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

摘要

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 tran- sition above a critical undercooling to an anomalous eutectic structure. This critical undercooling depends on the cooling rata. The transition is explained by a fragmentation of primary grown eutectic lamellas, with the reduction of the interfacial energy as its driving force. Fragmentation models are applied to describe quantitatively the observed processes.