OBSERVATION OF PHASE SELECTION FROM DENDRITE GROWTH IN UNDERCOOLED FE-NI-CR MELTS

The electromagnetic levitation method was applied to conduct in-situ o bservations of phase selection processes in undercooled metal drops. B y means of a high-speed photosensing diode with a sampling rate of 1 M Hz focused onto two adjacent areas of a droplet surface, the passing o f a solidification wave front can be recorded and thereby the growth v elocities calculated. The alloys studied were Fe69Cr31-xNix where x wa s varied between 12 and 24, and equilibrium are f.c.c. structure (aust enitic phase) but when undercooled sufficiently nucleate the b.c.c. (f erritic) phase. The dendrite growth velocities showed a clear break at the transition from the b.c.c. to the f.c.c. profile of the respectiv e growth velocity-undercooling curves. A two-fold explanation is found for the clear break; firstly there is a contrast between the lower un dercooling f.c.c. phase which travels relatively faster for the same u ndercooling. Secondly, the liquidus temperatures for the two phases ar e offset so that for the metastable b.c.c. phase, the b.c.c.-f.c.c. li quidi temperature difference increases significantly with increasing n ickel content. The result is two easily identifiable curves for the se parate phase growth modes; f.c.c. at lower undercoolings and metastabl e b.c.c. at higher undercoolings. It was also found that these alloys show a trend towards increasing undercooling necessary to nucleate the metastable b.c.c. phase with increasing nickel content. A number of n ucleation theories have been applied to explain this behaviour, namely Classical Nucleation Theory (CNT) and Diffuse Interface Theory (DIT). Two recent forms for the latter DIT theory have been proposed and bot h were applied to this system. The results show that by optimising the nucleus composition all predictions were seen to lie much closer to t he experimental results. The DIT theory by Granasy was found to give t he closest agreement to experiment after optimisation of the nucleus c omposition. (C) 1997 Acta Metallurgica Inc.