Recrystallization and the Development of Abnormally Large Grains After Small Strain Deformation in a Polycrystalline Nic
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INTRODUCTION
THE growth of abnormally large grains in superalloy disk components during thermomechanical processing causes a significant reduction in mechanical properties, particularly the fatigue life.[1–4] These grains may be the result of primary recrystallization, abnormal grain growth (AGG), or some combination of the two. Understanding the source and formation processes of the abnormal grains is essential for prediction of the microstructural and the corresponding property changes during service. Ultimately, this understanding can then be used to design polycrystalline superalloys that have more optimal combinations of properties across a wide range of processing conditions. Recovery, recrystallization, and grain growth occur in many polycrystalline materials in order to reduce the free energy of the system. Recovery involves the rearrangement and annihilation of deformation defects, typically dislocations, to lower-energy configurations. While it does not directly result in the formation of new grains, recovery can substantially influence the structure and mobility of moving boundaries, as well as in the deformed environment encountered by a recrystallization front. Recrystallization is the process by which new strain-free grains are VICTORIA M. MILLER, Graduate Student Researcher, CHRIS J. TORBET, Research Specialist, and TRESA M. POLLOCK, Professor, are with the Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106-5050. Contact e-mail: [email protected] ANTHONY E. JOHNSON, formerly Postdoctoral Researcher with the Materials Department, University of California Santa Barbara, is now Senior Process Modeling Engineer with the Alcoa Technical Center, 100 Technical Dr., New Kensington, PA 15068. Manuscript submitted August 24, 2015. Article published online January 21, 2016 1566—VOLUME 47A, APRIL 2016
formed from the deformed or recovered structure. All recrystallization processes considered in this study are discontinuous recrystallization mechanisms, where the recrystallized grains nucleate in discrete locations, then grow to consume the deformed structure.[5,6] Grain growth may occur after the impingement of recrystallized grains following deformation or independent of any deformation, in both cases achieving a reduction in grain boundary area upon coarsening.[6] It has been shown that in material-specific ranges of strain, strain rate, and temperature, some grains may experience enhanced growth and will become significantly larger than the average grain size, often resulting in a bimodal grain size distribution. This phenomena is called AGG.[6] However, it must be noted that some reported cases of AGG may actually be a recrystallization process, or ‘‘abnormal’’ recrystallization, in which only a few grains nucleate and grow through a deformed matrix, resulting in an exceptionally coarse grain size.[5] This possibility has been considered in other Ni-based superalloys by Bozzolo et al.,[7,8] who proposed selective growth of the grains having the lowest stored energy, resul
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