Characteristics of dynamic abnormal grain growth in commercial-purity molybdenum
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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.