Theory of Metastable Crystallization of Supercooled Eutectic Melt

Abstract

A new theory of crystallization of a supercooled metastable melt, based on the variational principles of mechanics, was developed. The theory takes into account the regularities of the dif-fusion growth of equilibrium crystals, as well as the regularities the diffusionless growth of meta-stable crystals. Mathematical modeling showed that in the supercooled melt Fe83B17 there is sim-ultaneous nucleation and growth of the stable Fe and Fe2B phases with the metastable Fe3B phase. The growth rate of crystals of near-critical sizes of the Fe3B phase exceeds the growth rate of the Fe and Fe2B crystals. For the Fe3B crystals the effect of diffusionless growth has been ob-served; in this case, the rapidly growing surface of the Fe3B crystal captures the boron atoms. On the basis of the developed theory, a quasi-equilibrium phase diagram for a supercooled Fe-B melt was constructed. The phase diagram takes into account both the equilibrium crystal growth and the growth of the metastable phase. The resulting diagram makes it possible to pre-dict the values of the concentration of components at the surface of growing crystals for the Fe and Fe2B crystals, for which the condition of local equilibrium is observed on their surface, and for the metastable Fe3B crystals, whose diffusionless growth is due to the high speed of the crys-tal surface movement. The performed mathematical modeling of the nucleation and growth of crystals in the over-cooled eutectic melt Fe83B17 showed that in a supercooled melt there is simultaneous nucleation and growth of the stable Fe and Fe2B phases with the metastable Fe3B phase. The regularities of nucleation and growth of the metastable Fe3B phase have been studied. The growth mechanism of the Fe3B nuclei differs from the growth of the Fe and Fe2B nuclei due to diffusionless capture of the boron atoms by the growing surface of the Fe3B crystals.