The morphology and microstructure of dynamic abnormal grain growth in commercial-purity molybdenum

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.