Pinning/depinning dynamics of trijunction lines during faceted/nonfaceted eutectic growth

We present an in situ experimental study of decoupled- and coupled-growth patterns in a binary faceted/ nonfaceted (f/nf) eutectic alloy. Real-time optical observation is carried out at low growth velocity $V$ ($<0.3\mu {\mathrm{ms}}^{-1}$) during directional solidification of the transparent AminoMethylPropaneDiol-Succinonitrile (AMPD-SCN) eutectic in thin samples. On a large scale, the two-phase growth front mostly exhibits irregularly spaced decoupled-growth patterns involving faceted fiber-like AMPD crystals that protrude in the liquid ahead of, and in weak diffusive interaction with the nonfaceted SCN solid. The morphological stability of low-velocity, steady-state decoupled-growth patterns is evidenced. In addition, three types of (f/nf) coupled-growth patterns with fiber-like, tubular (or “hollow”) and C-shaped morphologies, respectively, are identified. New light is cast on the pinning/depinning dynamics of trijunctions at which the two solid phases and the liquid meet in equilibrium. A discussion is initiated on the interplay between the interfacial kinetics of the more or less mobile facets and the solute diffusion field, and on the stability of triple junctions with strongly anisotropic interfaces.