Subgrains, Texture Evolution, and Dynamic Abnormal Grain Growth in a Mo Rod Material
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UCTION
THE rapid growth of one or a few grains at the expense of many others, i.e., abnormal grain growth (AGG), can significantly alter the properties of a material. In some cases, such as for silicon steels, the large grains produced by AGG enhance desirable properties.[1] In other cases, abnormally large grains can decrease yield strength or cause premature failure in components subjected to high-cycle fatigue.[2] Although significant efforts have been devoted to understanding AGG, a mechanistic description of this phenomenon has proved elusive. Fundamentally, the initiation and growth of abnormal grains depend on the mobility of the migrating boundary and the driving pressure for migration.[3] Boundary mobility is usually determined by the misorientation across the grain boundary, solutes at the boundary, and the size, shape, and distribution of second-phase particles on the boundary.[4] In most metallic systems, the driving pressure for boundary
PHILIP J. TALEFF is with the Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0889. Contact e-mail: [email protected] ERIC M. TALEFF is with the Department of Mechanical Engineering, The University of Texas at Austin, 204 East Dean Keeton St., Stop C2200, Austin, TX 78712. Manuscript submitted January 25, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
migration is controlled by grain boundary curvature and/or differences in dislocation density between neighboring grains.[4] Because of this, factors such as crystallographic texture, grain size, and the amount of cold work applied before annealing significantly influence the initiation and growth of abnormal grains. In particular, much work has been devoted to understanding the variables that govern the formation of abnormally large grains in Ni-based superalloys.[5–7] Recent work showed that concurrent plastic deformation also significantly affects the conditions under which AGG will occur and the rate at which abnormal grains grow.[8] For example, in molybdenum, concurrent plastic deformation increased the growth rate of abnormal grains by two orders of magnitude and decreased the initiation temperature for AGG by approximately 300 °C.[9–11] Because of this effect from plastic deformation, it is necessary to differentiate between AGG during static annealing, termed static abnormal grain growth (SAGG), and AGG during concurrent plastic deformation, termed dynamic abnormal grain growth (DAGG). DAGG is a recently discovered phenomenon that has been observed in the refractory metals molybdenum (Mo)[9] and tantalum (Ta).[12] To illustrate DAGG, Figure 1(a) shows a plot of engineering stress vs engineering strain for a tensile test on a Mo rod during which DAGG occurred. This test was performed at a true-strain rate of 103 s1 and a temperature of
Fig. 1—(a) Data from the tension test of Mo specimen R04 at 2023 K (1750 °C) and 103 s1 are plotted as engineering stress vs engineering strain. An image of the etched cross section from this specimen after testing is inset with the TD horizontal. (b) Data from the
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