RAPID DENDRITIC AND EUTECTIC SOLIDIFICATION OF UNDERCOOLED CO-MO ALLOYS

Bulk samples of Co-26.3%Mo hypoeutectic, Co-35%Mo hypoeutectic, Co-37.6%Mo eutectic and Co-40%Mo hypereutectic alloys were undercooled by up to 227 K, 112 K, 89 K and 81 K respectively. The crystal growth velocities in the Co-26.3%Mo hypoeutectic and Co-37.6%Mo eutectic alloys were measured as a function of the undercooling, and also were calculated on die basis of current dendritic and eutectic growth theories. A ''lamellar eutectic-anomalous eutectic'' microstructural transition was observed when undercooling exceeded a critical value of about 60 K. The most probable mechanism for this transition is that the alpha-Co phase grows substantially faster than the epsilon-muCo7Mo6 phase and destroys the coupled growth conditions at large undercoolings. In addition, the coupled zone of the Co-Mo eutectic system was calculated, providing a better explanation for the eutectic morphology transition.