Refuting a 70-year approach to predicting material microstructure

The dark blue shading is a boundary that separates two grains. As the boundary moves, some elements that were part of grain M become part of grain N. Credit to Carnegie Mellon University College of Engineering
Scientists at Carnegie Mellon University have discovered that a 70-year-old model for predicting the microstructure of materials is not applicable to today's materials. A Carnegie Mellon-Argonne National Laboratory microscopy technique yields evidence that contradicts the traditional model. This leads to the development of new types and characterizations for predicting properties and therefore safety and durability of new materials.

A metallurgist might discover an alloy that can dramatically improve the performance of an aircraft. It could take up to twenty years for a passenger to be able on a plane made from that alloy. Researchers use trial-and-error to determine a material's safety or durability. It is impossible to predict how a material will react to stressors such as daily use or processing. This long process is a major bottleneck in materials innovation.

Robert Suter and Gregory Rohrer, both from Carnegie Mellon University's Department of Materials Science and Engineering and Department of Physics, have discovered new information that will allow materials scientists to predict how materials properties change when exposed to elevated temperatures. They used near-field high-energy diffraction microscopy to determine that the existing model for predicting a material’s microstructure and property is not applicable to polycrystalline materials. A new model is required.

Most commonly used metals, alloys, and ceramics are uniformly solid to the eye. They are used in both industrial and consumer equipment. At the microscopic level they are polycrystalline. They are made up of different sizes, shapes, and crystal orientations. A network of grain boundaries connects the grains, which shifts when subjected to stressors. This alters the material's properties.

Scientists must control the microstructure of a material when they create it. This includes its grain boundaries. Materials scientists can alter the density of grain boundaries to meet specific needs. The structure around a car's passenger cabin is made from ultra-high strength steel, which has more grain boundaries than the body panels at the car's front crumple zone.

Researchers have used a theory to predict materials' behavior for the past 70 years. It states that the speed with which grain boundaries move through heated material is related to its shape. Rohrer, Suter and others have demonstrated that this theory, which was intended to be the best case scenario, is not applicable in real polycrystals.

Images of grain boundary curvatures, velocities, and computed mobilities are captured by high energy diffraction microscopy. The velocity does not correlate with other properties. Credit: Carnegie Mellon University College of Engineering

Polycrystals can be more complex than the ideal cases. Rohrer said, "If one looks at a single grain boundary within a crystal it can move uninterrupted, much like a car driving along an empty road. Each grain boundary in polycrystals is connected to on average ten other grains, so that the car can't drive as freely. This model is no longer valid." Rohrer and Suter also found that polycrystal grain boundaries were not moving in the predicted direction.

These discoveries were made possible by HEDM, a technique pioneered and used by Suter and his colleagues at the Argonne National Lab's Advanced Photon Source. Researchers can non-destructively image thousands upon thousands of crystals, and determine their orientations in opaque metals or ceramics using HEDM and the associated techniques. This technique is only possible at a handful of synchrotron sources worldwide.

Suter said, "It's almost like 3D Xray vision." "Before, it was impossible to look at the grains of a material without cutting them apart. HEDM allows us non-invasively to view the evolution of grain boundaries and orientations over time.

HEDM was developed around 20 years ago. It continues to be developed today. Suter's research group collaborated with scientists from APS to create procedures for the synchronized collection and analysis of thousands of images of Xray diffraction patterns taken from a material sample during its precision rotation under an intense incident beam. Suter's research team developed high-performance computer codes that convert sets of images into three-dimensional maps of the crystal grains that make up the material's microstructure.

Ten years ago, Suter's team (including Physics graduate students Chris Hefferan and Shiu-Fai Li) measured a nickel sample repeatedly after high temperature treatments. This resulted in the first observation of individual grain boundary movements. These motions did not show the same systematic behavior as the 70-year-old theory. Carnegie Mellon scientists in Science have correlated grain boundary structure and the systematic behaviors seen in the HEDM experiment data.

Although the current analysis was done on nickel, Xray diffraction microscopy can be used on many other materials. Rohrer, Suter and others believe that many materials will exhibit similar behavior to nickel. Other material processing conditions are also being investigated.

The Designing Materials to Revolutionize Engineer the Future program of the National Science Foundation funded this research. Effective October 1, 2021, the team received a $1.8 million dollar renewal of their four-year grant. The next steps of research into how and why polycrystals behave in different materials will be carried out by Carnegie Mellon's Kaushik dayal, Department of Civil and Environmental Engineering; Elizabeth Holm, Department of Materials Science and Engineering; and David Kinderlehrer (Department of Mathematical Sciences). The collaboration also includes Professors Carl Krill (University of Ulm in Germany) and Amanda Krause, University of Florida.

More information: Aditi Battacharya and colleagues, Grain boundary velocity is not related to curvature in Ni polycrystals. Science (2021). Information from Science Aditi Bhattacharya and colleagues, Grain boundary velocity (and curvature) are not correlated within Ni polycrystals. (2021). DOI: 10.1126/science.abj3210