Papers by Author: Dennis D. Keiser

Abstract: Using solid-to-solid couples investigation, this study characterized the reaction products evolved and quantified the diffusion kinetics when pure Mg bonded to AA6061 is subjected to thermal treatment at 300°C for 720 hours, 350°C for 360 hours, and 400°C for 240 hours. Characterization techniques include optical microscopy, scanning electron microscopy with X-ray energy dispersive spectroscopy, and transmission electron microscopy. Parabolic growth constants were determined for γ-Mg₁₇Al₁₂, β-Mg₂Al₃, and the elusive ε-phase. Similarly, the average effective interdiffusion coefficients of major constituents were calculated for Mg (ss), γ-Mg₁₇Al₁₂, β-Mg₂Al₃, and AA6061. The activation energies and pre-exponential factors for both parabolic growth constant and average effective interdiffusion coefficients were computed using the Arrhenius relationship. The activation energy for growth of γ-Mg₁₇Al₁₂ was significantly higher than that for β-Mg₂Al₃ while the activation energy for interdiffusion of γ-Mg₁₇Al₁₂ was only slightly higher than that for β-Mg₂Al₃. Comparisons are made between the results of this study and those of diffusion studies between pure Mg and pure Al [1] to examine the influence of alloying additions in AA6061.

Abstract: 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°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).

Abstract: 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.

Abstract: This study examined the growth kinetics of intermetallic phases that develop in solid-tosolid diffusion couples assembled with U-7, 10 and 12wt.%Mo vs. Al alloys (Al, Al-2wt.%Si, Al- 5wt.%Si, 4043 and 6061) after a diffusion anneal at 550°C for 24 hours. Based on interdiffusion microstructure and integrated interdiffusion coefficients, the addition of Si into the Al matrix alloy was observed to significantly reduce the growth rate of the intermetallic layer that primarily consisted of (U,Mo)Al4 phase. Growth rate of the (U,Mo)Al4 intermetallic layer also increases slightly with Mo content; however, it was not significant compared to the effect of alloying Si into Al alloys. Growth kinetics of (U,Mo)Al4 intermetallic layer appear highly sensitive to composition of U-Mo fuel alloy and Al cladding alloys, and must be an important criteria in alloy development/selection for optimum fuel performance with due consideration for compositiondependent multicomponent interdiffusion.

Abstract: To better understand interactions between fuel and cladding in research reactor fuels, diffusion couples between γ-phase U-7 wt% Mo and U-10 wt% Mo alloy fuels and a Si-bearing, Al alloy were fabricated using a friction stir welding technique. The advantage of such a fabrication technique is that it can potentially reduce the amount of aluminum-oxide that might be present at the diffusion couple interface. The presence of oxides at the interface can affect the interdiffusion process. These couples were annealed and characterized using a scanning electron microscope equipped with energy-dispersive and wavelength-dispersive spectrometers. Images were taken of the developed diffusion structures; x-ray maps were generated to identify partitioning behavior of the various components; and, point-to-point analysis was employed to generate composition profiles and to determine phase compositions. To try and determine how the presence of Si in an Al alloy affects the interdiffusion behavior of fuel and cladding components in research reactor nuclear fuels, the results from this study were compared to those from earlier diffusion studies using U-Mo alloys and Al. The formed diffusion zones in some samples annealed for 30 minutes are comprised of Si-rich aluminide phases that appear to be (U,Mo)0.9(Al,Si)4 and (U,Mo)(Si,Al)2, based on composition. The diffusion rates observed and the types of phases that form can be correlated to the stability of the γ-U phase, which is a metastable phase. For a sample annealed for a much longer time, large diffusion structures formed and no Si-rich phases were observed.