OBSERVATION OF PHASE SELECTION FROM DENDRITE GROWTH IN UNDERCOOLED Fe-Ni-Cr MELTS

Abstract

The electromagnetic levitation method was applied to conduct in-situ observations of phase selection processes in undercooled metal drops. By means of a high-speed photosensing diode with a sampling rate of 1 MHz focused onto two adjacent areas of a droplet surface, the passing of a solidification wave front can be recorded and thereby the growth velocities calculated. The alloys studied were Fe69Cr31−xNix where x was varied between 12 and 24, and equilibrium are f.c.c. structure (austenitic phase) but when undercooled sufficiently nucleate the b.c.c. (ferritic) 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 respective growth velocity-undercooling curves. A two-fold explanation is found for the clear break; firstly there is a contrast between the lower undercooling f.c.c. phase which travels relatively faster for the same undercooling. Secondly, the liquidus temperatures for the two phases are offset so that for the metastable b.c.c. phase, the b.c.c.-f.c.c. liquidi temperature difference increases significantly with increasing nickel content. The result is two easily identifiable curves for the separate phase growth modes; f.c.c. at lower undercoolings and metastable 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 nucleation 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 both were applied to this system. The results show that by optimising the nucleus composition all predictions were seen to lie much closer to the experimental results. The DIT theory by Gránásy was found to give the closest agreement to experiment after optimisation of the nucleus composition. © 1997 Acta Metallurgica Inc.