Low voltage electron back-scatter diffraction: enabling high resolution mapping of heavily deformed materials

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Non-technical Summary Modern engineering materials often have a complex microstructure that makes it difficult to accurately predict how they will behave under the influence of a applied forces. Many aspects of the deformation behavior of a material are still poorly understood. Since deformation occurs at a very small length scale, we need to use electron microscopes operated at a fairly high magnification to image the internal behavior of the material at this length scale. In this research project, cutting-edge microscopy techniques are advanced to quantitatively determine the local deformation in a number of metals and alloys; such measurements provide insight into the behavior of these materials, in particular when these metals are used in structural applications in our modern society. These algorithms are available to the community through publications and open source code. Open source text books are developed for the materials community and provide a mechanism for the on-demand creation of free open source text books. Technical Summary The thermomechanical processing of materials systems and the subsequent annealing and recrystallization processes have for more than a century now been the subject of numerous experimental and theoretical studies, in particular for metallic systems. Despite extensive work on recrystallization, many details of this process still remain unclear due to the difficulty to quantitatively and accurately characterize the deformed state of a material; this state can be considered as a precursor state, setting the stage for the subsequent annealing process, so understanding this state is of paramount importance. Extracting lattice strains from deformed samples is challenging but requires a renewed attention due to recent improvements in electron back-scattered diffraction (EBSD) detector systems. A physics-based EBSD prediction model is combined with the ability to incorporate the local deformation tensor in the pattern generation algorithm. In particular at lower microscope accelerating voltages, i.e., less than 10 kV, there is the potential to enhance the spatial resolution of EBSD measurements to a few tens of nanometers, especially when combined with sensitive direct electron detectors. Whole pattern matching of EBSD patterns against realistic simulated patterns for a deformed lattice make it possible to routinely reach strain sensitivities of better than 0.0001 over large sample areas. The proposed research consists of a tight combination of computational model development and in-situ experimental work. Portions of this work are carried out in collaboration with colleagues at UCSB and at Johns Hopkins University. All algorithms are made available to the community in peer-reviewed publications and as open source code. On the educational side, the OpenChapters project addresses the issue of rapidly rising cost of material science and engineering text books by creating an open source mechanism for the generation of free text books by a crowd-sourcing approach. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.