Materials World Network/Research in Undergraduate Institutions: The True Three-Dimensional Nature of Aligned Dislocation Boundaries in Deformed Metals

Harvey Mudd College, Claremont CA

Investigators

Abstract

This project is based on collaboration between Harvey Mudd College (HMC) and the School of Materials Science and Engineering and the Electron Microscope Unit at the University of New South Wales (UNSW) in Australia. It is also a Research in Undergraduate Institutions activity, as the award supports international materials science research experiences for HMC undergraduate students. The project focuses on resolving a longstanding controversy about the nature of arrays of aligned dislocation boundaries that are generated during rolling of deformed face-centered cubic and body-centered cubic metals with intermediate to high stacking fault energies. The structure and origin of these dislocation boundaries are of substantial interest due to their role in determining anisotropy in yield stress and strain hardening. Insight into the true character of these microscale structures is essential for advancing the predictive capabilities of physically-based models for macroscale mechanical properties. The two opposing theories of the structure and origin of these aligned boundaries in deformed polycrystals are: (i) they are oriented along certain crystallographic planes, or (ii) their alignment is dictated primarily by the macroscopic stress state during plastic deformation. Current evidence supporting these theories is based on two-dimensional data that does not necessarily reveal the true nature of deformation microstructures and can lead to erroneous interpretation. To definitively resolve the issue and to further modeling capabilities, three-dimensional (3D) orientations of the boundaries are required. Data in 3D is collected using focused ion beam-electron backscatter diffraction (FIB-EBSD) tomography. EBSD maps of FIB-generated serial sections are combined in post-processing to generate full crystallographic volumes capable of revealing many types of structural features at submicron resolution. HMC students lead the development of new computational tools required to efficiently and accurately analyze the large 3D data sets that result from this method. They also refine FIB-EBSD methods and present their findings in peer-reviewed journals and national or international conferences. At UNSW, the supported undergraduates and the principal investigator have extensive access to electron microscopy facilities and training unavailable at HMC. The project therefore provides an extraordinary opportunity for collaboration between UNSW's experts in physical metallurgy and electron microscopy and U.S.-based undergraduate students. This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering.