Rapid dendritic and eutectic solidification of undercooled CoMo 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 the 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 α-Co phase grows substantially faster than the ε-μCo₇Mo₆ phase and destroys the coupled growth conditions at large undercoolings. In addition, the coupled zone of the CoMo eutectic system was calculated, providing a better explanation for the eutectic morphology transition.