Solidification kinetics and phase formation of undercooled eutectic Ni-Nb melts

The non-equilibrium solidification behaviour of undercooled eutectic Ni84Nb 16 and Ni59.5Nb40.5 melts has been analysed by in situ observation of recal escence events during electromagnetic levitation of undercooled melts. Levi tated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. For Ni84Nb16 a maximum melt undercooling of 276 K has been achieved. A transition from coupled eut ectic to primary supersaturated alpha-Ni dendrite growth was revealed beyon d a critical undercooling of 30 K. Beyond 110 K the primary Ni3Nb phase occ urred for substrate quenching. The undercooling of Ni59.5Nb40.5 melt was li mited to 135 K. Bulk amorphous samples up to 2 mm thick have been prepared by quenching of undercooled Ni59.5Nb40.5 melts. On slow cooling the metasta ble phases decompose into an anomalous eutectic microstructure. (C) 1999 Ac ta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved .