A phase field model for spontaneous grain refinement in deeply undercooledmetallic melts

The origin of spontaneous grain refinement in deeply undercooled metallic m elts has been of enduring interest within the solidification literature. He re we present the results of phase held simulations of dendritic growth int o pure undercooled melts, at growth velocities up to 35 m s(-1). We End tha t, at low growth velocities, dendrite: morphologies are broadly self-simila r with increasing growth velocity. However, above approximate to 15 m s(-1) the initiation of side-branching moves closer to the dendrite tip with inc reasing growth velocity. This appears to be related to the level of kinetic undercooling at the tip. Once side-branch initiation begins to occur withi n 1-2 radii of the tip, profound morphological changes occur, leading to se vere thinning of the dendrite trunk and ultimately repeated multiple tip-sp litting. This process can be invoked to explain many of the observed featur es of spontaneous grain refinement in deeply undercooled metallic melts. (C ) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights r eserved.