Subgrains, Texture Evolution, and Dynamic Abnormal Grain Growth in a Mo Rod Material

The roles of subgrains, texture, and surface energy during dynamic abnormal grain growth (DAGG) were examined in a commercial-purity Mo rod material. DAGG was observed in this material during tensile deformation at 2023 K (1750 °C). Cooling of specimens after tensile testing was sufficiently rapid to preserve both subgrain structures developed during deformation and several abnormal grains at early stages of growth. These and other microstructural features were characterized to evaluate how subgrains and boundary character influence the early stages of DAGG. Subgrains were observed in the deformed polycrystalline material but were generally absent in newly formed abnormal grains. This was identified as the cause of the sudden drop in flow stress observed at the initiation of DAGG. It is proposed that subgrain intersections with abnormal grain boundaries provide a driving pressure for DAGG. Subgrains within the deformed polycrystals were observed to locally change the boundary curvature at their intersections with abnormal grain boundaries, which likely encouraged growth of the abnormal grains into the deformed polycrystals. Abnormal grains produced by DAGG retained crystallographic orientations and boundary characters that closely resembled those of the polycrystalline material from which they grew. This suggests that neither differences in orientation nor boundary character were important to DAGG in this material.