Mechanism of formation of the anomalous eutectic structure in rapidly solidified Ni-Si, Co-Sb and Ni-Al-Ti alloys

Electromagnetic levitation technique was applied to crystallize containerlessly bulk melts of eutectic Ni-Si, Co-Sb and Ni-Al-Ti alloys at various undercoolings and cooling rates. Complementary, the drop-tube technique was used to solidify droplets of the same alloys at high cooling rates as splats. At small undercoolings, a regular lamellar eutectic microstructure was found which undergoes a partial transition above a critical undercooling to an anomalous eutectic structure. This critical undercooling depends on the cooling rate. The transition is explained by a fragmentation of primary grown eutectic lamellae, with the reduction of the interfacial energy as its driving force. Fragmentation models are applied to describe quantitatively the observed processes. (C) 1998 Acta Metallurgica Inc.