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GRAIN size and crystallographic texture play crucial roles in determining the behavior of many metallic and ceramic materials. Grain growth often influences or determines these characteristics.[1] Normal grain growth produces a continuous, uniform increase in grain size that typically strengthens a material’s dominant crystallographic texture components.[1–3] Under some conditions, one or more grains may rapidly grow by consuming many others. This rapid growth of a few grains, termed abnormal grain growth (AGG), produces a bimodal size distribution consisting of a few large grains surrounded by many small grains.[1,4] The large grains produced by AGG may enhance desirable material properties, such as in the well-known case of the silicon steel used in electrical transformer cores.[5] Alternatively, the presence of a few abnormally large grains can significantly decrease the fatigue life of structural materials.[6–9] Because of its fundamental scientific and technological importance, AGG has been the subject of extensive study.[4,10–13] AGG during static annealing has been the subject of most studies.[2,4,10,11,13–15] However, recent observations of AGG in the body-centered-cubic (BCC) metals Mo and Ta during high-temperature plastic deformation indicate

PHILIP J. NOELL, Postdoctoral Fellow, and ERIC M. TALEFF, Professor, are with the Department of Mechanical Engineering, The University of Texas at Austin, 204 East Dean Keeton St., Stop C2200, Austin, TX 78712-1591. Contact e-mail: [email protected] Manuscript submitted March 30, 2016. Article published online July 14, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

that fundamental differences exist between AGG during dynamic and static conditions.[16–18] For example, AGG was observed at significantly lower temperatures in Mo and Ta during plastic deformation than during static annealing.[16–18] It is thus helpful to distinguish between static abnormal grain growth (SAGG), i.e., AGG during static conditions, and dynamic abnormal grain growth (DAGG), i.e., AGG during dynamic conditions.[16] While the discovery of DAGG provides new insight into the nature of AGG phenomena, many questions remain about the mechanisms that control DAGG. To better understand DAGG, the present study examines how DAGG in Mo is influenced by grain boundary character, grain size, crystallographic texture, and dislocation density. To illustrate important aspects of DAGG, data from one of the new tensile tests performed for this study are shown in Figure 1. This figure presents data from a tensile test at a true–strain rate of 104 s1 and a temperature of 1813 K (1540 °C) of the commercial-purity Mo sheet material of this study. Under these conditions, this material displays high-temperature polycrystalline plasticity typical of creep deformation until a critical strain, labeled ec in Figure 1(a), accumulates. At ec , the flow stress drops sharply. This initial drop in flow stress is produced by the initiation of DAGG. After initiation, continued plastic deformation drives DAGG until the ga