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 veloc ities in the Co-26.3%Mo hypoeutectic and Co-37.6%Mo eutectic alloys we re measured as a function of the undercooling, and also were calculate d 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. Th e most probable mechanism for this transition is that the alpha-Co pha se grows substantially faster than the epsilon-muCo7Mo6 phase and dest roys the coupled growth conditions at large undercoolings. In addition , the coupled zone of the Co-Mo eutectic system was calculated, provid ing a better explanation for the eutectic morphology transition.