PERITECTIC FORMATION IN THE NI-AL SYSTEM

Directional solidification studies have been carried out on Ni-Al allo ys at compositions in the two-phase region of the two phases, gamma an d gamma', that form the peritectic in this system. It was found that u nder certain conditions of G/V, the two phases form with the banded st ructure found in previous directional solidification studies of perite ctic systems. However, under other G/V conditions the two peritectic p hases were found to grow into the liquid along an isothermal front lyi ng just below the peritectic temperature. The growth front resembles t hose of lamellar and rod eutectic alloys because it lies as close to a n isotherm as the growth fronts observed in eutectic alloys. However, the two solid phases display a morphology resembling that of classical two phase cellular growth where a second phase forms in-between the p rimary cells. The structure has been termed cellular coupled growth, a nd its morphology and the conditions for its formation are characteriz ed and compared to eutectic and cellular growth morphologies.