Dendritic Growth tip velocities and radii of curvature in microgravity

Abstract

Dendritic growth is the common mode of solidification encountered when metals and alloys freeze under low thermal gradients. The growth of dendrites in pure melts depends on the transport of latent heat from the moving crystal-melt interface and the influence of weaker effects like the interfacial energy. Experimental data for critical tests of dendritic growth theories remained limited because dendritic growth can be complicated by convection. The Isothermal Dendritic Growth Experiment (IDGE) was developed specifically to test dendritic growth theories by performing measurements with succinonitrile (SCN) in microgravity, thus eliminating buoyancy-induced convection. The first flight of the IDGE in 1994 operated for 9 days at a mean quasi-static acceleration of 0.7 ϫ 10 Ϫ6 g 0 . The velocity and radius data show that at supercoolings above approximately 0.4 K, dendritic growth in SCN under microgravity conditions is diffusion limited. By contrast, under terrestrial conditions, dendritic growth of SCN is dominated by convection for supercoolings below 1.7 K. The theoretical and experimental Péclet numbers exhibit modest disagreement, indicating that transport theories of dendritic solidification require some modification. Finally, the kinetic selection rule for dendritic growth, VR 2 ϭ constant, where V is the velocity of the tip and R is the radius of curvature at the tip, appears to be independent of the gravity environment, with a slight dependence on the supercooling.

Dendritic growth velocities in an undercooled melt of pure nickel under static magnetic fields: A test of theory with convection

“…Due to the opaqueness of the nickel melt, it was not possible to measure dendritic tip radii in the present experiment. However, we may use literature data of dendritic growth kinetics measured in pure succinonitrile under normal gravity and microgravity conditions [13] to achieve this goal. We assume that the fluid flow in undercooled melts of pure succinonitrile is caused by natural convection only and that the flow at the dendritic tip is not parallel to the growth direction, but at angle of about 66 degrees.…”

“…The calculated dendritic tip radii are slightly larger than the measured dendritic tip radii. This discrepancy, however, can be understood if one accepts an enlarged error of about 20% in consideration of difficulties encountered in optical measurements and a non-ideal growth environment 2 [13]. We also compared the calculated growth Peclet numbers with those determined under different gravity conditions.…”

“…First, dendritic growth at high growth Peclet numbers is considered. under normal gravity and microgravity conditions [11][12][13], respectively. At this point, the AG theory bridges the discrepancy between the phase field modeling and the experimental observations.…”

“…The utilization of the static magnetic fields allowed us to tune forced convection in an undercooled melt continuously, thus providing much freedom for the test. Then we present a complementary test of the AG theory using literature data of dendritic growth velocities and dendritic tip radii of pure succinonitrile measured under normal gravity and microgravity conditions [13]. Finally, we compare the AG theory with phase field modeling of dendritic growth at high growth Peclet numbers [23].…”

A Phase-Field Model for the Diffusive Melting of Isolated Dendritic Fragments

“…Three IDGE experiments were flown [4,5,6] in the cargo bay of the space shuttle in order to study the dendritic solidification of the plastic crystals succinonitrile (SCN) and pivalic anhydride (PVA) in the very stable, low gravity environment provided by this platform. Therefore, unlike terrestrial melting experiments in which the fragments experience sedimentation due to the density difference between the solid and liquid phases, the dendritic fragments observed during the melting phase of the IDGE remained stationary within their parent melt.…”