Nonstationary Cell Shapes in Directional Solidification
In directional solidification, a sample, initially liquid, is pulled at a given velocity in a temperature gradient in such a manner as to solidify it progressively. If the material is not pure — in the present case we consider dilute binary alloys — the solid liquid interface presents a cellular instability above a certain critical pulling speed (Langer 1970). This deformation is associated with the segregation of solute in the cusps behind the cells. The mechanism behind the instability was explained by Mullins and Sekerka (1964): it is the competition between the destabilizing effect of solute diffusion and the stabilizing effects of the temperature gradient and the interfacial tension. To allow direct observation of the phenomenon, we use thin (50 μ) samples of a transparent material, tetrabromomethane, containing 0.12% excess brome. The experimental set-up is described elsewhere (de Cheveigné et al, 1986).
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