Ames Lab Scientists Say Small Effects Are Key To How Materials Evolve

The Ames Laboratory 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," said Rohit Trivedi, an Ames Laboratory physical metallurgist and an Iowa State University 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," he continued. "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."

Trivedi and Ames Laboratory collaborators Ralph Napolitano, physical metallurgist and an ISU assistant professor, and James Morris, theoretical physicist, are paying close attention to the effects of small factors on microstructural evolution. Innovative experimental techniques developed by Trivedi and Napolitano have provided the first reliable measurements of the minuscule variations in free energy at the liquid/solid interface in metallic systems. Morris’ computer simulations calculate these same quantities and show how the atoms behave at the interface. The combined efforts provide both a direct check between the experiment and the simulation and the opportunity to put forth new solidification theories that may lead to the ability to predict the development of microstructure.

"We’re investigating some very specific quantities, such as the variation of interfacial free energy with crystallographic orientation," said Napolitano. "By revealing the essential physical behavior of liquid/solid interfaces, these critical experiments are facilitating significant advancement in the theoretical prediction of microstructures."

In experiments designed to measure small effects on the property of interfacial free energy, Trivedi and Napolitano have developed a technique to selectively melt certain microscopic regions within an aluminum alloy single crystal, forming a dispersion of tiny liquid droplets trapped within the solid. (A single crystal is one in which all the atoms are oriented in a specific direction. The orientation is uniform throughout the material, creating a simple, symmetric structure.) The material is then heated to bring the droplet structures to equilibrium (the condition at which no change occurs in the state of a system unless its surroundings are altered). After rapid quenching, the droplet shapes are measured very carefully, and their equilibrium shapes are determined, providing the necessary link to interfacial properties. "Thermodynamics tells us how the equilibrium droplet shape is related to the interfacial free energy," said Napolitano. "These measurements provide a direct means for quantifying the subtle variation of this property with respect to crystallographic orientation. The challenge is to accurately measure the degree to which the droplet shapes deviate from being spherical, and they deviate only by a percent or so."

Getting a clearer picture of just how tiny that deviation might be is Morris’ job. "You’re looking at this droplet and saying, ‘Well, this droplet isn’t perfectly spherical; it’s got some small asymmetry.’ We want to measure that; we don’t want it influenced by dirt in the system or anything else," Morris said. "The deviation is a very small number, but it’s very important, and that’s where doing the calculations and modeling the atomic fluctuations of the liquid/solid interface have come in."

Trivedi added, "When you look at the droplets with your eyes, they look like spheres. It’s only when you magnify and measure them precisely that you find the spheres are altered in certain directions, so there’s a different energy in different directions."

Trivedi emphasized again that the small effects dominate throughout nature. "It’s very obvious in biology – think of gene expression in the development of a human embryo. Many things can go wrong, but by and large they don’t," he said. "The same thing is true for the development of microstructures in metals. There are very fundamental issues that govern what happens. We would like to know precisely how they influence microstructure formation. To do that, we need to understand the small effects. We study metals, but the principles we hope to generate will have much broader applications."

The research is currently funded by the Materials Science and Engineering Division of the DOE’s Office of Basic Energy Sciences and by the DOE BES Computational Materials Sciences Network. Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.