NONEQUILIBRIUM SOLIDIFICATION OF HYPERCOOLED CO-PD MELTS

Co-Pd alloy melts are characterized by a low heat of fusion and, as a consequence, by a comparably small critical undercooling for the hyper cooling Limit Delta T-hyp Of the order of 300 K. It is shown that cont ainerless processing of bulk melts by electromagnetic levitation offer s undercooling levels of Delta T approximate to 350 K thus exceeding t he hypercooling limit considerably. Solidification of undercooled melt s from the hypercooling regime leads to rapid crystallization of the e ntire sample under nonequilibrium conditions. The electromagnetic levi tation technique in combination with time-resolved recalescence detect ion was used to measure the growth velocity of Co-Pd alloy melts as a function of undercooling prior to solidification. The growth velocitie s of undercooled metallic melts were measured in an undercooling range exceeding the hypercooling limit. The experimental results are discus sed within current theory of dendritic growth. (C) 1998 American Insti tute of Physics.