Observation expérimentale de la morphologie "en algue" et d'états asymétriques appariés en solidification directionnelle de films minces d'alliages CBr4-C2Cl6

Groupe de Physique des Solides, Universités Paris 7 et Paris 6, URA 17 du CNRS, Tour 23, 2 place Jussieu, 75251 Paris cedex 05, France

Abstract

The morphology of the growth in the thin-film directional solidification of a CBr4 - 8 mol% C2Cl6 alloy is studied for a particular crystalline orientation, namely, where a <111> axis is perpendicular to the plane of the film. In this case, the effective surface tension as well as attachment kinetics anisotropies are very weak, and a disordered, "seaweed" structure is observed. In good agreement with recent numerical studies by Ihle and Müller-Krumbhaar (Phys. Rev. E 49 2972 (1994)), we show that this disordered structure is mainly composed of pairs of asymmetric fingers. This pairing phenomenon is in fact generic of all crystal orientations and is as well observed in standard, highly anisotropic grains, for higher velocities and thus higher undercoolings, as predicted numerically. In addition to the pulling velocity and the effective anisotropy in the plane of the experiment, we point out the relevance of the cellular spacing as a determinant parameter for the generation of symmetric or paired asymmetric structures. We also discuss the differences between the periodic patterns found numerically and the disordered morphologies observed experimentally.