Grain boundary faceting and abnormal grain growth in nickel
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I. INTRODUCTION
THE abnormal grain growth (AGG) or secondary recrystallization in alloys has been attributed to textures[1,2] or grain boundary pinning by precipitates[3,4,5] and, in thin films, to anisotropic surface energy.[6,7,8] But AGG has also been observed in highly pure bulk polycrystals of Ag,[9,10] Cd,[11] Cu,[10,12,13] Ni,[14] Fe,[15] Pb,[11] Pd,[10] and Sn.[16] It was also observed in the specimens where no texture was found or expected to exist.[10,13] The AGG in pure or single-phase bulk polycrystals has been attributed to segregation of impurity or additive solute atoms at grain boundaries.[17,18,19] The purpose of this work is to explore the possible relation of the grain boundary structure and its transformation to AGG in a bulk polycrystal of pure metal without any texture. During the last few decades, the possibility of a grain boundary roughening transformation has been proposed[20,21] and verified experimentally by mostly indirect methods.[22–27] But, except possibly for the rare cases of the grain boundaries with orientations (inclination angles) corresponding to cusps in the polar plot of the grain boundary energy (g) vs inclination angle, the grain boundaries are expected to undergo facet-defacet transformation, as pointed out by Cahn.[28] Indeed, both general grain boundaries in polycrystalline materials and grain boundaries with well-defined geometrical characteristics such as the coincidence site lattice (CSL) relations were observed to have faceted structures[29–34] and SUNG BO LEE, Postdoctoral Research Associate, and DUK YONG YOON, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305701, Korea. NONG MOON HWANG, Sub Group Leader, is with the National Creative Research Initiative Center for Microstructure Science of Materials, College of Engineering, Seoul National University, Seoul 151742, Korea; also Principal Investigator, Korea Research Institute of Standard and Science, Taejon 305-600, Korea. MICHAEL F. HENRY, Metallurgist, is with the General Electric Research and Development Center, Niskayuna, NY 12309. Manuscript submitted June 18, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
to undergo reversible defaceting transformation.[35] Impurities and additives were also found to induce grain boundary faceting.[36–42] If grain boundaries are faceted, the facet planes or a fraction of them can be singular, with ordered structures corresponding to cusp directions in the g plot vs the inclination angle. The grains with faceted boundaries may, thus, have a different growth behavior from those with defaceted rough boundaries. Various possible mechanisms for linking the AGG behavior to the grain boundary structure are examined in this work. Grain boundaries are likely to have a rough structure at temperatures close to the melting point, and oxygen has been reported to induce grain boundary faceting in Ni,[42] which is used in this work. There are indications from previous observations that AGG is indeed rel
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