PHASE SELECTION, GROWTH, AND INTERFACE KINETICS IN UNDERCOOLED FE-NI MELT DROPLETS

An electromagnetic levitation facility is used to containerlessly proc ess Fe-Ni droplets, undercoolings Delta T of up to 300 K were achieved . Thermal measurements during solidification showed two types of recal escence behaviour for alloys containing between 7.5 and 17 at% Ni: A s ingle recalescence step for Delta T< Delta T (primary growth of the s table ccp phase) and two recalescence steps for Delta T> Delta T (pri mary growth of the metastable bce phase, and the subsequent transforma tion of the bcc phase and solidification of the stable ccp phase). The critical undercooling Delta T strongly increases with the Ni-content . The growth velocities at which the primary dendrites propagate throu gh the droplet have been measured for a number of Fe-Ni alloy composit ions. The velocities reflect the phase selection, i.e. primary bcc pha ses grow markedly more slowly than primary ccp phases. Thermodynamic m odelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nuc leated in preference at high undercoolings because of its lower solid- liquid interface energy and that the kinetics at the bcc-liquid interf ace is considerably more sluggish than the kinetics at the ccp-liquid interface. (C) 1997 Elsevier Science S.A.