Rapid solidification of undercooled eutectic and monotectic alloys

Abstract

Bulk samples of Co-34.2wt.%Sn eutectic alloy and Ni-5wt.%Ag monotectic alloy were undercooled by up to 244 and 313 K respectively. The recalescence velocities were measured as a function of undercooling, and determine the upper limits of the actual crystal growth velocities. Theoretical calculations were also accomplished on the basis of the current eutectic and dendritic growth theories. A “lamellar eutectic-anomalous eutectic” microstructural transition was observed for undercoolings of more than 57 K, which agrees qualitatively with Trivedi-Magnin-Kurz's prediction that there is a maximum growth velocity for regular lamellar eutectic. In the highly undercooled monotectic alloy melt, liquid Ag phase seemed to nucleate preferentially. The α-Ni phase grew dendritically and facilitated the formation of macrosegregation at the bottom and top of the sample through coalescence of Ag droplets.