Role of back-diffusion studied by computer simulation

The solidification of Fe-Cr-C alloys was simulated with a program that allowed realistic treatment of diffusion in the liquid and the primary solid phase. It interacts with a thermodynamic databank and can evaluate the heat extraction continuously. Solidification under a constant rate of heat extraction can be simulated. In agreement with previous experience, diffusion of carbon in the fcc phase was found to be very effective, and it can be approximated by an infinitely fast diffusion. The same holds for carbon and chromium in the liquid. The back-diffusion of chromium in the fcc phase was examined in detail. Three different geometries were tested: the planar and cylindrical cases, as well as a new model based on two coupled cylinders. The simulation program allowed the effect of back-diffusion after the end of solidification to be included, and it was found to be important for the segregation ratio.