Symmetry-broken double fingers and seaweed patterns in thin-film directional solidification of a nonfaceted cubic crystal

Abstract

We present a detailed experimental and numerical investigation of the directional-solidification growth patterns in thin films of the CBr4-8 mol% C2Cl6 alloy, as a function of the orientation of the (fcc) crystal with respect to the solidification setup. Most experiments are performed with single-crystal samples about 10 mm wide and 15 mu m thick. The crystal sometimes contains small faceted gas inclusions, the shape of which gives us direct information about the orientation of the crystal. Numerical simulations by a fully dynamical method are carried out with parameters corresponding to the experimental system. We find experimentally that, in crystals with a (111) plane (nearly) parallel to the plane of the thin film, the growth pattern is nondendritic and unsteady over the explored velocity range (5Vc-50Vc; Vc approximately=1.9 mu m s-1 is the cellular threshold velocity). By studying the time evolution of this pattern, we establish that it is essentially similar to the "seaweed pattern" characteristic of vanishingly small capillary and kinetic anisotropies of the solid-liquid interface, recently studied numerically (T. Ihle and H. Muller-Krumbhaar, Phys. Rev. E 49, 2972 (1994)). The building blocks of this pattern are local structures-pairs of symmetry-broken (SE) fingers called "SE double fingers" or "doublons", and more complex structures called "multiplets"-whose lifetime is long but finite. We show experimentally that, in agreement with numerical findings, doublons obey selection rules, but do not have a preferential growth direction. We furthermore find that "dendritic" doublons also appear in crystals with a (100) axis close to the pulling direction (thus having a strong two-dimensional anisotropy) above a critical velocity ( approximately=20 Vc). The existence and stability of dendritic doublons in directional solidification at high velocity are confirmed by the simulations. Another crystal orientation of interest is that in which two (100) axes are symmetrically tilted at +or-45 degrees with respect to the pulling axis. We show experimentally and numerically that the nondendritic unsteady "degenerate" pattern observed in this case, and previously noticed by F. Heslot and A. Libchaber (Phys. Scr. T9, 126 (1985)), is qualitatively different from the seaweed pattern. In crystals close to this orientation, we find experimentally transitions between the degenerate and the two possible tilted dendritic states. The small amplitude of the sidebranches of dendrites in our system and the fact that the simulations with a purely capillary anisotropy do not reproduce these transitions lead us to attribute them to the increasing effect of kinetic anisotropy as the pulling velocity increase

Dendritic Growth Morphologies in Al-Zn Alloys—Part II: Phase-Field Computations

“…This phenomenon occurs because both anisotropy components are relatively weak, and for this case, their effects are also nearly equal in magnitude. This is the same mechanism that was found by Akamatsu et al [32] to produce seaweeds in 2-D thin samples.…”

Seaweed to Dendrite Transition in Directional Solidification

“…A long-standing problem has been to elucidate the mechanism of wavelength selection in such cellular or dendritic arrays. This problem has been extensively examined experimentally [6 -15], and theoretically [6,10,16 -20] using boundary integral methods, phase-field models, and semiempirical thermodynamic considerations.Another class of directionally solidified microstructures recently examined experimentally [6,15] and numerically [6,21] is known as seaweed. These structures are formed through successive tip splitting of primary branches of the solidification front.…”

“…Another class of directionally solidified microstructures recently examined experimentally [6,15] and numerically [6,21] is known as seaweed. These structures are formed through successive tip splitting of primary branches of the solidification front.…”

Surface effects in nucleation and growth of smectic-Bcrystals in thin samples

“…It then melts partly. The non-melted part of the sample is a polycrystal, but a large single crystal can be grown from it using funnel-shaped samples [10,24]. After a certain time (about 30 minutes) of maintain at rest (V = 0) in order to homogenize the liquid, the solidification is started at a given velocity.…”

“…The TDS method has been used extensively for the study of nonfaceted growth [24,26,27,28,29,30,31], but rarely for that of faceted growth. In general, faceted crystals exhibit many facets, the growth of which occurs far from equilibrium and is very sensitive to the structure of the interface on a molecular scale and to lattice defects [25].…”