Metal dendrites in microgravity.

It looks like "a forest of tiny metal pine trees," says Martin E. Glicksman, a materials scientist at Rensselaer Polytechnic Institute in Troy, N.Y.

Each time a molten alloy cools, changing from a liquid to a solid, a roughedged carpet of treelike spindles sprouts along the interface where the two states meet. Scientists call these tiny branching structures dendrites, derived from the Greek word for tree.

When observing this metal forest under a microscope, one sees metal trees growing so closely together "that their trunks and branches intermingle," Glicksman said this week at a meeting of the Materials Research Society in Boston.

Indeed, every engine block or soda can hardening from molten alloy forms such dendritic sheets along the edge where the liquid turns solid. Once the alloy has completely hardened, the interlocking dendrites imprint in the material a complex three-dimensional pattern called the microstructure.

That pattern, says Glicksman, strongly affects the material's strength, ductility, and electrical properties, as well as its tendency to crack and corrode. Thus, in order to improve alloy quality, scientists want to understand more thoroughly how metal dendrites grow during solidification.

A key factor influencing dendritic growth is the convective currents of heat flowing in molten metal. On Earth, with its strong gravitational fields, those currents are difficult to control or avoid. So Glicksman and his colleagues proposed to measure dendritic growth in space.

On March 4, the space shuttle Columbia carried the Isothermal Dendritic Growth Experiment into orbit for 9 days, soaring 163 nautical miles above sea level in a low-Earth circular orbit. Still and video cameras surrounding a thermostatic chamber recorded the growth of dendrites in succinonitrile, a transparent, nonmetallic material with a conveniently low melting point that mimics metal in the way it solidifies.

With gravity lowered to less than one-thousandth of Earth's, the team repeatedly raised and lowered the temperature of the succinonitrile, watching it melt and resolidify. Slow-scan video cameras captured the emergence of dendrites on the end of a "stinger tip" immersed in the gently freezing fluid, while two 35-millimeter cameras snapped 400 pictures of sprouting crystal trees. Other sensors measured physical details during dendritic growth.

With the low-gravity experiments completed, scientists compared the data generated in space with similar information obtained on Earth. As a result, they can now see more precisely how convection currents, which alter the way heat diffuses through a cooling metal, can influence the growth rate and curvature of dendrites.

Such information, the scientists maintain, may now give metallurgists on Earth an added edge as they strive to make lighter, stronger, more reliable alloys for the machinery needed in the next century.