Free growth and instability morphologies in directional melting of alloys.

The dynamics of melting morphologies, namely, liquid droplets in the bulk solid and liquid dendrites due to morphological instability of the phase boundary, is observed in situ and in real time during directional melting of transparent succinonitrile-acetone alloys in a cylinder. Specific patterns are associated to grain boundaries. A model based on free growth but with time-dependent superheating is proposed for the lateral growth of the liquid inclusions. Contrary to what is largely believed, it is shown that free melting is not the mere reversal of free crystal growth, basically because solute diffusion is much lower in the solid, which imposes a boundary layer approach.