Dual effects of co-electrodeposition of CeO 2 nanoparticles on the grain growth of nanocrystalline Ni matrix
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Xiao Pengb) Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; and School of Material Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China (Received 8 December 2016; accepted 10 April 2017)
Thermal stability up to 400 °C of nanocrystalline (NC) Ni electrodeposits (EDs) with a mean grain size of 28 nm and dispersions of a small amount of CeO2 nanoparticles has been investigated by comparing with two CeO2-free NC Ni counterparts, one with a slightly smaller mean grain size of 18 nm and the other with a slightly larger mean grain size of 34 nm. The results show that the co-deposition of CeO2 particles has dual effects on the thermal stability of the NC Ni EDs, i.e., it promotes the grain growth at the beginning but retards subsequently. It is proposed that the CeO2 co-deposition leads to a decrease in sulfur level and an increase in the plane defects as a result of introduction of incoherent Ni/CeO2 interfaces, which play dominant roles in the grain growth at low temperatures; while the drag effect of CeO2 dispersions becomes dominant at higher temperatures. I. INTRODUCTION
The existence of numerous grain boundaries (GBs) in nanocrystalline (NC) materials with respect to their conventional coarse-grained (CG) counterparts leads to a variation (mostly an improvement) of their mechanical, chemical, and physical properties.1–4 However, NC materials, because of considerable energy stored in GBs, become thermodynamically unstable with increases in temperature. This allows the stored energy to be released by grain growth. The microstructural evolution with temperature irreversibly degrades the unique mechanical, chemical, and physical properties that only the NC materials have. Hence, an understanding of the thermal stability of a typical NC material is essential for its practical application, particularly in an environment under high temperatures. In the past decades, the thermal stability of NC Ni electrodeposits (EDs), due to their ease of fabrication and promising applications in recording heads and soft magnetic disk components, micro-electromechnical systems (MEMS), parts acquired for repairing nuclear steam generator tubes,5–7 etc., has been studied. Normal growth of NC Ni grains occurred quickly at approximately 290 °C but was significantly retarded in the presence of Contributing Editor: Susan B. Sinnott Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.163
a small amount of impurities and/or solute atoms.8–18 The retardation of grain growth has been proposed to be the result of GB segregation of the impurities and/or solid solute elements, which normally reduce the GB energy from a thermodynamic point of view and/or allow the GB mobility to be more difficult from a kinetic point of view due to a solute drag effect.19–22 Moreover, based on the thermodynamic calculations by Kirchheim,23 the GB energy of some NC materials can be decreased furth
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