Imaging the Recalescence Behavior of Undercooled Nickel

The thermal front accompanying recalescence of undercooled pure nickel melts was characterized using a high speed digital imaging technique. Nucleation was either spontaneous or was stimulated using a physical trigger to induce heterogeneous nucleation at a known site at the desired undercooling. Undercoolings ranged from 30 to 300 *C, with measured velocities between 0.3 and 50 rn/sec using a computer data analysis routine to predict the nucleation position and time. Results were compared with those of other workers and with the LKT solidification model assuming a kinetic parameter of 1.0. For levitation processed samples, positive deviation of the propagation velocity with respect to LKT was observed below undercoolings of 80 *C while above a critical undercooling of AT* = 170 *C, the observed propagation velocity showed negative deviation. Glassencased samples showed no deviation at low undercoolings but the deviation was more pronounced at higher undercoolings than observed using containerless processing. At these high velocities, the front morphology was smooth and a model assuming isotropic spherical expansion provided the best prediction of growth behavior. At low undercoolings, the angular front was observed to result from complex interaction between the shape of the growing prismatic dendritic array and its orientation relative to the surface. Bifurcation of the vertices behind the apex of the pyramid was commonly observed. Similarity in crystallographic orientation between competing arrays within the same sample indicates subsurface connection; the occurrence of multiple nucleation was never observed. Thesis Supervisor: Professor Merton C. Flemings Title: Toyota Professor of Materials Processing