From Acta

November 28, 2010

Modelling of the influence of the vacancy source and sink activity and the stress state on diffusion in crystalline solids

Svoboda and Fischer

Diffusion in solids is a well-known phenomenon that has many consequences in technology and material science. Modelling of diffusion-controlled processes requires both a reliable theory of diffusion and reliable kinetic coefficients, as well as other thermodynamic data. Often the classical Darken theory, valid for stress-free systems with ideal vacancy source and sink activity, is generalized to multicomponent systems with ideal vacancy source and sink activity. Nazarov and Gurov presented a theory for stress-free systems with no vacancy source and sink activity. Recently we published a general theory of diffusion that accounted for the role of non-ideal vacancy source and sink activity, as well as the stress state. Since diffusion theories are tested and diffusion coefficients measured usually on diffusion couples, this paper presents evolution equations based on that general theory for a diffusion couple. In the limit, the equations of the Darken theory and the Nazarov and Gurov theory are valid for ideal vacancy source and sink activity and no vacancy source and sink activity, respectively. Simulations for binary and ternary diffusion couples demonstrate the influence of the vacancy source and sink activity and the stress state on evolution of site fraction profiles of components and vacancies, and on the Kirkendall effect.

Influence of anisotropy on heterogeneous nucleation

Mariaux and Rappaz

Heterogeneous nucleation is governed by the interplay of interfacial energies between a substrate, a solid and a liquid. Although the intensity of these energies can strongly change with the orientation of the nucleus for anisotropic media, this parameter is not taken into account in the available nucleation theories. In this paper, the Gibbs free energy barrier for nucleation is computed for an arbitrary solid–liquid interface energy. It is shown that anisotropy favors particular orientations of the nucleus on the substrate. Experimental evidence from the zinc–aluminum system is given as an application of this extended nucleation theory. It also sheds new light on the texture of galvanized steel sheets.

On the role of initial conditions in the selection of eutectic onset mechanisms in directional growth

Serefoglu and Napolitano

The early-stage dynamics and onset mechanisms for eutectic solidification are investigated experimentally using slab-geometry slides of succinonitrile–(D)camphor (SCN–DC) transparent organic eutectic material. By specifically focusing separately on the pre-growth or holding period and the growth or pulling period, the critical roles of each in the establishment of initial conditions and the competition between eutectic initiation mechanisms, leading to the development of a steady-state eutectic front, are examined. It is found that a single-phase layer forms and increases in thickness monotonically with time during the holding period with a corresponding increase in the interface temperature. Because the thickness of this layer is observed to influence subsequent eutectic initiation mechanisms, it is concluded that the pre-existing structure, holding period duration, single-phase identity and thickness, and specimen slide geometry should all be reported as standard practice, along with the pulling velocity and thermal gradient, for a complete description of a gradient-zone directional solidification experiment.

Interaction between an edge dislocation and a circular inclusion with interface slip and diffusion

Wang and Pan

We investigate in detail the transient response induced by an edge dislocation near a circular elastic inclusion with simultaneous interface slip and diffusion. A rigorous solution to the interaction problem is derived in series form. As the time approaches infinity, our solution just recovers the classical one derived by Srolovitz et al. (Acta Metall 1984;32:1079) for fully relaxed boundary conditions. In addition, we observe that the edge dislocation will induce a uniform rigid-body rotation in the inclusion as the time approaches infinity. When the dislocation is far away from the inclusion, simple asymptotic expressions of the glide and climb forces on the dislocation are also obtained. Furthermore, five extreme cases for the imperfect interface are discussed; in particular, we derive approximate closed-form expressions of the decaying internal stress field within the inclusion and the image force on the dislocation for long-range stress relaxations when the interface diffusion occurs much faster than the interface slip and vice versa. Some interesting physical behaviors are observed.

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