Many properties of crystalline materials, such as metals, ceramic, and electronic materials, are controlled by crystal defects. Dislocations are crystal defects containing long lines of disrupted atomic arrangements of their various crystal structures. The motion of these dislocations is responsible for controlling the strength, ductility, and fracture behavior of structural metals and alloys, while in functional materials dislocations are often responsible for breakdown in optical and electron properties, often leading to reduction in lifetimes. This research program is developing a new approach to mapping dislocations. The research uses electron channeling contrast imaging (ECCI) in scanning electron microscopy (SEM) to image near surface dislocations. This imaging is combined with focused ion beam (FIB) milling to cut a series of nano-scale sections through volumes of interest. The images of each section are combined into 3-dimensional maps of the dislocations through volumes on the scale of tens of cubic micrometers. Mapping of the dislocations associated with nano-indentations is facilitating a better understanding of the crystal-to-crystal dislocation motion necessary for deformation of arrays of crystals and responsible for fracture initiation at crystal boundaries. Combining this enhanced understanding with computer simulations of dislocation motion is enabling enhanced prediction of material fracture behavior and lifetimes. The research program supports the broad professional training of a postdoctoral research associate, carried out under a formal mentoring plan, in complement with undergraduate research. The postdoctoral and undergraduate research support is leveraged to expose K-12 students and educators to science and engineering through the SEM Education Program (MSU-SEMED) program at MSU, which allows K-12 students and their teachers to be introduced to materials science though a hands-on experience with electron microscopy.
This research project is developing a novel approach for tomographic mapping of the nano- to micro-scale distribution of dislocations in small volumes. The technique combines focused ion beam (FIB) milling with electron channeling contrast imaging (ECCI), a scanning electron microscopy (SEM) technique that allows dislocations to be imaged in the near surface region of bulk materials, to collect serial sections of dislocation images through a volume of interest, which are then reconstructed into a 3-D map of the dislocation structure. Mapping at this scale addresses a gap in our current technology, with smaller volumes being assessable with transmission electron microscopy (TEM) with similar dislocation densities and x-ray tomography assessing much larger volumes with much lower dislocation densities. The development is focused on three technical tasks: 1) establishment of the FIB sectioning parameters necessary to achieve an optimal combination of high quality FIB polished surfaces with a high FIB sectioning throughput, 2) collection of serial ECCI images through deformed volumes under optimized electron channeling conditions, and 3) development of image recognition techniques for identifying the dislocations in the ECC images and subsequent reconstruction of the 3-dimensional dislocation maps. The technique is being used to map the dislocation structures in plastic fields developed under nano-indentations in non-cubic metals in order to facilitate a robust assessment of crystal plasticity finite element (CPFE) models of small-scale plastic deformation. Indentations near grain boundaries are being studied to understand the nature of plastic deformation transfer across the boundaries in order to elucidate the mechanisms associated with grain boundary failure and damage nucleation in polycrystals.
