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Experimental results are presented concerning the influence of primary intercrystalline defects, i. e., grain boundaries and triple junctions, on the properties of nanocrystalline nickel and its alloys. The properties considered are largely based on nanostructured materials relatively free of secondary defects introduced during synthesis, such as porosity, residual amorphous regions, lattice dislocations and extrinsic grain boundary dislocations. These property studies include thermal stability and grain growth, mechanical, electrical, corrosion and hydrogen transport.

Abstract

Magnetic properties of nanocrystalline Ni and Ni-Fe alloys produced by electrodeposition have been studied at 2K and at 298K. Ni and Ni-15%Fe alloy deposits show nano-grain structure with the average grain size of 23 nm and 12 nm, respectively. Both materials exhibit soft magnetic properties. Nanocrystalline Ni at 2K shows saturation magnetization, coercive force, and remanent magnetization of 57 emu/gr, 101 Oe, and 16 emu/gr respectively. Nanocrystalline Ni-15%Fe alloy exhibits superior soft magnetic properties than Ni with corresponding saturation magnetization, coercive force, and remanent magnetization at 2K of 96 emu/gr, 6 Oe, and 4 emu/gr respectively. The magnetic properties and their dependence upon temperature data are interpreted in terms of the Herzer random anisotropy model for nanocrystalline materials.

Nanocrystalline metals are often produced in a state of stress which can adversely affect certain properties, e.g. corrosion resistance, wear, fatigue strength, etc. This stress is referred to as internal or “intrinsic” stress since it is not directly caused by applied loads. The structural causes of these stresses in nanocrystalline materials are not fully understood and are therefore an area of particular interest. The internal stresses of nanocrystalline Ni and Ni-16wt%Fe were measured and found to increase with the addition of iron. Characterization using HR-TEM revealed no signs of porosity, second phase particles, or a high density of dislocations. Both materials possessed well defined high-angle grain boundaries. The main structural difference between the two materials was found to be grain size and correspondingly, a decrease in grain size resulted in an increase in internal stress which supports the applicability of the coalescence theory. The current study also provides evidence to rule out the effect of voids (or porosity), dislocations, and second phases as possible causes of internal stress.

An analysis of grain-size dependent structure-sensitivity of various physical and mechanical properties of nickel is presented, covering the crystal size range from 10 nm to 10 μm. It is shown that the well established general trends regarding defect-sensitivity observed for conventional polycrystalline materials also apply to porosity-free nanomaterials with grain sizes less than 100 nm produced by the electrodeposition process.

In this study, the surface structure of a self-cleaning, superhydrophobic leaf was examined using electron microscopy and optical methods, and its wetting properties were measured using a contact angle goniometer. Using the micro/nanostructural surface features of this leaf as a blueprint, an inexpensive surface structuring technique was developed by modifying the surface of nanocrystalline nickel to create a template. These templates were then pressed into softened polyethylene at elevated temperatures and pressures, thereby transferring the structured surface to the polymer samples. All templates and pressed polymers were characterized in the same manner as the leaves. This method increased the wetting angle for polyethylene from 96° to 151° and reduced the tilt angle from 38° to <5°.

The grain-size dependences of thermal conductivity and electrical resistivity of polycrystalline and nanocrystalline nickel were measured by the flash method and four-point probe method, respectively. Nanocrystalline nickel (grain size: 28 nm) was made by the pulsed-current electrodeposition process, while polycrystalline nickel (grain size: 57 μm) was the same material in fully annealed condition. Noticeable differences in thermal conductivity and electrical resistivity were observed for both materials. These results can be explained on the basis of the rapid increase in the intercrystalline grain boundary and triple junction volume fractions at very small grain sizes. The relationship between thermal conductivity and electrical resistivity of nanocrystalline nickel follows the classic Wiedemann-Franz law.

The work hardening behavior of electrodeposited nanocrystalline (grain size: 100 nm) and fully annealed polycrystalline nickel (grain size: 160 µm) was examined by hardness indentation analysis. First, plastic strain was introduced into the materials through large Rockwell hardness indentations. A series of Vickers micro-hardness traces below and away from the Rockwell indentation then measured the change in hardness as a function of distance from the plastic zone. The results showed that polycrystalline nickel exhibited considerable strain hardening, with micro-hardness values closest to the Rockwell indentation averaging twice the hardness value of the bulk material. On the other hand, for the nanocrystalline nickel the Vickers micro-hardness values changed only by a few percent indicating a limited strain hardening capacity.

In this study, alternatives to corrosion-resistant cadmium coatings on high strength steel fasteners are explored. Nanocrystalline Zn-Ni and Zn-Ni-Co electrodeposits, synthesized by electrochemical deposition, were analyzed by potentiodynamic polarization in a 1M NaCl aqueous solution, to determine the corrosion behaviour of the alloys. Sample coatings were characterized using scanning electron microscopy to examine differences in surface morphology that can affect corrosion resistance. Energy dispersive x-ray spectroscopy was used to relate composition to corrosion resistance and to characterize the corrosion behavior during polarization. All coatings showed corrosion potentials at equivalent, or lower values than that of cadmium, suggesting that both alloys are viable as cadmium replacements. Specific compositions showed slightly passivating regions, but the brittle film formed during polarization is unstable. Comparatively, Zn-Ni shows a better resistance to corrosion than Zn-Ni-Co. The effect of composition on the corrosion resistance can be explained on the basis of the sacrificial depletion of Zn over Ni.

Monolithic and multilayered iron electrodeposits were successfully synthesized by the pulse plating electrodeposition method. Electron microscopy and Vickers microhardness measurements were used to investigate the microstructure and mechanical properties of the iron electrodeposits produced. Two types of monolithic iron coatings were produced, one with a coarse grained, columnar structure and the other with an ultra-fine grained structure. Hall-Petch type grain size strengthening was observed in these monolithic coatings. Multilayered iron coatings composed of alternating layers of coarse grained and fine grained structures were also produced. The hardness value of the multilayered coatings falls between the hardness values for the two types of monolithic coatings produced. This study has demonstrated the possibility of applying a multilayered structure design to tailor the microstructure and mechanical properties of electrodeposited iron coatings.

Commercially available polycrystalline nickel (grain size: 30 µm) and electrodeposited nanocrystalline nickel (grain size: 30 nm) were analyzed for the effect of stress-induced heat generation during plastic deformation at room temperature. Tensile coupons in conformance to ASTM E8 standard were tested at a strain rate of 10-1/s to record the amount heat dissipated using a high resolution infrared detector. The maximum temperature increases recorded for nanocrystalline and polycrystalline nickel close to sample fracture were 58℃ and 70℃, respectively. Grain growth in nanocrystalline nickel due to stress-induced heat generation is unlikely since the maximum temperature during deformation is below the previously reported onset temperature for grain growth in nanocrystalline nickel.

This paper reviews size effects in nanocrystalline metals and alloys made by electroforming, a particular form of electrodeposition. The main size effects in these fully dense, three-dimensional nanomaterials come from grain size reduction to less than 5 nm and down towards the limit of the amorphous structure. Details will be given on the synthesis of such materials and their microstructural characteristics including reduced crystal size and intercrystalline defect density. Properties which show either weak or strong grain size dependence will be discussed and compared with the structure-sensitivity of various properties in conventional polycrystalline materials.

Due to their excellent wear and corrosion properties, nanocrystalline cobalt and several cobalt alloys made by electrodeposition are currently being developed as environmentally benign replacement coatings for hard chromium electrodeposits. The focus of this study is on the mechanical properties of nanocrystalline cobalt, which are currently not well understood. A comparison is presented for hardness, tensile properties, Charpy impact properties and fracture surface analysis of both nanocrystalline (grain size: 12 nm) and conventional polycrystalline (grain size: 4.8 μm) cobalt. It is shown that the hardness and tensile strength of nanocrystalline cobalt is 2–3 times higher than for polycrystalline cobalt. However, in contrast to other nanocrystalline materials tested previously, nanocrystalline cobalt retains considerable ductility with elongation to fracture values up to 7%. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A comparison is presented of the cross-sectional mechanical properties (i.e. hardness and Young's modulus) for electrodeposited microsystem components with conventional polycrystalline and nanocrystalline structures. It is shown that nanocrystalline electrodeposits exhibit considerabily improved cross-sectional properties both in terms of uniformity (hardness and Young's modulus) and overall hardness level. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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The influence of the structural disorder at grain boundaries on the local magnetic moments has been investigated using molecular dynamics and the tight-binding-linear-muffin-tin-atomic-sphereapproximation (TB-LMTO-ASA) method. The correlation between the local structural disorder of grain boundaries and the local magnetic moment is discussed. While there is some variation in the values of the local moments, the average magnetic moment is very insensitive to the type of boundary. These results correlate extremely well with the recent observation that the saturation magnetization of fully dense electrodeposited nickel depends very little on grain size, down to about 10 nm. The local magnetic moments of the strong itinerant ferromagnets are, however, strongly dependent on alloying elements, which are used commonly to produce smaller grains in nanocrystalline materials. Alloying effects are discussed for Ni-P, Ni-Al and Co-W alloys. The influence of atom substitution on the host grain boundary is studied in the case of the Σ = 5 nickel grain boundaries by replacing nickel atoms with aluminium.

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