Lamellar eutectic growth of CBr 4-C 2Cl 6: effect of crystal anisotropy on lamellar orientations and wavelength dispersion

Abstract

Directional solidification samples of eutectic alloys generally appear divided into numerous “eutectic grains”, i.e. regions of uniform orientation of the two solid phases. Differences of behavior from grain should provide information about the effect of crystal anisotropy on solidification dynamics of eutectics. In thin samples of CBr 4-C 2Cl 6, we observed two types of grains: “locked”, or epitaxial, grains and “floating” grains, with no epitaxy between the two solid phases. We present a systematic experimental study of the stationary basic state in floating grains. The parity of this state appears generally broken: within a grain, the lamellae are tilted to a small (⪅ 5°), but definite angle Φ S with respect to the pulling direction. Φ S and the wavelength λ S are grain dependent. The interpretation of this tilting as an effect of the weak anisotropy of the interface energies is supported by a perturbative analytical calculation, completely developed in the limit of small triple-point pinning angles. The phase diffusion coefficient ¢D(λ) is also calculated in this limit to account for the observed slowness of phase diffusion. Eutectic grain boundaries are permanent sources of instabilities and of λ-gradients which do not spread out by phase diffusion for the duration of experiments, and the principal source of λ dispersion.