Effects of Nanoscale Grain Boundary Composition Fluctuations on Mechanical Behavior of Metals and Alloys

Lehigh University, Bethlehem PA

Investigators

Abstract

TECHNICAL: Changes in the local composition of metals and alloys are key to many fundamental aspects of materials science and engineering (MS&E). These variations may be large in compositional difference, but they are usually small in spatial extent, often occurring over a nanometer or less. When composition fluctuations are associated with defects in polycrystalline materials, they may induce deleterious property changes, such as the catastrophic brittle failure of metals and alloys via, for example, hydrogen-, temper-, or irradiation-enhanced embrittlement. Until recently, it has not been possible to measure these composition changes directly and relate them to the mechanical behavior, the crystallography, the electronic structure, and the defect structure of the material. However, such measurements are now possible. In a previous NSF grant, the PI has developed methods for mechanical testing of small volumes of material, including samples that can contain a single grain boundary. In two prior NSF grants, the co-PI has participated in the acquisition of two aberration-corrected transmission electron microscopes (TEMs) and has developed computer-processing methods to detect and quantify sub-nanometer scale composition changes at large numbers of grain boundaries (GBs) and precipitates. Initial data have been obtained in model systems and commercial alloys. The objective of this project is to explore and accurately quantify nanoscale composition changes associated with GB segregation in metals and alloys, and to establish the connections between GB character and mechanical behavior through direct testing and through modeling. The assembled team of international collaborators has combined skills that will enhance understanding of the complex interplay between GB crystallography and local chemistry via a unique combination of the most advanced electron microscopy/spectroscopy and 3D atom-probe methods with the latest first-principles simulations and in-situ mechanical testing techniques. The research will generate hitherto-unobtainable composition and mechanical behavior data associated with grain-boundary segregation, selecting specific interfaces from hundreds or even thousands that can now be analyzed for the first time. NON-TECHNICAL: The broader impact of this research will arise through the potential to revise the basic understanding of the role of composition variations on segregation and mechanical properties, which are central to the education and training of all MS&E students. These latest advances in materials characterization will enhance the research and education infrastructure and will be taught to many classes of Lehigh students and hundreds of industrial attendees at Lehigh?s Annual Microscopy School, now in its 38th year. The results will be disseminated to the materials community through technical presentations, publications and a new textbook. Improvements in the measurement of nanoscale elemental changes in metals and alloys have the potential to modify basic theories of segregation, and will lay the groundwork for similar studies of precipitation in the future. In turn, this knowledge may permit the re-design of standard fabrication and processing methods that control the properties of materials. Thus the long-term result may affect society in the broadest sense through improved metals and alloys for the physical infrastructure, the hydrogen economy, aerospace and automobiles.