Browsing by Subject "Abnormal grain growth"
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Item Characteristics of dynamic abnormal grain growth in commercial-purity molybdenum(2011-12) Worthington, Daniel Lee; Taleff, Eric M.; Ditmire, Todd; Kovar, Desiderio; Rabenberg, Llewellyn K.; Ralls, Kenneth M.Dynamic abnormal grain growth (DAGG) in commercial-purity molybdenum sheets was investigated through a series of tensile tests at temperatures between 1450°C and 1800°C. DAGG is abnormal grain growth (AGG) which requires the presence of concurrent plastic strain. Most AGG phenomena previously documented in the literature can be categorized as static abnormal grain growth (SAGG) because they occur during static annealing, sometimes following plastic strain, but do not occur during plastic deformation. The DAGG boundary migration rate is much faster than the SAGG boundary migration rate, and DAGG may be utilized to obtain large single crystals in the solid state. Dynamic abnormal grains were found to exhibit a crystallographic orientation preference with respect to the specimen geometry, generally described as derivative from a <101> fiber texture. DAGG was found to prefer growth on the surface of the specimen rather than the interior. The growth of dynamic abnormal grains, which initiated and grew during plastic straining, generally ceased when the application of plastic strain was removed. The DAGG boundary migration rate was found to be a direct function of plastic strain accumulation, regardless of the strain-rate. Therefore, it is hypothesized that the rapid boundary migration rate during DAGG results from an enhanced mobility of certain boundaries. A model is proposed based on the rate of boundary unpinning, as mediated by the emission of dislocations from pinning sites.Item Dynamic abnormal grain growth of selected refractory metals(2013-08) Pedrazas, Nicholas Alan; Taleff, Eric M.Dynamic abnormal grain growth (DAGG) is a phenomenon by which single crystals up to centimeters in length are produced at elevated temperature during the application of strain. DAGG was previously demonstrated in commercial-purity molybdenum (Mo) materials. This is the first investigation to confirm DAGG in another material, tantalum (Ta). Previous experiments initiated and propagated DAGG using constant true-strain rate tensile tests, but this study demonstrates that DAGG can also occur under constant true-stress tensile conditions. A Mo material was tested under constant true stress, and two Ta materials were tested under constant true-strain rate. The effects of temperature, stress, strain rate, initial microstructure and texture on tensile test data and the resulting microstructures are examined. The microstructures of the Ta materials are analyzed using electron backscatter diffraction (EBSD) data to quantify the orientation, deformation, grain boundary character, and slip properties of the DAGG grains and unconsumed microstructure. The DAGG grains were found to be relatively undeformed compared to the unconsumed microstructure following DAGG and to not be oriented favorably, or unfavorably, for slip. The grain boundaries between DAGG grains in one Ta material were found to commonly have [sigma]3 character. This was likely due to a strong initial <111>-fiber texture. Previous investigations of DAGG in Mo indicated that DAGG grains commonly grow along the surface of the specimen, but this was not observed with significant frequency in Ta. Results suggest that the distance the DAGG grain boundary travels is proportional to the accumulated strain during DAGG, and the velocity of the DAGG grain boundary is proportional to the applied strain rate but is not related to the orientation of the DAGG grain or its slip properties.Item The morphology and microstructure of dynamic abnormal grain growth in commercial-purity molybdenum(2014-05) Noell, Philip James; Taleff, Eric M.Dynamic abnormal grain growth (DAGG) is a phenomenon that produces abnormal grain growth at elevated temperatures during plastic deformation. It is distinct from classically studied static abnormal grain growth phenomena in that it only occurs during plastic deformation. Previous investigations of DAGG in a Mo sheet material produced using powder metallurgy techniques observed DAGG grains to grow more rapidly near the sheet surface than near the sheet center. This phenomenon is explored in the present study. A Mo sheet material produced using arc melting techniques is also studied to determine the morphology of DAGG grains. A preference for growth near the sheet center is observed in this material. The through-thickness variations in texture and grain size for both the arc-melted and powder-metallurgy Mo sheet materials are investigated. The preference for growth near the surface in the powder-metallurgy material is due to a through-thickness variation in grain size, with smaller grains near the surface and larger grains near the center. The preference for DAGG grain growth at the center of the arc-melted sheet material is because of very large grains that grow near the sheet surface. These large grains may be the product of multiple abnormal grains occurring near the sheet surface because of texture variation through the sheet thickness. Regardless, the DAGG grain cannot consume these large grains and leaves them as island grains decorating the region near the sheet surface. These results suggest that DAGG is driven primarily by grain boundary curvature. Microstructures that include DAGG grains are investigated with electron backscatter diffraction (EBSD). A new method to evaluate geometrically necessary dislocation densities using EBSD data is derived. DAGG grains are relatively undeformed compared to the polycrystalline microstructure. DAGG grains are not oriented either favorably or unfavorably for slip. Results of the analysis of the grain boundaries between DAGG grains and normal grains do not indicate any special character preference for these grain boundaries.