The Initiation and Propagation of Dynamic Abnormal Grain Growth in Molybdenum
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NTRODUCTION
ABNORMAL grain growth (AGG) is the process of one or a few grains rapidly growing in size by consuming many surrounding smaller grains.[1–3] This typically produces a microstructure that contains one or a few grains many times larger than the original grain size.[4–6] In the most extreme case, AGG can produce a single crystal.[7,8] For some applications, such as directionally oriented silicon steels, the large grains produced by AGG are very desirable.[9] However, AGG is typically undesirable in structural materials because the resulting large grains reduce part strength.[10] These effects motivate our desire to understand AGG phenomena sufficiently to both predict and control them. The current understanding of these phenomena is, unfortunately, incomplete.[11] The present study endeavors to extend our understanding of the subset of AGG phenomena that occur during plastic straining at elevated temperatures. Two distinct categories of AGG phenomena are known: static abnormal grain growth (SAGG) and dynamic abnormal grain growth (DAGG).[12–14] SAGG is defined as AGG that occurs without any concurrent plastic straining, i.e., under static conditions.[12] SAGG in bulk metallic materials is controlled primarily by a PHILIP J. NOELL, Current Student, and ERIC M. TALEFF, Professor of Mechanical Engineering, are with the University of Texas at Austin, 204 East Dean Keeton St., Stop C2200, Austin, TX 787121591. Contact e-mail: [email protected] DANIEL L. WORTHINGTON, Former Student, is with Fujifilm Dimatix, Inc., 2250 Martin Avenue, Santa Clara, CA 95050. Manuscript submitted June 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
combination of grain-boundary energy,[15,16] stored dislocation energy,[17,18] and texture.[1,19,20] Most investigations of AGG described in the literature deal exclusively with SAGG. DAGG is a recently discovered phenomenon[12] defined as AGG that occurs during concurrent plastic straining, i.e., under dynamic conditions. Several important features distinguish DAGG from SAGG. The most striking are that DAGG occurs at much lower temperatures and proceeds more rapidly than SAGG.[12–14] DAGG has been definitively observed in the two BCC refractory metals Mo and Ta.[12–14] In Mo, abnormal grains were grown by DAGG at rates of up to 0.5 mm s1 and to lengths as long as 10 cm.[13] This maximum size was restricted only by the physical limitations of the testing apparatus. DAGG is illustrated through the example shown in Figure 1. These data are from tensile tests of a commercial-purity Mo sheet conducted at a constant true-strain rate of 10 4 s1 and a constant temperature of 1923 K (1650 C). As the true-stress vs true-strain plot in Figure 1(a) demonstrates, this material initially displays the high-temperature polycrystalline creep deformation expected for these conditions.[21] However, after accumulating a critical strain, ec , a dramatic drop in flow stress occurs. The drop in flow stress from the value labeled as rc corresponds with the initiation of DAGG. If tensile straining is ter
