Effect of Solid-Solution W Addition on the Nanostructure of Electrodeposited Ni
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Effect of Solid-Solution W Addition on the Nanostructure of Electrodeposited Ni Hajime Iwasaki, Kenji Higashi1 and T. G. Nieh2 Department of Materials Science & Engineering, Himeji Institute of Technology 2167 Shosha, Himeji, Hyogo 671-2201, Japan 1 Department of Metallurgy and Materials Science, College of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan 2 Lawrence Livermore National Laboratory, L-350, PO Box 808, Livermore, CA 94551, USA ABSTRACT Electrodeposition method was employed to produce freestanding Ni-W alloy foils. The foils consist of nanograins. The structure of the foil, e.g. texture, grain morphology, size distribution, and the nature of grain boundaries, were characterized using X-ray diffraction and high-resolution electron microscopy. The deposited foils exhibit an equiaxed nanocrystalline structure having a grain size value of about 6 nm. Two types of grain boundary structure were observed. One type of grain boundary is essentially one atomic layer thin and another type consists of a structureless layer of about 0.5-1 nm in thickness. Angular dark field (Z-contrast) image of the deposited foils showed an inhomogeneous distribution of W solutes. In some local regions, the W content actually exceeds the equilibrium solid solution limit. Many grain boundaries with a structureless layer of about 0.5-1 nm are probably a result of local supersaturation of W. INTRODUCTION The technological development of nanocrystalline alloys has been driven by the promise of exceptional properties. Scientifically, the study of nanocrystalline materials is also of great interest because the potential breakdown of classical scaling laws and the accompanying need for new materials physics in the nanostructured limit. However, materials with grain sizes below about 10 nm are still extremely difficult to produce. Electrodeposition is a viable method to produce fine-structured material. However, to produce nanocrystalline materials using electrodeposition, fine grain sizes apparently require a compromise in purity. For example, the addition of a nucleation agent during electrodeposition can reduce the grain size of the nanocrystalline product, but it also increases the carbon and sulfur content of the alloy [1,2]. Alternatively, finer grain sizes can be achieved via alloying, as demonstrated in Fe-Ni [3,4] and other systems [5,6]. Alloying has long been recognized as an effective method to increase glassforming ability and, in fact, is critical for the production of bulk amorphous metals and devitrified nanocrystalline alloys [7]. Considered collectively, the above studies suggest that fine-structured nanocrystalline metals (from amorphous to ~10 nm grain size) are achievable through the use of a low-temperature single-step production process and a careful selection of alloy composition. In the present work, we study the grain and grain boundary structures of nanocrystalline NiW alloys with grain sizes below 10 nm produced by direct current (DC) electrodeposition. Prior research has shown that Ni-W a
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