Orientation selection in solidification patterning

Abstract

Crystal growth has been recognized as a paradigm for non-equilibrium pattern formation for decades. Scientific interest in this field has focused on the growth rates and curvature of branches in snowflake-like structures patterned after a solid’s crystallographic orientations. In reality, there exists a much richer variety of crystal patterns in nature. Investigations of dendritically solidifying alloys reveals structures that continuously change orientation between different growth directions, some of which are not along preferred crystallographic directions. The selection mechanism of such patterns is poorly understood. In this paper we demonstrate computationally and experimentally that a material’s surface tension anisotropy can compete with anisotropies present in processing conditions during solidification to produce a continuous transition from dendritic to seaweed and fractal-like structures. The phase space of such morphologies is characterized and the selection principles of the various morphologies explored are explained. These results have direct relevance to microstructure formation in commercial lightweight metal castings.