Grain boundaries, the interfaces between crystallites, often control important physical properties in advanced materials. For example, mechanical strength is often determined by grain boundary cohesion; electronic transport can be dominated by boundary scattering; and corrosion often proceeds rapidly along boundaries. Because of this central role, understanding how to engineer grain morphologies and chemistries that can produce higher-strength alloys would impact energy applications ranging from fuel-efficient automobiles to radiation-resistant vessels.
ORNL has developed unique, new tools to quantitatively investigate grain boundary effects over multiple-length scales ranging from atomic to macroscopic. At the nanoscale, electron microscopy and atom probe tomography provide two- and three-dimensional images of atom positions, including changes near grain boundaries. This capability is essential for understanding the role of segregation, which can act either as strengthening glue or as an embrittling layer. Thus, controlling segregation and precipitation is essential for progress in important materials such as superalloys, phase-transition materials and radiation-resistant materials.
In addition to nanoscale characterization, longer-range interactions between neighboring grains and inhomogeneities inside grains require mesoscale studies covering microns to millimeters. Using pioneering work on synchrotron x-ray focusing, ORNL has developed a unique capability for nondestructively mapping the 3D mesoscale microstructure inside polycrystals. X-ray measurements have demonstrated that the evolution of grain shapes, orientations and internal dislocation densities during thermal annealing can be mapped with submicron 3D spatial resolution.
The emerging new data covering multiscale lengths from atomic to mesoscale will be essential for guiding development of computational models aimed at predicting advances in grain boundary “engineering.”
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