Progression of Late Stage Abnormal Grain Growth of Electroformed Nanocrystalline Ni Without the Addition of Grain Refine
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INTRODUCTION
NANOSTRUCTURE science and technology have gained considerable interests in the scientific community since the early 1980s. Among other beneficial properties, it is well known that the submicron grain size of ultrafine grained (ufg) and nanocrystalline (nc) metals, with mean grain size d < 1 lm and d < 100 nm, respectively, greatly increases its mechanical strength as predicted by the Hall–Petch relationship. However, the ufg and nc structures are often considered to be thermodynamically unstable due to the excess energy arising from the high volume fraction of the intercrystalline constituents (i.e., grain boundaries, triple junctions, etc.). Consequently, vast transformation of the nc structure to a large-grained polycrystalline structure can easily occur upon thermal activation (such as by annealing, or at elevated service temperature) and hence jeopardizing the strengthening offered by the initial small grain size. Therefore, the thermal stability of the microstructure in ufg and nc metals is one of the major concerns that must be addressed prior to its practical application. CHARLES C.F. KWAN, formerly Ph.D. Candidate with the Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, ON M5S 3E4, Canada, is now Lecturer with the Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. ZONGSHU LI, M.A.Sc. Candidate, and ZHIRUI WANG, Professor, are with the Department of Materials Science and Engineering, University of Toronto. Contact e-mail: [email protected] Manuscript submitted November 4, 2014. Article published online July 8, 2015 4636—VOLUME 46A, OCTOBER 2015
Numerous fabrication techniques have been developed to synthesize ufg and nc materials in order to take advantage of the beneficial properties it offers. Among these techniques, electrodeposition is a cost-effective process that employs electrochemistry to produce a number of metals, alloys, and even composites with ufg and nc structures.[1] Furthermore, the microstructure and grain size of the electrodeposited material can be easily manipulated, and hence optimized, by adjusting the processing conditions.[2–5] Ni is often one of the model materials for studies of electrodeposited nc materials due to the relatively wellknown electrolyte chemistry. As such, the thermal stability of nc Ni produced by electrodeposition has been rigorously studied and reported in literature.[6–23] A consolidation of the observations from these literatures shows that the growth of the nc Ni grains occur first in an abnormal manner from the initial nc microstructure to submicron-sized ufg structure, which some researchers have termed as Stage I abnormal grain growth, or Stage I AGG for short.[7,10,11,13,15,17,19,22,24] A Stage II normal grain growth stage has been reported to follow, which consists of a relatively slow-paced uniform growth of the ufg structure.[11,15] Last yet most intriguing is the secondary abnormal growth often termed as
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