TRANSITIONS BETWEEN TYPE-A FLAKE, TYPE-D FLAKE, AND CORAL GRAPHITE EUTECTIC STRUCTURES IN CAST IRONS

Directional solidification experiments were used to measure the transi tion velocities between the type A and coral eutectic structures in hi gh-purity cast irons and between the type A and type D eutectic struct ures in S and Te doped cast irons. Introduction of O into the gas atmo sphere was found to have little effect on the A double right arrow D t ransition velocities in S doped alloys, but it produced a strong reduc tion in the A double right arrow coral transition velocities in high-p urity irons. Transmission electron microscopy revealed interesting var iations in the defect structures of the graphite in the flake irons vs the type of flake (A or D) and the type of doping element. Scanning A uger microscopy demonstrated that both S and Te segregate to the iron/ graphite interface. In the S doped alloys, type A flakes are generally covered with a monolayer of S with patches of O in the form of iron o xide having a thickness on the order of 2 nm. A series of experiments, including examination of fracture surfaces at the quenched solid/liqu id growth front, have shown that S segregates to the iron/graphite int erfaces from the liquid at the growth front, but O forms at these inte rfaces during the cooldown. These results are discussed in relation to current models of eutectic growth in cast irons.