Metastable phase formation in undercooled eutectic Ni78.6Si21.4 melts

Non-equilibrium solidification processes in bulk undercooled eutectic Ni78. 6Si21.4 melts are investigated with respect to metastable phase and microst ructure formation. Melt undercooling levels up to 250 K have been achieved by an electromagnetic levitation technique. The in-situ observation of reca lescence events was accomplished by a fast responding silicon diode. Levita ted drops of controlled undercooling prior to solidification were quenched onto chill substrates. A transition from coupled eutectic to primary dendri te growth beyond a critical undercooling level of 30-50 K was revealed from double-recalescence events. Quenched samples displayed a featureless zone of the metastable hexagonal high-temperature phase Ni25Si9, which replaces the equilibrium phases. The metastable dendrites gradually decompose into a n anomalous eutectic microstructure on slow post-solidification cooling. (C ) 1999 Elsevier Science S.A. All rights reserved.