Crystallographic Analysis of Nucleation at Hardness Indentations in High-Purity Aluminum
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TRODUCTION
NUCLEATION of recrystallization in the bulk of metals is very difficult to control and uncertainties exist on active nucleation mechanisms (e.g., References 1 and 2). However, it is well known that surface imperfections such as scratches and hardness indentations stimulate nucleation. The present work deals with the latter. Extensive studies have been done to understand the deformation mechanism occurring during indenting (e.g., References 3 through 5). Early work focused on the visualization of the deformation microstructures of the volume underneath indentations using split samples and optical microscopy.[6,7] Newer studies use scanning electron microscopy and conventional electron backscattering diffraction (EBSD)[8] as well as 3D-EBSD[9] to investigate the size of the affected zone underneath indentations. It is generally found that hardness indentations cause a high local dislocation density. Also several studies have investigated nucleation at hardness indentations (e.g., References 10 through 13). Upon annealing, the increased dislocation density in indentation zones provides a higher driving force for
CHAOLING XU, Joint Ph.D. Student, is with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China, and also with the Section for Materials Science and Advanced Characterization, Department of Wind Energy, Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark. Contact e-mail: [email protected] YUBIN ZHANG, Senior Scientist, FENGXIANG LIN, Postdoctoral Fellow, and DORTE JUUL JENSEN, Professor, are with the Section for Materials Science and Advanced Characterization, Department of Wind Energy, Technical University of Denmark. GUILIN WU, Researcher, and QING LIU, Professor, are with the College of Materials Science and Engineering, Chongqing University. Manuscript submitted November 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleation and subsequent growth during recrystallization. It has been found that recrystallization depends strongly on the distribution of stored energy below the indentations and that the recrystallized volumes closely match the indentation zones.[11] It has also been found that the shape of the indenter has an effect on the nucleation potentials: a sharp indenter leads to more nucleation than a ball-shaped one.[13] Although it is well documented that hardness indentations stimulate nucleation, much less is known about crystallographic effects. Open questions include (1) (2) (3)
Do grains of different crystallographic orientations have the same potential for stimulating nucleation at hardness indentations? Do all nuclei that form at different hardness indentations within a given grain have the same crystallographic orientation? What orientation relationships exist between nuclei and matrices?
Questions No. 1 addresses the nucleation potentials and thus the nucleation densities which determine the average recrystallized grain size. Question Nos. 2 and 3 address crystallographic orientation relationships and thus the form
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