Isothermal dendritic growth— a proposed microgravity experiment

Abstract

Dendritic growth is one of the most common forms of crystallization in supercooled metals or al-loys. The isothermal dendritic growth experiment (IDGE) is a microgravity flight-oriented scientific experiment aimed at testing and developing dendritic growth theory at small supercoolings. In the case of dendrites grown from pure, supercooled melts, growth is controlled by diffusion-limited transport of heat, which causes temperature gradients to be present in the liquid phase. Thermal gradients can excite anisotropic convection which affects the growth velocity, overall crystal mor-phology, and distribution of heat and solute. Dendritic growth, by its nature, does not permit inde-pendent manipulation of parameters which would reduce the vigor of melt convection under terrestrial conditions. The reduction of gravity through free fall is the only practical way to allow observation of “convection free” growth and thereby provide a test of dendritic growth theory. The IDGE is currently being developed at our laboratory, in collaboration with the NASA Lewis Research Center. The apparatus consists of a controlled thermostatic bath capable of millikelvin stability, a photographic data collection system, a crystal growth chamber ensuring “free” dendritic growth, and an optical RAM crystal growth detection system to initiate data collection. The ex-periment will be essentially autonomous, since it will be located aboard the Materials Science Laboratory in the cargo bay of the space shuttle, where direct astronaut intervention is not possible. Limited interaction from the ground is planned through a number of preprogrammed computer com-mands. Previously conducted ground based studies will be described and the current approach to performing these studies in low earth orbit will be discussed.