Abstract: In most schools of engineering, this is among the first interdisciplinary courses that third-year undergraduate students are likely to attend. This presents formidable challenges because any discussion of diffusion phenomena draws heavily on prior knowledge of physics, chemistry, and mathematics. In our traditionally inadequate way of teaching, these disciplines are presented as self-contained, autonomous units. Yet it should be the purpose of any instructor of diffusion theory and practice to show how they may be integrated. Heuristic arguments are certainly appealing — thus recommended — but the methods and tools to be developed must be robust enough to not immediately crumble with use. In that connection, attention to a known and consistent notation is vital. Furthermore, one cannot expect these students to be fully familiar with partial differential equations, and yet, that’s the very nature of the diffusion equation. Its properties must be explained. Finally, diffusion in solids suffers from a bewildering variety of “diffusion coefficients." These must be carefully defined and distinguished. A version of my class-notes will be included in my web-site within a few weeks (just type my name in Google). You are most welcome.
Abstract: Monolithic fuel plates have been developed utilizing low enriched U alloyed with 10 wt.% Mo to replace highly enriched fuels in research and test reactors, in accordance with the goals of the Materials Management and Minimization Reactor Conversion Program. The fuel plates consist of U10Mo fuel, Zr diffusion barrier, and AA6061 cladding. They are fabricated by co-rolling the U10Mo and Zr, which are then encapsulated via hot isostatic pressing of the entire U10Mo/Zr/AA6061 assembly. During fabrication, the metal constituents of the fuel plates undergo phase transformations as well as interdiffusion and reactions at interfaces. The areas of interest are the U10Mo fuel, U10Mo/Zr interface, U10Mo/AA6061 interface, Zr/AA6061 interface, and AA6061-AA6061 bond line. Knowledge of the transformations and growth in the plates is necessary to optimize fabrication parameters and predict behavior as they relate to irradiation performance. Numerous studies have been conducted to analyze these reactions in monolithic fuel plates, and a summary of their observations is provided in this paper.
Abstract: Atomic redistribution of W and Fe in Si were studied using secondary ion mass spectrometry and transmission electron microscopy. W diffusion experiments performed during isothermal annealing and during Si oxidation show that W atoms should use at least two different diffusion mechanisms. Experimental diffusion profiles can be well simulated by considering the simultaneous use of three different W diffusion mechanisms: the dissociative and the kick-out mechanisms, as well as an original mechanism based on the formation of a W-Si self-interstitial pair located on the interstitial Si sub-lattice. Fe redistribution was studied during the oxidation of a Fe-contaminated Si wafer. Fe is shown to be first pushed-out in Si by the mobile SiO2/Si interface, and thus to form Fe silicides precipitates at this interface. The silicide precipitates, which can exhibit a core-shell structure, appear to move with the SiO2/Si interface thanks to an oxidation/dissolution mechanism in the SiO2 and a nucleation/growth mechanism in the Si matrix. Furthermore, the rate difference between Si and Fe silicide precipitate oxidation leads to the formation of Si pyramidal defects at the SiO2/Si interface.
Abstract: Diffusion couple technique is an efficient tool for the estimating the chemical diffusion coefficients. Typical experimental uncertainties of the composition profile measurements complicate a correct determination of the interdiffusion coefficients via the standard Boltzmann-Matano, Sauer-Freise or the den Broeder methods, especially for systems with a strong compositional dependence of the interdiffusion coefficient. A new approach for reliable fitting of the experimental profiles with an improved behavior at both ends of the diffusion couple is proposed and tested against the experimental data on chemical diffusion in the system Fe-Ga. An extension of the approach for reliable description of the up-hill diffusion phenomenon in multicomponent systems is presented.
Abstract: The study of diffusion processes in the aluminum - copper system was carried out at the temperature 350 and 520 °C. Special attention was paid on the chemical composition of the system near Al/Cu interface. It was determined that the intermediate phases in the system, corresponding to the equilibrium phase diagram, were not formed at low temperature. At high temperature the intermediate phases forms starting with Cu - rich phases. In both cases supersaturated solid solution of copper in aluminum could be observed near the interface.
Abstract: In this report, a combination of the diffusion multiple technique and the recently developed numerical inverse method was employed for a high–throughput determination of interdiffusivity matrices in Ni–Al–Cr alloys. A face–centered cubic (fcc) quinary Ni–Al–Cr diffusion multiple at 1173 K was carefully prepared by means of the hot–pressing technique. Based on the composition profiles measured by the field emission electron probe micro analysis (FE–EPMA), the composition–dependent interdiffusivity matrices in ternary Ni–rich Ni–Al–Cr system at 1173 K were then efficiently determined using the numerical inverse method.
Abstract: Sorption and desorption of moisture in epoxy composites reinforced with triangular and V-shaped bar fillers were studied. Epoxy was reinforced with vinyl ester bar assemblies fabricated according to 4 varied factors: bar orientation relative to diffusion direction (pointed or base side), bar alignment (parallel or staggered), spacing between bars (1 or 5 mm), and bar cross-sectional shape (triangular or V-shaped). Unlike previous studies, the bar fillers were initially coated with small amount of epoxy resin to improve wetting during fabrication of composites. Moisture uptake and loss of composites during one-side exposure to water vapor (50% relative humidity at 60°C) and hot air (60°C), respectively, were monitored with time. Experimental results show weight change of composites during moisture sorption and desorption varies linearly with square root of exposure time. Diffusion coefficients of composites were determined by assuming the material to be semi-infinite and fitting a mathematical solution to Fick’s second law of diffusion to weight change data. Results show diffusion coefficient of composites during moisture sorption is increased when bars are oriented pointed relative to diffusion direction, aligned parallel, spaced at 1 mm, and has triangular cross-sectional shape. However, during desorption, the diffusion coefficient is increased when base side of bars are exposed and when the bars are aligned staggered. The observed effects of factors on moisture diffusion coefficients of epoxy composites during sorption and desorption are discussed in relation to path length, available diffusion area, and status of epoxy/vinyl ester interphase before and after moisture sorption in composites.
Abstract: The cellular automata method offers a promising approach to describe diffusion and diffusion-controlled precipitation processes at high temperatures. During high temperature exposure, technical components like gas-turbine blades, furnaces, or exhaust systems, are operating in corrosive atmospheres. The resulting material-degradation processes are diffusion‐controlled, and corrosive species penetrate into the material leading to the formation of embrittling precipitates. Cellular automata (CA) represent distributed dynamical systems whose structure is particularly well suited to determine the temporal evolution of the system. In this study, it is shown that the model is able to consider diffusion, nucleation and growth aspects, interdiffusion between scales, and high diffusivity paths like grain boundaries. This has been demonstrated by applying CA to (i) nitrogen diffusion, (ii) internal intergranular oxidation of nickel-based alloy, and (iii) interdiffusion of a binary diffusion couple.
Abstract: Recently developed method of atomistic modelling (SKMF) is applied to order-disorder transitions in FCC alloys and to tracer diffusion in the ordered L12 structure. Results correlate with Kinetic Mote-Carlo modelling. Difference of diffusion activation energies of two species is found. Activation energy of ordering is close to one of minority component diffusion.