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INTRODUCTION

THE co-deposition of nanosized particles in the nanocrystalline metal matrix by the pulse electrodeposition procedure improves the mechanical, tribological, and anticorrosion properties.[1,2] Qu et al.[3] reported that the Ni-CeO2 composite exhibits higher microhardness, better wear resistance, improved corrosion resistance, and enhanced high-temperature oxidation resistance compared to pure Ni. The mechanical property of electroplated Ni composite films increases due to the particle-strengthening mechanism.[4] For nanocrystalline materials, thermal stability is of significant importance since the large excess free energy associated with the intercrystalline region, which provides a significant driving force for grain growth, is inversely proportional to the grain size for the ideal case of grain boundary curvature-driven growth.[5,6] The thermal stability of the nanostructured metal matrix is improved by the co-deposited nanosized ceramic particles present in the matrix.[7] According to the Zener equation, a large number of dispersed nanosized particles have the ability to inhibit the grain growth of the matrix at high temperatures.[8,9] A common finding in the thermal stability studies of electrodeposited nanocrystalline Ni is the significantly increased rate of grain growth at relatively low temperatures.[10,11] However, nanocrystalline Ni-Fe[12] and Ni-P[13] electrodeposits have increased thermal stabilities with respect to pure Ni.[14] Zhang et al.[15] studied the thermal stability of Cu-(2.5-10) vol pct Al2O3 nanocomposite powders and found that the thermal stability of the nanocomposite structure increases with the increase in volume fraction of Al2O3 nanoparticles in the composite. Ferkel and Mordike[16] RANJAN SEN, Research Scholar, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, Kharagpur 721302, West Bengal, India. SIDDHARTHA DAS and KARABI DAS, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur. Contact e-mail: [email protected] Manuscript submitted August 13, 2011. Article published online July 11, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

studied the strengthening of Ni-Al2O3 nanocomposites after heat treatment at 1123 K (850 C) and found that the hardness improves due to grain stabilization and dispersion hardening of the Ni grains by Al2O3 nanoparticles. Lin et al.[17] studied the electrodeposited NiTiO2 coatings and found that the Ni-TiO2 coating has higher hardness and lower recrystallization temperature than the Ni coating due to the higher density of lattice defects, finer grain size, and increased number of interfaces in the composite coatings. However, the growth of the recrystallized composite matrix is effectively inhibited by the presence of TiO2 particles up to 873 K (600 C). The annealing beyond 873 K (600 C) deteriorates the properties of the composite coatings arising from the interfacial reaction between the matrix and reinforce