Variational theory of crystal growth and its application for analysis of forming processes for metastable phases in overcooled metallic melts with eutectic composition

Abstract

The new crystallization theory for overcooled metastable melt is developed; it is based on variational mechanical principles and takes into account regularities of forming and diffusion growth of equilibrium crystals as well as diffusion-free growth of metastable crystals. Calculations for the melt Fe83B17 were conducted on the model; they displayed that simultaneous nucleation and growth of Fe and Fe2B with metastable phase Fe3B are observed in overcooled melt, and growth speed of near-critical dimensional crystals of Fe3B exceeds crystal growth speed of Fe and Fe2B. The effect of diffusion-free growth is observed for Fe3B crystals, when quickly growing Fe3B crystal surface catches boron atoms. Quasi-equilibrium phase diagram for overcooled Fe-B melt was built on the base of the developed theory; it takes into account both equilibrium crystal growth and metastable phase growth. The obtained diagram allows to predict the values of components concentration near the surface of growing crystals both for Fe and Fe2B crystals meeting the requirements of local equi-librium condition on their surface and for Fe3B metastable crystals which are characterized by diffusion-free growth stipulated by high motion speed of crystal surface.