EUTECTIC GROWTH - A MODIFICATION OF THE JACKSON AND HUNT THEORY

The Jackson and Hunt theory of eutectic growth is modified in order to get a better understanding of the physical mechanisms driving the solidification process. The density difference between the eutectic phases is taken into account when calculating the diffusion field, and the isothermal interface coupling condition is replaced by an equilibrium criterion for the three-phase junction. Although the problem can be solved in the general case, a simpler solution is derived for the case of nearly isothermal interface, which differs from that of Jackson and Hunt only by a corrective factor reflecting the density differences between the phases, the correction being quite large in the case of cast iron. The volume fractions of the eutectic phases are shown to be adjusted by the movement of the three-phase junctions. The boundary layer composition, which depends on this mechanism, is shown to be responsible for the well-known occurrence of austenite halos surrounding the primary graphites in cast iron. The curvature undercooling is written as the ratio of the volumetric interface energy contained in the eutectic microstructure to an average volumetric entropy of melting, which allows a quantitative evaluation of the effect of an anistropy in the alpha/beta interface energy. Reliable values of the relevant physical constants are provided for several important eutectic alloys.