Authors: Lyudmila N. Paritskaya, Yuri S. Kaganovsky, V.V. Bogdanov
Abstract: The phenomenon of low-temperature homogenization (LTH) during interdiffusion is studied under condition a t Dv £ 2 / 1 ) ( (Dv is the bulk diffusion coefficient, a is the lattice parameter) using nano-objects of binary Cu-Ni and Cr-Ni systems compacted from nano-powders and produced by mechanical alloying. Two stages of LTH were detected: at the first stage (t £ 103 s) the volume fraction of solution rapidly grows; at the second stage (t > 103 s) the volume fraction of solutions grows slowly with practically constant average solution concentration. The first stage of LTH correlates with active grain growth caused by small size (l) of structural element and nonequilibrium structure of nano-objects. Obtained results are analyzed theoretically in terms of interdiffusion along migrating GBs due to grain growth at the first stage and DIGM mechanism at the second stage. It is shown that the GB concentration distribution during interdiffusion along migrating GBs and the kinetics of LTH depend on a parameter l/l where 2 / 1 ) / ( b b V sD d l= is the
characteristic diffusion length. The mechanisms and criteria of LTH are proposed.
123
Authors: Jie Dong, Z.F. Li, Xiao Qin Zeng, Wen Jiang Ding
Abstract: Intermediate phase growth in Mg-Al diffusion couples were studied with different intensity
of a strong static magnetic field from 0 to 10 Tesla. Thickness measurement of the intermediate
phases (Mg17Al12 and Al3Mg2) shows that with the increasing of magnetic field intensity, the growth
rate of both intermediate phases is retarded. The decrease of the phase growth rate is ascribed to the
suppressed Al, Mg atom interdiffusion in the diffusion couple under the static magnetic field. It is also
found that the orientation relationship between couple interface and magnetic field direction has no
influence on the growth of intermediate phases.
491
Authors: Vladimir V. Popov
Abstract: Possibilities of grain-boundary diffusion and segregation studies using nuclear gammaresonance
spectroscopy (NGR) are considered. It is shown that the results of the Mössbauer
investigations testify the necessity to specify the classical Fisher’s model of grain-boundary
diffusion, and a possible way of such specification is suggested. It is demonstrated that
investigation of grain boundaries using emission Mössbauer spectroscopy appreciably supplement
the information obtained from the diffusion profiles analysis. In particular, Mössbauer
investigations make it possible to evaluate directly the grain-boundary segregation factor, to
determine the grain-boundary diffusion mechanism, to estimate the rate of the diffusant pumping
from a grain boundary core into the bulk, etc.
497
Authors: Michail Michailov
Abstract: The present study deals with diffusion behavior of adsorbed atoms on stepped crystal surfaces. In volume-immiscible systems, two-dimensional (2D) atomic intermixing at epitaxial interface could be completely blocked on step-free surface domains. This is a result of high diffusion barrier for direct atomic exchange between adsorbed layer and substrate. In that case, diffusion takes place exclusively across the steps of atomic terraces. In such systems, dynamic competition between energy gain by mixing and substrate strain energy results in diffusion scenario where adsorbed atoms form alloyed stripes in the vicinity of terrace edges. At high temperatures, the stripe width increases and finally completely destroys the terraces. This process leads to formation of alloyed 2D atomic islands on top of pure substrate layer. The atomistic Monte Carlo simulations reveal vacancy-mediated mechanism of diffusion inside atomic terraces as a result of spontaneous generation of vacancies at high temperature. Being in agreement with recent experimental findings, the observed surface-confined alloying opens up a way various surface pattern to be configured at different atomic levels on the crystal surface.
121
Authors: Alain Portavoce, Ivan Blum, Lee Chow, Jean Bernardini, Dominique Mangelinck
Abstract: The measurement of diffusion coefficients in today’s materials is complicated by the down scaling of the studied structures (nanometric effects in thin films, nano-crystalline layers, etc.) and by the complex production process conditions of industrial samples or structures (temperature variations, complex solute and point defect distributions, stress gradients, etc.). Often diffusion measurements have to be performed in samples for which initial experimental conditions do not offer the possibility of using conventional diffusion analytical solutions. Furthermore, phenomena involved with diffusion are sometimes so numerous and complex (stress, matrix composition inhomogeneities, time dependence of point defect generation sources, electrical effects, clustering effects, etc…) that the use of analytical solutions to solve the observed diffusion behavior is difficult. However, simulations can be of use in these cases. They are time consuming compared to the use of analytical solutions, but are more flexible regarding initial conditions and problem complexity. The use of simulations in order to model physical phenomena is quite common nowadays, and highly complex models have been developed. However, two types of simulations have to be considered: i) simulations aiming to understand and predict phenomena, and ii) simulations for measurement purposes, aiming to extract the (average) value of a physical parameter from experimental data. These two cases have different constrains. In the second case, that is the subject of this article, one of the most important stress is that the simulation has to precisely scale the experiment (sample size, experiment duration, etc.), sometimes preventing the measurement due to computational time consumption. Furthermore, the simpler the model (small number of parameters) used in the simulation, the more relevant the measurement (minimum error). In this paper, examples of recent works using two- and three-dimensional finite element simulations for diffusion coefficient measurements in thin polycrystalline films and nano-crystalline layers are presented. The possible use of simulations for diffusion coefficient measurements considering GB migration, GB segregation, or triple junctions is also discussed.
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