Diffusion Phenomena in Engineering Materials

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作者: Mysore A. Dayananda
摘要: There exist several interesting phenomena and observations reported in literature for isothermal diffusion in multicomponent systems. Such phenomena include uphill diffusion, development of zero-flux planes and flux reversals for individual components, flux reversals at interfaces, and instability at interfaces and multiphase layer development. In addition, uncommon diffusion structures exhibiting unusual diffusion paths can develop in both single phase and multiphase diffusion assemblies. An overview of such phenomena is presented to highlight the role of interactions among diffusing components with the aid of selected diffusion studies carried out in multicomponent alloy systems, aluminides, silicides, and nuclear fuels.
作者: T.R. Paul, I.V. Belova, E.V. Levchenko, A.V. Evteev, G.E. Murch
摘要: The self-or tracer diffusivity of one component in a binary alloy is often required when there is knowledge of the other component’s self-or tracer diffusivity and the interdiffusivity (and the thermodynamic factor). In the present paper, this problem is addressed for the random alloy model by applying three possible approximations having different levels of accuracy: Darken (low level of accuracy), Manning (medium level of accuracy) and Moleko, Allnatt and Allnatt (MAA) (high level of accuracy). There are unexpectedly large differences between the results of these approximations that sometimes are reflected in the high sensitivity of the vacancy-wind factor to the level of approximation. Generally, for the application of Manning and the MAA approximations, it is found that there is a difference in the number of self-diffusivity roots depending on whether the tracer diffusivity is available for the faster diffuser or for the slower diffuser and depending on how close the composition is to the forbidden (according to Manning’s description) region. Provided that the interdiffusion coefficient (divided by the thermodynamic factor) is greater than the available self-diffusion coefficient multiplied by its complementary composition, the application of the Darken approximation always results in one self-diffusivity root.
作者: A.B. Lidiard
摘要: This article reviews the subject of the Soret effect and Thermodiffusion in solids more generally. In doing so it draws upon computer simulations made with a method (the Grout-Gillan method) derived from the Green-Kubo approach to transport coefficients in solids. The insights into the make-up of heats of transport parameters, Q*, so obtained are described and used to provide additional insight into measured heats of transport in situations where no reliable theories or simulations exist. These insights also point to the relations between heats of transport on the one hand and phonon thermal conductivity and focussed collision sequences on the other. These relations point to circumstances where the heat of transport may be small (e.g. low coordination in the lattice) or can be estimated from heats of activation for atom movements. In other cases the Grout-Gillan simulation method may offer the most reliable approach. These new insights are expected to be useful in materials modelling.
作者: Gregory Kozlowski, V. Chaudhary, Z. Turgut, H. Kosai, A. Sheets, T. Bixel, A. Wheatley, P. Abdulkin, B. Knappett, David Jefferson, M. Matusiewicz, J. Kasperczyk, M. Makowska-Janusik
摘要: The surface effects on the critical dimensions of ferromagnetic nanoparticles have been studied. Iron nanoparticles with different mean diameter from 5.9 nm to 21.4 nm were prepared by thermal decomposition of iron pentacarbonyl in the presence of oleic acid/octyl ether. The heating response of these ferromagnetic nanoparticles suspended in water were measured experimentally during which the same amount of iron nanoparticles and di-ionized water were irradiated by an alternating magnetic field and the increase in temperature of the system was measured. The heating performance of the nanoparticles was described in terms of Specific Absorption Rate (SAR) which depends on the heating rate. The heating rate was calculated from the initial slope of the heating curve at an inflection point whereby there is minimum heat loss to the surrounding. Results were analyzed to find the critical diameters for the transition from single-domain to superparamagnetic regime and from single-domain to multi-domain regime. Also, the frequency and current dependence of SAR were studied. The maximum value of SAR was obtained when the applied frequency and current were at 175 kHz and 15 A, respectively. An equation for the critical radius for the transition from single-domain to multi-domain regime with low anisotropy was derived and numerically solved by using a program written in C++ and results were analyzed to find the effect of surface parameters on the critical diameter of nanoparticles. The SAR as a function of nanoparticle’s diameter shows two maxima which can be connected with the two critical dimensions. One is DC1 at 18 nm for the transition from single-domain to multi-domain configuration and the second is DC2 at 10 nm for the transition from single-domain to superparamagnetic regime. Comparison of these experimental results with the bond order-length-strength correlation theory was discussed.
作者: Hasan Güleryüz, Erdem Atar, Fared Seahjani, Hüseyin Çimenoğlu
摘要: In this paper, diffusional surface hardening processes utilized to overcome the poor tribological performance of titanium and its alloys is briefly introduced. More specifically, surface treatments known as thermal oxidation, nitriding and boriding offering the advantage of producing graded surfaces comprising hard compound layer and diffusion zone by diffusion of interstitial atoms (oxygen, nitrogen and boron) are overviewed.
作者: D.A. Morton-Blake
摘要: A molecular dynamics simulation is conducted to describe the behaviour of sodium and chloride ions as they enter a synthetic ion channel (mounted in a bilayer membrane) from aqueous NaCl solutions on either side of the membrane. The channel consists of an α-helical peptide chain scaffold with six aligned crown ether (CE) rings (18-CE-6) as side groups, forming a molecular chamber between each neighbouring pair. Responding to the channel’s axial −2 to −1 V electric potential, the Na+ ions, but not Cl, enter the channel spontaneously but they do not proceed beyond the first chamber formed between CE rings 1 and 2. The application of an axial electric field promotes the entry of a Na+ ion and its migration over the internal length of the channel. The forces that drive the migration phenomena are predominantly coulombic. Although the same electric field simultaneously allows a Cl ion initially to access the channel the ion is subsequently expelled from the first chamber into the bilayer. Although a Na+ ion may make a facile or even spontaneous entry to the channel it requires an energy estimated from Coulomb forces as ~5 eV to pass subsequent CE rings, and considerably more to exit the channel. An important role is found for the vibrational activity of the ether rings’ C-O-C units in their facilitating contribution to the migration of Na+ in the channel.
作者: Viswanathan Neelakantan Nurni, Bharath Nidambur Ballal, Seshadri Seetharaman
摘要: Diffusion phenomena are of great importance in materials processing wherein atomic, molecular or ionic species are distributed within a phase or among different phases. Though the phenomenological equation describing the diffusion phenomena including the bulk flow arising out of diffusion in fluid and the phenomena of Kirkendall shift in substitutional solids are the same, these processes are often treated independently. Some discussion on this aspect is presented in the theoretical aspects of diffusion. Owing to the complexity of atomic interactions, prediction of diffusion coefficients in condensed systems from first principles may not be that reliable; Experimental determination of diffusion coefficients is essential. In the second section, some novel experimental techniques developed recently to measure diffusion coefficients in the solid state as well as liquid systems including those in slags are described. In the last section, two case studies on application of diffusion phenomena in process metallurgy are presented emphasizing the importance of these in metallurgical processing.