Effect of Annealing on Hardness and the Modulus of Elasticity in Bulk Nanocrystalline Nickel
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INTRODUCTION
NANOCRYSTALLINE (nc) materials are defined as single- or multiphase polycrystals that are characterized by grain sizes in the range 1 to 100 nm. Because of their small grain size, grain boundaries, junction lines, and nodes have significant volume fractions that influence properties far more strongly than in more conventional microstructures. The mechanical properties of nc materials have been the subject of a number of studies.[1,2] In general, these studies have been motivated in part by a growing interest in assessing the potential of nc materials as materials for engineering applications. Primary among the mechanical properties that have received considerable attention are the hardness and the modulus of elasticity.[3–13] Studies of the hardness in nc materials focused in part on the identification of parameters that might influence hardness in terms of softening or strengthening as a result of annealing. Fougere et al.[11] reported an increased hardness with decreasing grain size for nc-Cu and nc-Pd when samples were annealed at 423 and 569 K (150 and 296 C) (~0.315 Tm, where Tm is the melting point), respectively. Such strengthening was explained in terms of the reduction of porosity during annealing. Chen et al.[12] studied Ni-P hardness behavANNA TORRENTS and HEATHER YANG, Graduate Research Assistants, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical Engineering and Materials Science, University of California-Irvine (UCI), Irvine, CA. Contact e-mail: [email protected] Manuscript submitted March 18, 2009. Article published online January 28, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
ior after heat treatment in the temperature range 623 to 873 K (350 to 600 C). Their results[12] indicated that hardening occurred below a critical temperature, located between 623 and 673 K (350 and 400 C), followed by softening after annealing at higher temperatures. Chen et al.[12] associated the increase in hardness with an increasing annealing temperature with the formation of nanocrystals. Yang and Vehoff[13] annealed nc-Ni samples at 573 and 673 K (300 and 400 C) for different holding times to produce grain sizes in the range 30 lm to 30 nm. They reported[13] that the hardness increased with decreasing grain size and that the hardness scaled with the dislocation density within the grains. Early studies on the moduli of elasticity in nc materials were carried out using materials prepared by the gas condensation method. These studies revealed that the values of the modulus of elasticity for nc materials were significantly lower than for coarsegrained counterparts.[14,15] The lower values of the moduli of elasticity in nc materials prepared by the gas condensation method were explained in terms of the presence of porosity.[15,16] In the present study, the variations in hardness and in the modulus of elasticity in nc-Ni prepared by the electrodeposition technique as a function of the annealing temperature were studied. The investigation was motivated by two primary considerations. Firs
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