The Distribution of Grain Boundary Planes in Interstitial Free Steel
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INTRODUCTION
INTERSTITIAL free (IF) steels are widely employed for automobile panels as they offer excellent deep drawability. This property is mainly attributed to the control of a unique texture development (i.e., c-fiber) through the recrystallization process.[1] Surprisingly, less attention has been given to the role of grain boundary characteristics to the outstanding property in IF-steels, though the presence of grain boundaries in polycrystalline materials significantly controls their mechanical behavior. The grain boundaries can, for instance, act as the source of dislocations and the resistance to the dislocation motion from one grain to another. This results in an increase in the strength with a decrease in the grain size (i.e., dislocation mean free path) as illustrated by the Hall-Petch relationship.[2,3] Interestingly, ferritic steels generally exhibit an abnormally high Hall-Petch slope (i.e., typically on the order of 20 MPa/mm0.5) compared with other metals (e.g., austenitic steels). This is primarily linked to the presence of interstitial elements (i.e., carbon) at the grain boundaries.[4] However, a similar trend was reported for IF steels[5] where the interstitial elements are negligible. This encourages investigation of the characteristics of the grain boundary plane (i.e., distribution and energy) in IF steels; because the grain boundaries are active structural elements, certain properties of crystalline materials are controlled to a large extent by their characteristics. Five grain boundary parameters are required to quantitatively characterize the grain boundary plane HOSSEIN BELADI, Senior Research Academic, is with Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] GREGORY S. ROHRER, W.W. Mullins Professor and Head, is with Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted June 17, 2012. Article published online September 8, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
distribution, consisting of three misorientation parameters to describe the orientation relationship across the grain boundary, and two parameters specifying the orientation of the boundary plane.[6] Although, the former can be readily obtained through conventional two-dimensional electron back-scattered diffraction (EBSD), the latter, indeed, needs new tools such as the dual beam focused ion beam scanning electron microscope (i.e., serial sectioning) and high energy X-ray tomography, to precisely resolve and visualize the threedimensional internal microstructures of materials. However, limited work has been undertaken to date for the grain boundary characterization using advanced threedimensional analysis techniques[7,8] due to their complexity and time constraints. Recently, all five grain boundary parameters can statistically be measured through the conventional EBSD orientation mapping technique. This unique procedure, which is described in detail elsewhere,[6] enables
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