Dynamical polygonization below the cellular-bifurcation threshold in thin-sample directional solidification

We show that a polygonization of the crystal (formation of grain subboundaries) takes place during the growth in thin-sample directional solidification of a transparent nonfaceted alloy at solidification rates lower than the cellular-bifurcation threshold. We give experimental evidence that this phenomenon is due to a dynamical coupling between the lattice defects intersecting the growth front and the diffusive modes of instability of the front. We present a theoretical model of this process in two-dimensional systems.