Sweating the small stuff matters.

• Position: Metallurgy - Microstructures

In an effort to understand better how microstructures develop in materials, scientists at the U.S. Department of Energy's Ames (Iowa) Laboratory are investigating certain properties that exist in metals at the interface between the liquid and solid phases during solidification. The basic research effort may one day allow scientists to tailor microstructural development, providing the basis for new and improved materials.

The researchers have shown that there are many subtle variations in microscopic properties near the liquid-solid interface as the solid is "freezing out." The small variations depend upon which crystal face is in contact with the liquid. Different faces (orientations) give slightly different values for properties such as free energy, mobility, and stiffness (surface tension), and these properties play a crucial role in how the microstructure of a metal evolves during solidification.

"There are some properties that are extremely small, but they have a profound influence," explains Rohit Trivedi, a physical metallurgist and an Iowa State University, Ames, distinguished professor. "For example, the way a snowflake forms depends on very small factors. It turns out that some of these small factors are really the essential ones in determining shape. The same thing is true not only for materials, but for humans, animals, plants--anything that grows. People generally ignore this, but we're finding out that they simply cannot."

In experiments designed to measure small effects on the property of interfacial free energy, the researchers have developed a technique to melt selectively certain microscopic regions within an aluminum alloy single crystal, forming a dispersion of tiny liquid droplets trapped within the solid. (A single crystal...