 Link Metallic uranium alloy fuels cladded in stainless steel are being examined for fast reactors that operate at high temperature. In this work, solid-to-solid diffusion couples were assembled between pure U and Fe, and annealed at 853 K, 888 K and 923 K where U exists as orthorhombic α, and at 953 K and 973 K where U exists as tetragonal β. The microstructures and concentration profiles developed during annealing were examined by scanning electron microscopy and electron probe microanalysis, respectively. U6Fe and UFe2 intermetallics developed in all diffusion couples, and U6Fe was observed to grow faster than UFe2. The interdiffusion fluxes of U and Fe were calculated to determine the integrated interdiffusion coefficients in U6Fe and UFe2. The extrinsic (KI) and intrinsic growth constants (KII) of U6Fe and UFe2 were also calculated according to Wagner’s formalism. The difference between KI and KII of UFe2 indicate that its growth was impeded by the fast-growing U6Fe phase. However, the thin UFe2 played only a small role on the growth of U6Fe as its KI and KII values were determined to be similar. The allotropic transformation of uranium (orthorhombic α to tetragonal β phase) was observed to influence the growth of U6Fe directly, because the growth rate of U6Fe changed based on variation of activation energy. The change in chemical potential and crystal structure of U due to the allotropic transformation affected the interdiffusion between U and U6Fe. Faster growth of U6Fe is also examined with respect to various factors including crystal structure, phase diagram, and diffusion.
		"Interdiffusion between Potential Diffusion Barrier Mo and U-Mo Metallic Fuel Alloy for RERTR Applications" Ke Huang, Young Joo Park, Dennis Keiser, Yongho Sohn, Journal of Phase Equilibria and Diffusion Vol. 34 2013 307-312 Link U-Mo alloys are being developed as low enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor Program. Previous investigation has shown that the interdiffusion between U and Mo in γ(bcc)-U solid solution is very slow. This investigation explored interdiffusional behavior, especially in regions with high Mo concentration, and the potential application of Mo as a barrier material to reduce the interaction between U-Mo fuel and Al alloys matrix. Solid-to-solid U-10wt.%Mo versus Mo diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for 960, 720, 480, 240, 96 h, respectively. The interdiffusion microstructures and concentration profiles were examined via scanning electron microscopy and electron probe microanalysis, respectively. As the Mo concentration increased from 22 to 32 at.%, the interdiffusion coefficient decreased while the activation energy increased. The growth rate constant of the interdiffusion zone between U-10wt.%Mo versus Mo was also determined and compared to be 104-105 times lower than those of U-10wt.%Mo versus Al and U-10wt.%Mo versus Al-Si systems. Other desirable physical properties of Mo as a barrier material, such as neutron adsorption rate, melting point and thermal conductivity, are also highlighted.
		"Interdiffusion Between Zr Diffusion Barrier and U-Mo Alloy" Ke Huang, Young Joo Park, Dennis Keiser, Yongho Sohn, Journal of Phase Equilibria and Diffusion Vol. 33 2012 443-449 Link U-Mo alloys are being developed as low-enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Significant reactions have been observed between U-Mo fuels and Al or Al alloy matrix. Refractory metal Zr has been proposed as barrier material to reduce the interactions. In order to investigate the compatibility and barrier effects between U-Mo alloy and Zr, solid-to-solid U-10wt.%Mo versus Zr diffusion couples were assembled and annealed at 600, 700, 800, 900, and 1000 °C for various times. The microstructures and concentration profiles due to interdiffusion and reactions were examined via scanning electron microscopy and electron probe microanalysis, respectively. Intermetallic phase Mo2Zr was found at the interface, and its population increased when annealing temperature decreased. Diffusion paths were also plotted on the U-Mo-Zr ternary phase diagrams with good consistency. The growth rate of interdiffusion zone between U-10wt.%Mo and Zr was also calculated under the assumption of parabolic diffusion and was determined to be about 103 times lower than the growth rate of diffusional interaction layer found in diffusion couples U-10wt.%Mo versus Al or Al-Si alloy. Other desirable physical properties of Zr as barrier material, such as neutron adsorption rate, melting point, and thermal conductivity, are presented as supplementary information to demonstrate the great potential of Zr as the diffusion barrier for U-Mo fuel systems in RERTR.
		"Interdiffusion, Intrinsic Diffusion, Atomic Mobility, and Vacancy Wind Effects in γ(bcc) Uranium-Molybdenum Alloy" Ke Huang, Dennis Keiser, Yongho Sohn, Metallurgical and Materials Transactions A Vol. 44 2012 738-746 Link U-Mo alloys are being developed as low enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) Program. In order to understand the fundamental diffusion behavior of this system, solid-to-solid pure U vs Mo diffusion couples were assembled and annealed at 923 K, 973 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 800 °C, 900 °C, and 1000 °C) for various times. The interdiffusion microstructures and concentration profiles were examined via scanning electron microscopy and electron probe microanalysis, respectively. As the Mo concentration increased from 2 to 26 at. pct, the interdiffusion coefficient decreased, while the activation energy increased. A Kirkendall marker plane was clearly identified in each diffusion couple and utilized to determine intrinsic diffusion coefficients. Uranium intrinsically diffused 5-10 times faster than Mo. Molar excess Gibbs free energy of U-Mo alloy was applied to calculate the thermodynamic factor using ideal, regular, and subregular solution models. Based on the intrinsic diffusion coefficients and thermodynamic factors, Manning’s formalism was used to calculate the tracer diffusion coefficients, atomic mobilities, and vacancy wind parameters of U and Mo at the marker composition. The tracer diffusion coefficients and atomic mobilities of U were about five times larger than those of Mo, and the vacancy wind effect increased the intrinsic flux of U by approximately 30 pct.
		"Microstructural Analysis of As-Processed U-10wt.%Mo Monolithic Fuel Plate in AA6061 Matrix with Zr Diffusion Barrier" Emmanuel Perez, B. Yao, Dennis Keiser, Yongho Sohn, Journal of Nuclear Materials Vol. 402 2010 8-14 Link For higher U-loading in low-enriched U–10 wt.%Mo fuels, monolithic fuel plate clad in AA6061 is being developed as a part of Reduced Enrichment for Research and Test Reactor (RERTR) program. This paper reports the first characterization results from a monolithic U–10 wt.%Mo fuel plate with a Zr diffusion barrier that was fabricated as part of a plate fabrication campaign for irradiation testing in the Advanced Test Reactor (ATR). Both scanning and transmission electron microscopy (SEM and TEM) were employed for analysis. At the interface between the Zr barrier and U–10 wt.%Mo, going from Zr to U(Mo), UZr2, γ-UZr, Zr solid-solution and Mo2Zr phases were observed. The interface between AA6061 cladding and Zr barrier plate consisted of four layers, going from Al to Zr, (Al, Si)2Zr, (Al, Si)Zr3 (Al, Si)3Zr, and AlSi4Zr5. Irradiation behavior of these intermetallic phases is discussed based on their constituents. Characterization of as-fabricated phase constituents and microstructure would help understand the irradiation behavior of these fuel plates, interpret post-irradiation examination, and optimize the processing parameters of monolithic fuel system.
		"Microstructural Characterization of U-7Mo/Al-Si Alloy Matrix Dispersion Fuel Plates Fabricated at 500 C" Emmanuel Perez, Dennis Keiser, Jan-Fong Jue, Bo Yao, Yongho Sohn, Curtis Clark, Journal of Nuclear Materials Vol. 412 2011 90-99 Link The starting microstructure of a dispersion fuel plate will impact the overall performance of the plate during irradiation. To improve the understanding of the as-fabricated microstructures of U–Mo dispersion fuel plates, particularly the interaction layers that can form between the fuel particles and the matrix, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses have been performed on samples from depleted U–7Mo (U–7Mo) dispersion fuel plates with either Al–2 wt.% Si(Al–2Si) or AA4043 alloy matrix. It was observed that in the thick interaction layers, U(Al, Si)3 and U6Mo4Al43 were present, and in the thin interaction layers, (U, Mo) (Al, Si)3, U(Al, Si)4, U3Si3Al2, U3Si5, and possibly USi-type phases were observed. The U3Si3Al2 phase contained some Mo. Based on the results of this investigation, the time that a dispersion fuel plate is exposed to a relatively high temperature during fabrication will impact the nature of the interaction layers around the fuel particles. Uniformly thin, Si-rich layers will develop around the U–7Mo particles for shorter exposure times, and thicker, Si-depleted layers will develop for the longer exposure times.
		"Microstructural Characterization of U-Nb-Zr, U-Mo-Nb, and U-Mo-Ti Alloys via Electron Microscopy" Emmanuel Perez, Ashley Ewh, Dennis Keiser, Yongho Sohn, Journal of Phase Equilibria and Diffusion Vol. 31 2010 216-222 Link Ternary uranium molybdenum alloys are being examined for use as dispersion and monolithic nuclear fuels in research and test reactors. In this study, three such ternary alloys, with compositions U-10Nb-4Zr, U-8Mo-3Nb, and U-7Mo-3Ti in wt.%, were examined using scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) with high angle annular dark field (HAADF) imaging via scanning transmission electron microscopy (STEM). These alloys were homogenized at 950 °C for 96 h and were expected to be single-phase bcc-γ-U. However, upon examination, it was determined that despite homogenization, each of the alloys contained a small volume fraction precipitate phase. Through SEM and XRD, it was confirmed that the matrix retained the bcc-γ-U phase. TEM specimens were prepared using site-specific focused ion beam (FIB) in situ lift out (INLO) technique to include at least one precipitate from each alloy. By electron diffraction, the precipitate phases for the U-10Nb-4Zr, U-8Mo-3Nb, and U-7Mo-3Ti alloys were identified as bcc-(Nb,Zr), bcc-(Mo,Nb), and bcc-(Mo,Ti) solid solutions, respectively. The composition and phase information collected in this study was then used to construct ternary isotherms for each of these alloys at 950 °C.
		"Microstructural development from interdiffusion and reaction between U-Mo and AA6061 alloys annealed at 600° and 550 °C" Dennis Keiser, Emmanuel Perez, Yongho Sohn, Journal of Nuclear Materials Vol. 477 2016 178-192 Link The U.S. Material Management and Minimization Reactor Conversion Program is developing low enrichment fuel systems encased in Al-alloy for use in research and test reactors. Monolithic fuel plates have local regions where the UMo fuel plate may come into contact with the Al-alloy 6061 (AA6061) cladding. This results in the development of interdiffusion zones with complex microstructures with multiple phases. In this study, the microstructural development of diffusion couples, U7 wt%Mo, U10 wt%Mo, and U12 wt%Mo vs. AA6061, annealed at 600 °C for 24 h and at 550 °C for 1, 5, and 20 h, were analyzed by scanning electron microscopy with x-ray energy dispersive spectroscopy. The microstructural development and kinetics were compared to diffusion couples UMo vs. high purity Al and binary AlSi alloys. The diffusion couples developed complex interaction regions where phase development was influenced by the alloying additions of the AA6061.
		"Microstructure characterization of as-fabricated and 475ºC annealed U-7wt.%Mo dispersion fuel in Al-Si alloy matrix" Emmanuel Perez, Bo Yao, Dennis Keiser, Jan-Fong Jue, Curtis Clark, Nicolas Woolstenhulme, Yongho Sohn, Journal of Alloys and Compounds Vol. 509 2011 9487-9496 Link High-density uranium (U) alloys with an increased concentration of U are being examined for the development of research and test reactors with low enriched metallic fuels. The U–Mo fuel alloy dispersed in Al–Si alloy has attracted particular interest for this application. This paper reports our detailed characterization results of as-fabricated and annealed (475 °C for 4 h) U–Mo dispersion fuels in Al–Si matrix with a Si concentration of 2 and 5 wt.%, named as “As2Si”, “As5Si”, “An2Si”, “An5Si” accordingly. Techniques employed for the characterization include scanning electron microscopy and transmission electron microscopy with specimen prepared by focused ion beam in situ lift-out. Fuel plates with Al–5 wt.% Si matrix consistently yielded thicker interaction layers developed between U–Mo particles and Al–Si matrix, than those with Al–2 wt.% Si matrix, given the same processing parameters. A single layer of interaction zone was observed in as-fabricated samples (i.e., “As2Si”, “As5Si”), and this layer mainly consisted of U3Si3Al2 phase. The annealed samples contained a two-layered interaction zone, with a Si-rich layer near the U–Mo side, and an Al-rich layer near the Al–Si matrix side. The U3Si5 appeared as the main phase in the Si-rich layer in “An2Si” sample, while both U3Si5 and U3Si3Al2 were identified in sample “An5Si”. The Al-rich layer in sample “An2Si” was amorphous, whereas that in sample “An5Si” mostly consisted of crystalline U(Al,Si)3, along with a small fraction of U(Al,Si)4 and U6Mo4Al43 phases. The influence of Si on the diffusion and reaction in the development of interaction layers in U(Mo)/Al(Si) is discussed in the light of growth-controlling mechanisms and irradiation performance.
		"Phase Constituents and Microstructure of Interaction Layer Formed in U-Mo Alloys vs Al Diffusion Couples Annealed at 873 K (600 °C)" Emmanuel Perez, Dennis Keiser, Yongho Sohn, Metallurgical and Materials Transactions A Vol. 42 2011 3071-3083 Link U-Mo dispersion and monolithic fuels are being developed to fulfill the requirements for research reactors, under the Reduced Enrichment for Research and Test Reactors program. In dispersion fuels, particles of U-Mo alloys are embedded in the Al-alloy matrix, while in monolithic fuels, U-Mo monoliths are roll bonded to the Al-alloy matrix. In this study, interdiffusion and microstructural development in the solid-to-solid diffusion couples, namely, U-15.7 at. pct Mo (7 wt pct Mo) vs pure Al, U-21.6 at. pct Mo (10 wt pct Mo) vs pure Al, and U-25.3 at. pct Mo (12 wt pct Mo) vs pure Al, annealed at 873 K (600 °C) for 24 hours, were examined in detail. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) were employed to examine the development of a very fine multiphase interaction layer with an approximately constant average composition of 80 at. pct Al. Extensive TEM was carried out to identify the constituent phases across the interaction layer based on selected area electron diffraction and convergent beam electron diffraction (CBED). The cubic-UAl3, orthorhombic-UAl4, hexagonal-U6Mo4Al43, and cubic-UMo2Al20 phases were identified within the interaction layer that included two- and three-phase layers. Residual stress from large differences in molar volume, evidenced by vertical cracks within the interaction layer, high Al mobility, Mo supersaturation, and partitioning toward equilibrium in the interdiffusion zone were employed to describe the complex microstructure and phase constituents observed. A mechanism by compositional modification of the Al alloy is explored to mitigate the development of the U6Mo4Al43 phase, which exhibits poor irradiation behavior that includes void formation and swelling.
		"Phase constituents of Al-rich U–Mo–Al alloys examined by transmission electron microscopy" Emmanuel Perez, Ashley Ewh, Jinwei Liu, Dennis Keiser, Yongho Sohn, Journal of Nuclear Materials Vol. 394 2009 160-165 Link To supplement the understanding of diffusional interactions involving Al-rich region of the U–Mo–Al system, alloys with composition 85.7Al–11.44U–2.86Mo and 87.5Al–10U–2.5Mo in at.%, were examined to determine the equilibrium phase constituents at 500 °C. These alloys were triple arc-melted, homogenized at 500 °C for 200 h, and water-quenched to preserve the high temperature microstructure. X-ray diffraction, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (XEDS), and transmission electron microscopy (TEM) with high angle annular dark field (HAADF) imaging via scanning transmission electron microscopy (STEM) were employed for the characterization. Alloy specimens for TEM/STEM were prepared using site-specific focused ion beam (FIB) in situ lift-out (INLO) technique. Despite the homogenization time and temperature, five different phases, namely fcc-Al solid solution, cubic-UAl3, orthorhombic-UAl4, hexagonal-U6Mo4Al43 and diamond cubic-UMo2Al20, were observed. Based on U–Al, U–Mo and Al–Mo binary phase diagrams, previously proposed U–Mo–Al isotherms, and the solidification microstructure of these alloys, the Al-rich region of the equilibrium ternary isotherm at 500 °C was constructed. The fcc-Al solid solution, orthorhombic-UAl4, and diamond cubic-UMo2Al20 which were determined to be the equilibrium phases in 85.7Al–11.44U–2.86Mo and 87.5Al–10U–2.5Mo alloys.
		"Phase decomposition of ?-U (bcc) in U-10 wt% Mo fuel alloy during hot isostatic pressing of monolithic fuel plat" Nicholas Eriksson, Dennis Keiser, Ryan Newell, Young Joo Park, Yongho Sohn, Journal of Nuclear Materials Vol. 480 2016 271-280 Link Eutectoid decomposition of γ-phase (cI2) into α-phase (oC4) and γ′-phase (tI6) during the hot isostatic pressing (HIP) of the U-10 wt% Mo (U10Mo) alloy was investigated using monolithic fuel plate samples consisting of U10Mo fuel alloy, Zr diffusion barrier and AA6061 cladding. The decomposition of the γ-phase was observed because the HIP process is carried out near the eutectoid temperature, 555 °C. Initially, a cellular structure, consisting of γ′-phase surrounded by α-phase, developed from the destabilization of the γ-phase. The cellular structure further developed into an alternating lamellar structure of α- and γ′-phases. Using scanning electron microscopy and transmission electron microscopy, qualitative and quantitative microstructural analyses were carried out to identify the phase constituents, and elucidate the microstructural development based on time-temperature-transformation diagram of the U10Mo alloy. The destabilization of γ -phase into α- and γ′-phases would be minimized when HIP process was carried out with rapid ramping/cooling rate and dwell temperature higher than 560 °C.
		"Phase development in a U–7 wt.% Mo vs. Al–7 wt.% Ge diffusion couple" Emmanuel Perez, Dennis Keiser, Yongho Sohn, Journal of Nuclear Materials Vol. 441 2013 159-167 Link Fuel development for the Reduced Enrichment for Research and Test Reactors (RERTR) program has demonstrated that U–Mo alloys in contact with Al develop interaction regions with phases that have poor irradiation behavior. The addition of Si to the Al has been considered with positive results. In this study, compositional modification is considered by replacing Si with Ge to determine the effect on the phase development in the system. The microstructural and phase development of a diffusion couple of U–7 wt.% Mo in contact with Al–7 wt.% Ge was examined by transmission electron microscopy, scanning electron microscopy and energy dispersive spectroscopy. The interdiffusion zone developed a microstructure that included the cubic-UGe3 phase and amorphous phases. The UGe3 phase was observed with and without Mo and Al solid solution developing a (U,Mo)(Al,Ge)3 phase.
		"Radiation effects on interface reactions of U/Fe, U/(Fe + Cr), and U/(Fe + Cr + Ni)" Bulent Sencer, Lin Shao, Yongho Sohn, Di Chen, Chaochen Wei, Michael Martin, Xuemei Wang, Young Joo Park, Ed Dein, Kevin Coffey, Rory Kennedy, Journal of Nuclear Materials Vol. 456 2015 302-310 Link We study the effects of radiation damage on interdiffusion and intermetallic phase formation at the interfaces of U/Fe, U/(Fe + Cr), and U/(Fe + Cr + Ni) diffusion couples. Magnetron sputtering is used to deposit thin films of Fe, Fe + Cr, or Fe + Cr + Ni on U substrates to form the diffusion couples. One set of samples are thermally annealed under high vacuum at 450 °C or 550 °C for one hour. A second set of samples are annealed identically but with concurrent 3.5 MeV Fe++ ion irradiation. The Fe++ ion penetration depth is sufficient to reach the original interfaces. Rutherford backscattering spectrometry analysis with high fidelity spectral simulations is used to obtain interdiffusion profiles, which are used to examine differences in U diffusion and intermetallic phase formation at the buried interfaces. For all three diffusion systems, Fe++ ion irradiations enhance U diffusion. Furthermore, the irradiations accelerate the formation of intermetallic phases. In U/Fe couples, for example, the unirradiated samples show typical interdiffusion governed by Fick’s laws, while the irradiated ones show step-like profiles influenced by Gibbs phase rules.
		"Role of Si on Diffusional Interaction between U-Mo and Al-Si Alloys at 823K (550 C)" Emmanuel Perez, Dennis Keiser, Yongho Sohn, Metallurgical and Materials Transactions A Vol. 44 2012 584-595 Link U-Mo dispersions in Al-alloy matrix and monolithic fuels encased in Al-alloy are under development to fulfill the requirements for research and test reactors to use low-enriched molybdenum stabilized uranium alloys fuels. Significant interaction takes place between the U-Mo fuel and Al during manufacturing and in-reactor irradiation. The interactions products are Al-rich phases with physical and thermal characteristics that adversely affect fuel performance and lead to premature failure. Detailed analysis of the interdiffusion and microstructural development of this system was carried through diffusion couples consisting of U-7wt.%Mo, U-10wt.%Mo and U-12wt.%Mo in contact with pure Al, Al-2wt.%Si, and Al-5wt.%Si, annealed at 823K for 1, 5 and 20 hours. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for the analysis. Diffusion couples consisting of U-Mo vs. pure Al contained UAl3, UAl4, U6Mo4Al43, and UMo2Al20 phases. The addition of Si to the Al significantly reduced the thickness of the interdiffusion zone. The interdiffusion zones developed Al and Si enriched regions, whose locations and size depended on the Si and Mo concentrations in the terminal alloys. In the couples, the (U,Mo)(Al,Si)3 phase was observed throughout interdiffusion zone, and the U6Mo4Al43 and UMo2Al20 phases were observed only where the Si concentrations were low. (PDF) Role of Si on the Diffusional Interactions Between U-Mo and Al-Si Alloys at 823 K (550 °C). Available from: https://www.researchgate.net/publication/255813658/download [accessed Sep 06 2018].
		"Selected Observations in Phase Constituents, Growth Kinetics and Microstructural Development of Aluminides in U-Mo vs. Al and 6061 Diffusion Couples Annealed at 600°C" Emmanuel Perez, Dennis Keiser, Yongho Sohn, Defects and Diffusion Forum Vol. 289-292 2009 41-49 Link This paper presents selected experimental observations of phase constituents, growth kinetics, and microstructural development of aluminide phases that develop in solid-to-solid diffusion couples assembled with U-7wt.%Mo, U-10wt.%Mo and U-12wt.%Mo vs. Al and 6061 alloy after a diffusion anneal at 600°C for 24 hours. Scanning electron microscopy coupled with energy dispersive spectroscopy, electron microprobe analysis, and transmission electron microscopy via focused ion beam in-situ lift-out were employed to characterize the interaction layer that develops by interdiffusion. While concentration profiles exhibited no significant gradients, microstructural analysis revealed the presence of extremely complex and nano-scale phase constituents with presence of orthorhombic--U, cubic-UAl3, orthorhombic-UAl4, hexagonal-U6Mo4Al43 and diamond cubic-UMo2Al20 phases. Presence of multi-phase layers with microstructure, which suggest a significant role of grain boundary diffusion, was observed.
		"Thermotransport in γ(bcc) U-Zr Alloys: A Phase-Field Model Study" Rashmi Mohanty, J. Bush, Maria Okuniewski, Yongho Sohn, Journal of Nuclear Materials Vol. 414 2011 211-216 Link Atomic transport in the presence of a temperature gradient, commonly known as thermotransport or the thermomigration phenomenon, was simulated for U–Zr alloys using a phase-field model derived from irreversible thermodynamics. The free energy of the U–Zr system, a necessary ingredient for the phase-field-model, was directly incorporated from the available thermodynamic database. Kinetic parameters such as atomic mobility and heat of transport terms were obtained from experimental values reported in the literature. The model was applied to a single-phase (bcc-γ phase) alloy and to a diffusion couple consisting of two single-phase (bcc-γ phase) alloys of different compositions, both subjected to a constant temperature gradient. Constituent redistribution in the absence and presence of a compositional gradient was examined. An enrichment of Zr with a corresponding depletion of U was observed at the hot end of the initially homogeneous single-phase alloy. A similar atomic transport behavior was observed in the diffusion couple, where the magnitude and direction of the final composition gradient was dictated by the combined influence of atomic mobility and heat of transport terms.
		"Understanding the Phase Equilibrium and Irradiation Effects in Fe-Zr Diffusion Couples" Assel Aitkaliyeva, Bulent Sencer, Lin Shao, Yongho Sohn, Chao-Chen Wei, Zhiping Luo, Ashley Ewh, Rory Kennedy, Michael Myers, Joseph Wallace, M. J. General, Michael Martin, Journal of Nuclear Materials Vol. 432 2013 205-211 Link We have studied the radiation effects in Fe–Zr diffusion couples, formed by thermal annealing of a mechanically bonded binary system at 850 °C for 15 days. After irradiation with 3.5 MeV Fe ions at 600 °C, a cross sectional specimen was prepared by using a focused-ion-beam-based lift out technique and was characterized using scanning/transmission electron microscopy, selected-area diffraction and X-ray energy dispersive spectroscopy analyses. Comparison studies were performed in localized regions within and beyond the ion projected range and the following observations were obtained: (1) the interaction layer consists of FeZr3, FeZr2, Fe2Zr, and Fe23Zr6; (2) large Fe23Zr6 particles with smaller core particles of Zr-rich Fe2Zr are found within the a-Fe matrix; (3) Zr diffusion is significantly enhanced in the ion bombarded region, leading to the formation of an Fe–Zr compound; (4) grains located within the interaction layer are much smaller in the ion bombarded region and are associated with new crystal growth and nanocrystal formation; and (5) large a-Fe particles form on the surface of the Fe side, but the particles are limited to the region close to the interaction layer. These studies reveal the complexity of the interaction phase formation in an Fe–Zr binary system and the accelerated microstructural changes under irradiation.

Presentations:

	"Diffision Couple Expereiments: Opportunities and Challenges in Determining Thermo-Kinetic and Functional Properties" Yongho Sohn, 13th International Conference on Diffusion in Solids and Liquids June 26-30, (2017)
	"Interdiffusion, Reactions and Phase Transformations Observed during Fabrication of Low Enriched Metallic Fuel System for Research and Test Reactors" Nicholas Eriksson, Dennis Keiser, Ryan Newell, Young Joo Park, Yongho Sohn, 10th International Conference on Diffusion in Materials (DIMAT-2017) May 7-12, (2017)

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Low Fluence Behavior of Metallic Fuels - FY 2010 Fall Solicitation for User Proposals, #242

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