Cellular Morphological Instabilities During Directional Solidification: Thin Sample Experiments

Abstract

Metallurgists are familiar with the existence of morphological instabilities₁which deform the solid-liquid interface of binary alloys during directional solidification. In this situation, samples are pulled at velocity V in a temperature gradient G so that they are progressively solidified. The equilibrium concentration of the solute in the liquid is usually different from that of the solid - often higher, as with the compounds studied here, but sometimes lower. Then an excess or a lack of solute builds up in front of the moving interface and must be evacuated to allow solidification to progress. Above a threshold pulling speed, for given temperature gradient and concentration, the interface becomes unstable and breaks down into a regular, periodic pattern ( Fig. 1 ).