Defects and Diffusion in MetalsAn Annual Retrospective - VII

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Authors: A. Benmakhlouf
Abstract: In this work we develop an analytical method for resolution of the reaction-diffusion equations which govern impurity diffusion by the dissociative mechanism in a finite-thickness sample and from a deposit of solute atoms on the surface. This method is based upon the perturbation of basic solutions corresponding to limiting cases and the choice of suitable small parameters. The solutions obtained and their comparison to those of numerical studies are also presented in this paper.
Authors: A. Benmakhlouf
Abstract: The analytical treatment of dissociative diffusion by using the matched perturbation method given in the literature deals with a virtually infinite foreign-atom source producing a constant^concentration at the boundary. In this paper, a new mathematical model is developed for analysing the dissociative diffusion of the solute atoms in the case of finite-source conditions. The mathematical model combines the reaction-diffusion equations which govern solute atom diffusion by the dissociative mechanism and the boundary condition expressing the fact that the rate at which solute leaves the source is always equal to that at which it enters the sheet over the surface x=0. Solutions obtained by applying the matched perturbation method and their comparison with those of the numerical study are also presented in this paper.
Authors: Karem Boubaker
Abstract: This study develops a theoretical and experimental method for atomic structure characterization. The mirage technique has been performed to prospect unknown material structures in order to diagnose eventual treatment or defect. The use of Gaussian Laser beam made photothermal deflection signal more appropriate to build-up a simple algorithm in order to investigate metal structure frequential response to a modulated heating excitation and thus identify its more probable structure. Some applications on steel quenched or annealed samples yielded interesting results such as delimitation of martensitic structure . Method is actually to be tested on differently treated material.
Authors: Kyosuke Kishida, Masahiko Demura, Satoru Kobayashi, Ya Xu, Toshiyuki Hirano
Abstract: We have studied the texture and microstructure evolution during cold rolling of Ni3Al single crystals as a function of the initial crystal orientations and revealed that the cold rolling behavior of the single crystals are strongly dependent on the initial crystal orientations, especially on the initial rolling direction (RD). An optimum condition for thin foil fabrication is determined that the initial RD is close to <001>. According to the conditions we have successfully fabricated the wide and thin foils of binary Ni3Al by cold rolling the single crystalline ingots. The thinnest foils obtained so far are about 20µm in thickness and 50mm in width. This document reviews the current status of our research on the thin foils of intermetallic compound Ni3Al.
Authors: Sergey V. Dmitriev, Ju Li, Nobuhiro Yoshikawa, Yoshihisa Tanaka, Yutaka Kagawa, Takayuki Kitamura, Sidney Yip
Abstract: We apply the lattice instability analysis to homogeneously strained single crystal with a flat or defected surface to demonstrate that, in the process of emission of dislocation by an unstable surface, prior to the breaking of atomic bonds, a vibrational mode localizes, foretelling the location and the manner of the impending microscopic catastrophic event.
Authors: A.A. Kodentsov, A. Paul, F.J.J. van Loo
Abstract: There is now a considerable body of experimental evidence to indicate that in a volumediffusion controlled interaction the Kirkendall plane need not be unique. The Kirkendall plane can microstructurally be stable as well as unstable (it does not exist!). Under predictable circumstances, it can also bifurcate and even trifurcate. This can be rationalised in terms of Kirkendall velocity construction as well as from a purely chemical point of view considering diffusion-controlled interactions at the interphase interfaces. The physico-chemical approach is also used to explain significance of the Kirkendall effect in the morphogenesis of interdiffusion systems.
Authors: Alexandre Legris
Abstract: We highlight some of the most salient recent advances in point defects studies obtained from atomic-scale simulations performed in the framework of the density functional theory. The refinement of the theory, combined with its efficient numerical implementations and the (until now) everlasting growth of computer power allowed the transition from qualitative (in the beginning of the 90’) to quantitative results. Some of the longstanding controversies in the field have been tackled, and as far as aluminum is concerned, it has been shown that the curvature in the Arrheniusplot is due to anharmonic effects rather than to a two-defect diffusion mechanism. The anomalous diffusion in the b (bcc) phase of the group-IV elements has been related to the strong structural relaxation around vacancies, which significantly reduces their formation energy. Self-interstitials have been studied in materials of technological interest, their structure and mobility have been analyzed allowing a better interpretation of experimental results and an improved understanding of processes occurring under irradiation. Dilute interstitial solid solutions have been investigated. The strong binding between C and vacancies in bcc Fe may partially explain the observed influence of low amounts of C on Fe self-diffusion; the attraction of H to stacking faults in a Zr should favor planar dislocations glide. Intermetallics involving Fe (Fe-Al, Fe-Co) behave like highly correlated systems requiring methodological improvements of the DFT for a quantitative description. However, valuable trends concerning the structural point defects (those that allow nonstoichiometric compositions at low temperature) as well as the temperature dependence of point defects concentrations have been obtained.
Authors: A. Kellou, Thierry Grosdidier, H. Aourag
Abstract: Atomistic modeling based on Density Functional Theory (DFT) within the framework of the Generalized Gradient Approximation (GGA) is used to show the effects of defects such as vacancy, boron, carbon, nitrogen and oxygen substituting Fe or Al atoms in the B2-FeAl structure. The site preference of each type of defect is determined from a comparison of total energycalculations using a supercell structure, consisting of 16-atoms, within which each the various defects are introduced. The changes in lattice parameter and bulk modulus associated to the presence of the defects in the FeAl matrix are also studied.
Authors: Henrik Strandlund, Henrik Larsson
Abstract: Some different approaches to diffusion process simulations are briefly presented. Their varying areas of applicability are discussed. Example simulations using the phase-field method, the DICTRA software, and random-walk based approaches are presented.
Authors: Yoshihiro Yamazaki, Yoshiaki Iijima
Abstract: Diffusion in a metal under an elevated hydrogen pressure is interesting in view of the fact that the diffusion is enhanced owing to the injection of a large amount of vacancies into the metal. This is peculiar to an elevated hydrogen pressure because diffusion in a metal is generally suppressed under a hydrostatic pressure. In the present article, the effect of an elevated hydrogen pressure on interdiffusion and impurity diffusion is reviewed in the Au-Fe system which has a large difference in the hydrogen solubilities between g -Fe and Au under an elevated hydrogen pressure.

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