June 23, 2013
June 23, 2013
June 26, 2013
23.1345.1 - 23.1345.15
Using the Processing, Properties and Characterization of Brass to Teach the Differencesbetween Crystal Structure and MicrostructureAnastasia D. Micheals, Christina S. Peters, Emily L. Allen and Robert B. HerringDepartment of Materials Engineering, San Jose State University, U.S.A.Microstructure and crystal structure are two critically important concepts in materials scienceand engineering, especially in polycrystalline systems. However in teaching materialsengineering, specifically materials characterization, we have found that students often confusethe two concepts, failing to grasp notions related to order on different scales. Crystal structuredescribes the periodic arrangement of atoms in a solid on the atomic level (nanometer scale),while microstructure describes the order, including both phases and defects, on a larger scale(nanometers to centimeters).The difficulty seems to stem from previously acquired understanding of crystal structure. Inintroductory materials science, students learn about crystals as areas of repeating atomic units ofidentical orientation interrupted by disordered grain boundaries. This simple model is furtherreinforced by characterization techniques like metallography, where the macroscopicallyobservable structure is treated as a mirror of the underlying atomic arrangement. Crystal grainsand grain boundaries are revealed through sample preparation (grinding, polishing and etching,and viewing under magnification). The nuance of crystallite orientation is absent from thismodel.In X-ray diffraction, crystal structure is studied not by direct observation, but through theinteraction of atomic planes with electromagnetic radiation. The results must be interpretedmathematically, rather than visually. The Scherrer equation model, while allowing abstractdetermination of crystallite size, does not effectively describe the concept. Clouding the situationis the fact that peak broadening can also result from instrument optics and residual strain in thematerial.Electron Backscatter Detection (EBSD) is a technique performed in conjunction with a scanningelectron microscope. In EBSD, the top atomic layers of a polished material diffract the incidentelectron beam. As in transmission electron microscopy, the resulting electron diffraction patternat any point is uniquely determined by the crystal structure that the beam is incident upon.Computer models allow easy indexing of the patterns to the corresponding crystal structure andorientation. Previously, use of this technique was constrained the time required to process thepatterns and derive the structure. Computing power was the barrier to automating this process.Now, it is possible to collect and process many patterns per second using computer automation.The benefit of EBSD is that it maps each point on a sample’s surface to a specific crystalorientation. Metallography, electron microscopy, elemental characterization and crystalorientation can be presented in image form, making the information easy to conceptualize.A teaching module was developed from the characterization of -brass at different stages of coldwork, recovery, recrystallization and grain growth. Characterization was performed bymetallography, X-ray diffraction, and electron back-scatter diffraction. The characterizationresults were incorporated into a module for two materials engineering courses at San Jose StateUniversity, X-Ray Diffraction Laboratory (MatE 144) and Principles of Scanning ElectronMicroscopy (MatE 143). Student learning was measured using a concept inventory test, andfound to improve following completion of the modules.
Micheals, A. D., & Allen, E. L. (2013, June), Using the Processing, Properties and Characterization of Brass to Teach the Differences Between Crystal Structure and Microstructure Paper presented at 2013 ASEE Annual Conference & Exposition, Atlanta, Georgia. https://peer.asee.org/22730
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