OSCILLATIONS INDUCED BY PURELY DIFFUSIVE PROCESSES IN PLANAR DIRECTIONAL SOLIDIFICATION FROM THE MELT

The planar growth of directional solidification from a binary melt is analyzed taking into account thermodiffusive transport. Both stable an d overstable planar growth regimes are considered in the case of dilut e alloys and as far as convective process does not modify the extent o f the solute transition zone (the diffusion boundary layer) that appea rs in the absence of convection. Under such conditions, thermodiffusio n appears as a thermophoretic transport process. The extent of the dif fusion boundary layer is divided by a factor mu(c) = 1 + D(T)G(l)/V, w here D-T is the thermodiffusion coefficient, G(l) the interface therma l gradient in the melt and V the growth rate. The solute profile is mo dified accordingly. Planar overstable modes under the form of striatio ns, are predicted. They appear with a period of the order of the ratio of the isothermal diffusion coefficient divided by the square of the growth rate. Such instabilities are expected in constitutionally super cooled melts for systems with large positive liquidus slopes and Soret coefficient for a threshold value of the solute concentration.