At the core of materials science is the study of material structure on the atomic scale. To understand how to build the next generation of cellphones or sustainable batteries for electric cars, materials science engineers zoom in on metals, polymers and other substances to understand how their component atoms and molecules are arranged. By interrogating these materials on that level, engineers can learn how to manipulate and improve their physical properties, such as strength or ductility, the ability to be shaped into sheets or wires without breaking.
Most metals, for example, are crystalline, meaning their atoms are arranged in a repeating lattice. A perfect crystal would have that same repeating structure throughout, but most are composed of distinct “grains,” which all have the same basic pattern but in different orientations to one another.
Understanding the boundaries where those grains meet is of particular interest to materials scientists, as they are essentially defects where corrosion or other forms of damage are more likely to occur. Knowing how, and how quickly, these boundary lines move when the material is subjected to outside forces is fundamental to microengineering the desired properties.
A group of Penn Engineers studying grain boundaries has now developed a deeper mathematical understanding of the factors that govern their mobility. Using molecular dynamics simulations and statistical-mechanics-based models of copper, they “demonstrate the fundamental nature” of grain boundary mobility, “reconciling a wide range of observations in a consistent model.”
The new study was led by David J. Srolovitz, Adjunct Professor in Materials Science and Engineering (MSE), and Kongtao Chen, an MSE graduate student. They collaborated with Jian Han, a postdoctoral researcher in MSE, and Xiaoqing Pan, Professor and Henry Samueli Endowed Chair in Engineering at the University of California at Irvine.
It was published in the Proceedings of the National Academy of Sciences of the United States of America.
The researchers’ primary findings are that that grain boundary mobility should be considered a tensor, rather than a scalar. Tensors are algebraic constructs that describe the relationships between multiple variables, whereas scalars represent the magnitude of single quality. This essentially means that while previous research believed that the speed with which grain boundaries moved depended solely on the size of the external forces that were acting on the material, the researchers have shown that the type and direction of that driving force also matters to the velocity of grain boundaries.
Additionally, the researchers showed that when grain boundaries migrate, there are other molecular rearrangements occurring at the same time, all of which affects the material’s overall properties.
A three-dimensional, multi-variable understanding of these dynamics will enable researchers to better control grain boundary migration.
“Grain growth is good for superalloy turbine blades and silicon photovoltaics,” Chen says, “but grain growth is bad for nanocrystalline materials in the lab.”
More fundamentally, being able to fully relate the behavior of grain boundaries to external forces that get them moving will allow materials scientists and engineers to determine which of the two to target in efforts to get the properties they desire.
“We care whether the grain boundary’s motion arises more from material properties or the driving force because the ways to manipulate them are different,” Chen says. “If it’s the property of a material, we need to change its structure, which is hard; but if it’s not, we can manipulate it by changing something in the environment, like the temperature or stress it’s being subjected to.”
Though the simulations and models the researchers used in their study were of copper with a particular crystal structure pattern, the insights they generated are about underlying physics and thus are broadly applicable.
“Most engineers in metals and ceramics industries, no matter what they’re making, will benefit from manipulating these defects in materials, as well as from choosing better stress and temperature when processing these materials,” Chen says.
This research was sponsored by Army Research Office Grant W911NF-19–1–0263.