Dynamic Abnormal Grain Growth in Molybdenum
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INTRODUCTION
ABNORMAL grain growth occurs when a few ‘‘abnormal’’ grains grow rapidly larger than their neighboring grains in a polycrystalline microstructure.[1–3] This process leads to a bimodal microstructure containing isolated large grains and can produce large single crystals in the most extreme case.[4–6] Abnormal grain growth is used in various ‘‘strain-annealing’’ techniques for solid-state growth of single crystals.[7–9] Abnormal grain growth has also been called secondary recrystallization[1] because it is a distinct recrystallization process observed following primary recrystallization. Thus, abnormal grain growth may also be considered as a type of discontinuous recrystallization. Traditionally, abnormal grain growth has been studied only under static conditions, i.e., without concurrent plastic straining. We classify classical abnormal grain growth phenomena observed under static conditions as static abnormal grain growth (SAGG).[10] This definition is necessary following discovery of the dynamic abnormal grain growth (DAGG) phenomenon,[10,11]
DANIEL L. WORTHINGTON, formerly with the Materials Science and Engineering Program, The University of Texas at Austin, 204 East Dean Keeton St., Stop C2200, Austin, TX 78712-1591, is now Engineer with the Fujifilm Dimatix Inc., 2250 Martin Avenue, Santa Clara, CA 95050. NICHOLAS A. PEDRAZAS and PHILIP J. NOELL, Graduate Students, and ERIC M. TALEFF, Professor, are with the Department of Mechanical Engineering, The University of Texas at Austin. Contact e-mail: taleff@utexas.edu Manuscript submitted April 3, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
which occurs only during plastic straining at elevated temperature, i.e., dynamic conditions. Although DAGG is a recently discovered phenomenon, it may be considered related to other dynamic grain growth and recrystallization phenomena. Most notable among these are the ‘‘elephant grain growth’’ of Musiol[12] and the boundary migration and recrystallization observed in lead (Pb) during creep deformation.[13–17] We previously reported DAGG in commercial-purity molybdenum (Mo) and used DAGG to produce large single crystals.[10] The present study expands upon that work by addressing the roles of microstructure, crystallographic texture, temperature, strain rate, and DAGG grain orientation. The DAGG phenomenon is most easily described through an illustrative example, such as that shown in Figure 1. This figure demonstrates results from the present investigation of DAGG in a wire of commercialpurity Mo material. The force–elongation plot of Figure 1 presents data from a simple tension test of this wire, conducted at 2013 K (1740 C) and an approximate true-strain rate of 104 s1. Primary recrystallization occurred while heating to the testing temperature. During tensile testing, the force rapidly reaches a steady-state value as is expected for creep of polycrystalline Mo under these conditions.[18] When the elongation reaches ~20 mm the force rapidly drops. The tensile test was intentionally terminated near the bottom
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