The Mullins-Sekerka instability in directional solidification of quasi-azeotropes

2014 We investigate the stability of the planar front during directional solidification of a quasi-azeotrope. We show that the corresponding shape of the phase diagram induces a strong reduction of the scale of the bifurcation curve which makes its complete exploration feasible in standard directional solidification experiments. We show that the finite concentration of azeotropic mixtures results in a « chemical capillary term » in the equation relating interface concentrations. This effect, which turns out to be negligibly small for metallic azeotropes, should be significant for the 3He-4 He azeotrope where it might give rise to a qualitative change of the bifurcation curve. We study the nature (normal or inverted) of the bifurcation with the help of a third order amplitude expansion. We show that, for metallic alloys, thanks to the reduction of scale of the instability diagram, the normal bifurcation regime 2014 which cannot be easily reached with dilute mixtures 2014 becomes accessible to standard experiments. J. Physique 47 (1986) 1077-1090 JuIN 1986, Classification Physics Abstracts 61.SOC 64.70