Stability of lamellar eutectic growth in thick samples

The growth front of a transparent non-faceted eutectic (CBr4–C2Cl6) is observed from the top in real time in thick samples during directional solidification. We study the stability of the lamellar patterns at near-eutectic concentrations as a function of the spacing λ and the pulling velocity V. The domain of stability of the symmetrical (straight) lamellar patterns is limited by an Eckhaus instability followed by eliminations of lamellae at λ/λ m ≈ 0.7, where λ m ∝ V −0.5 is the minimum-undercooling spacing, and a zigzag bifurcation at λ/λ m ≈ 0.85. A lamella break-up instability occurs at λ/λ m ≈ 1.1. Wall effects and topological defects play a major role during transients. No important anisotropy effect is observed, except near eutectic grain boundaries.