Papers by Keyword: Grain Boundary Diffusion

Abstract: The diffusion of a B element into an A matrix was studied by the random walk theory. Considering that concentration of B element in the A matrix is very low, the jumps of diffusing atoms are independent of each other. The A matrix is a two-region material with different properties, such as a two-phase material, a single crystal with dislocations, or regions influenced by other solute and a polycrystalline material.It is assumed that material B has a penetration that allows it to cross each region of material A several times. This implies that jumps across the surface between those regions have an average frequency and, as a consequence, there is an interdiffusion coefficient between them. The interdiffusion coefficient between those regions is different than the coefficient of the diffusion in each region.Expressions were obtained that allow to delimit the ranges of validation with greater precision than the corrected Hart-Mortlock equation for solute diffusion. In addition, an original relationship was obtained between the segregation coefficient and parameters specific to the diffusion. New powerful tools were also found that can help to understand diffusion in nanocrystalline materials, diffusion in metals influenced by impurities and diffusion produced by different mechanisms.

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: Formation of microstructure in Ni under equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), its thermal stability and diffusion properties of grain boundaries are investigated. Grain boundary diffusion in the ultrafine-grained Ni is found to be significantly faster than in the coarse-grained Ni, which indicates a 'non-equilibrium' (deformation-modified) state of grain boundaries in the former. The effect of non-equilibrium state of grain boundaries on the level of internal stresses is analyzed.

Abstract: In this article new experimental evidences of anomalous grain boundary diffusion (GBD) of Fe and Co in Cu were describe. To demonstrate that the brief describing of results of grain boundary diffusion in Cu with following formulation of rules which can be established on the base of the analysis of the results is presented. Experimental results which are described here concern the attempt to change the effect negative segregation by microalloying by sulfur which did not change the situation and the diffusion through foil which allowed to demonstrate the absence of accelerated GB diffusion without specific sample preparation. It is shown that GBs do not give the additional effect to the flux of Fe and Co through the foil. The extended model of surface tension gradient as an additional driving force is described.

Abstract: According to different experimental data the grain boundary diffusion the triple product (P) can change in opposite directions after alloying. In this paper the analysis of the effect of alloying for different systems is proposed. It was shown that in Al-based system (Cu diffusion in Al, Zn diffusion in Al) the P value increases with alloying while the solidus temperature according to the phase diagram decreases, in other systems no such tendency can be seen. Estimations based on the segregation factor value demonstrate that some structural effect must be proposed in addition to describe the experimental results.

Abstract: The model of diffusional mass transfer in the medium with a strong spatial dependence of diffusivity and its application to the grain boundary (GB) diffusion problem is presented. A significant decrease of diffusion activation energy is shown to take place takes place in the vicinity of non-equilibrium grain boundary, which leads to the formation of a region of enhanced diffusion. The generalization of grain boundary diffusion theory is given which accounts for the diffusion enhancement near GB. An original mathematical approach is developed and general asymptotic solutions of the one-and two-dimensional diffusion problems are derived for two types of diffusant source — constant and instant. The applicability domain of presented model is discussed.

Abstract: The classical Fisher model of grain-boundary diffusion and the traditional method of determination of grain-boundary diffusion parameters by radiotracer technique combined with the serial-sectioning method are analyzed. The Fisher model specification based on the data of the emission Mössbauer spectroscopy is considered, and the additional information which can be extracted from the Mössbauer studies is discussed. The possibility of determination of grain-boundary diffusion parameters based on the combined analysis of the radiotracer technique and Mössbauer spectroscopy with the application of the specified Fisher model of grain-boundary diffusion is considered. This approach is demonstrated by an example of determination of grain-boundary diffusion of Co in W and Mo.

Abstract: It is generally well recognized that in the course of a grain boundary (GB) diffusion experiment the diffusion of solute atoms in grain boundaries must exhibit a strong time-dependent segregation. But there has been no clear understanding of exactly how this time dependence develops. In this chapter, we review and analyse transient solute GB diffusion by means of the computer simulation technique of Lattice Monte Carlo (LMC). This technique has been successfully used on numerous occasions for the purposes of systematically studying the GB transition regimes that occur between the principal well-defined Harrison GB kinetics regimes (A, B and C-Types). Recently, the analysis using LMC has been extended to the case of solute GB diffusion when the segregation factor is independent of time. In the present paper, we analyse two cases of solute segregation in GB diffusion: first, where the solute atoms are homogeneously distributed along the tracer source plane but their mobility is not high at this plane; and the second, where the mobility of the solute atoms along the tracer source plane is comparable to their mobility along the GB. It is shown that the time dependence of the segregation can contribute significantly into the resulting values of the triple-product that is usually obtained experimentally in the Harrison Type-B kinetics regime.

Abstract: The influence of Co as an alloying element on grain boundary diffusion (GBD) in Cu attracts particular interest due to anomalous GBD of Co in Cu. Ni as a neutral to Co and Cu element was chosen for GBD study. The triple products of Ni GBD in Cu and Cu-Co alloys (with concentration up to 2.9 wt. %) were determined in temperature range 500 – 700 °C by X-ray microprobe analysis. It was shown, that in spite of some scattering the triple product does not depend on Co concentration at all temperatures of experiments. From the obtained results it follows that Co does not change the GB structure.

Abstract: Grain boundary diffusion of Co in Mo has been studied by radiotracer analysis and emission Mössbauer spectroscopy. The experimental results are treated based on the specified Fisher model. The mechanism of grain boundary diffusion of Co in Mo and temperature dependences of segregation factor and grain boundary diffusivity have been determined.