ANISOTROPY-DRIVEN DYNAMICS OF CELLULAR FRONTS IN DIRECTIONAL SOLIDIFICATION IN THIN SAMPLES

We present an experimental investigation of the influence of interfaci al anisotropy on the cellular growth patterns observed in directional solidification of the CBr4-8 mol % C2Cl6 alloy at pulling velocities l ess than twice the cellular-threshold velocity. The experiments ate pe rformed with single-crystal samples about 8 mm wide and 12 mu m thick. In such samples, the solidification dynamics is essentially two dimen sional, and the effective anisotropy of the system can be varied by ch anging the orientation of the (face centered cubic) crystal with respe ct to the solidification setup, as was previously established by S. Ak amatsu, G. Faivre, and T. Ihle [Phys. Rev. E, 51, 4751 (1995)]. We fin d that the cellular pattern is unstable at all values of the spacing l ambda for crystal orientations corresponding to a vanishing effective interfacial anisotropy. For the other crystal orientations, i.e., for a nonvanishing effective interfacial anisotropy, stable cellular patte rns are found over a finite width lambda range. The various modes of i nstability limiting this range are described. In particular, we show t hat a homogeneous tilt bifurcation exists for some orientations of the ''degenerate'' type (i.e., such that a {110} plane of the crystal is parallel to the growth direction and perpendicular to the sample plane ). This bifurcation is not spontaneous, however, but a consequence of the particular symmetry of the effective interfacial anisotropy of the system for this crystal orientation.