Morphological stability of the planar solid-liquid interface

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Abstract
An experimental investigation of the morphological stability of the planar solid-liquid interface was carried out in which, for the first time, all pertinent solidification parameters were determined quantitatively and the effect of convenction was evaluated. The microscopic growth rate and interface morphology were determined through solidification by interface demarcation; thermal characterization of the melt was achieved by direct probing, and compositional profiles were obtained by single point spreading resistance. solidification was carrried out in a stationary vertical Bridgman-type apparatus designed for minimal convective interference. It was found that the interface morphology at the onset of instability is characterized by near-sinusoidal undulations with amplitudes that increase exponentially with time and undergo harmonic distortion as instability develops. Compositional maxima and minima at depressions and protrusions, respectively, across the unstable interface were observed consistent with steady-state equilibrium behavior. When the tangent to the unstable interface became parallel to 〈111〉 planes, microfaceting took place which effectively arrested the development of the instability in the light of current theoretical models, it was concluded that dynamic rather than equilibrium (thermodynamic) criteria control the stability-to-instability transition. Measured dynamic parameters, i.e., the wavelength of the fastest growing perturbation and the time constant, were found to be in very good agreement with theory. However, an inconsistency between experimental and theoretical values of the diffusivity parameter was found which indicates delocalized rather than localized initial perturbations not appropriately described by approximate theory. The heat of fusion and the difference in thermal conductivity of the solid and liquid were shown to promote morphological stability in the presence of constitutional supercooling. The development of a transient instability superimposed on the original instability not described by existing linear or nonlinear theories was attributed to the growth of a second harmonic undulation.