Relation Between Bulk and Interface Descriptions of Alloy Solidification

From a simple bulk model for the one-dimensional steady-state solidification of a dilute binary alloy we derive an interface description, allowing arbitrary values of the growth velocity. Our derivation leads to exact expressions for the fluxes and forces at the interface and for the set of Onsager coefficients. We, moreover, discover a continuous symmetry, which appears in the low-velocity regime, and there deletes the Onsager sign and symmetry properties. An example is the generation of the sometimes negative friction coefficient in the crystallization flux-force relation

Diffusion-Induced Oscillations of Extended Defects

From a simple model for the driven motion of a planar interface under the influence of a diffusion field we derive a damped nonlinear oscillator equation for the interface position. Inside an unstable regime, where the damping term is negative, we find limit-cycle solutions, describing an oscillatory propagation of the interface. In case of a growing solidification front this offers a transparent scenario for the formation of solute bands in binary alloys, and, taking into account the Mullins-Sekerka instability, of banded structures

Capillary-Wave Description of Rapid Directional Solidification

A recently introduced capillary-wave description of binary-alloy solidification is generalized to include the procedure of directional solidification. For a class of model systems a universal dispersion relation of the unstable eigenmodes of a planar steady-state solidification front is derived, which readjusts previously known stability considerations. We, moreover, establish a differential equation for oscillatory motions of a planar interface that offers a limit-cycle scenario for the formation of solute bands, and, taking into account the Mullins-Sekerka instability, of banded structures

Capillary-Wave Model for the Solidification of Dilute Binary Alloys

Starting from a phase-field description of the isothermal solidification of a dilute binary alloy, we establish a model where capillary waves of the solidification front interact with the diffusive concentration field of the solute. The model does not rely on the sharp-interface assumption, and includes non-equilibrium effects, relevant in the rapid-growth regime. In many applications it can be evaluated analytically, culminating in the appearance of an instability which, interfering with the Mullins-Sekerka instability, is similar to that, found by Cahn in grain-boundary motion.Comment: 17 pages, 12 figure

Vorwort

Bei der Diskussion über die Förderung und Ausbildung von institutionell gesteuerten Mehrsprachigkeitsprofilen nehmen Formen des bilingualen Lehrens und Lernens bzw. des Einsatzes von Fremdsprachen als Arbeitssprachen im Sachfachunterricht eine vorrangige Stellung ein. Bilinguale Lehr- bzw. Lernangebote zielen darauf ab, im Vergleich zum herkömmlichen Fremdsprachenunterricht intensivierte und zugleich sach- bzw. fachorientierte - und damit differenzierte - Fremdsprachenkenntnisse zu vermitteln