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In general, there are still some characteristics and properties of these materials which require an advanced understanding. Microstructural studies on electrodeposited nanocrystalline materials have often been limited to the X-ray diffraction (XRD) and conventional transmission electron microscopy (TEM) methods. Only a few studies have used high-resolution transmission electron microscopy (HR-TEM) with a narrow scope to examine the microstructure of electrodeposited nanocrystalline materials. For example, Mehta et al. (1995) reported the presence of low angle grain boundaries and twin boundaries in electrodeposited nanocrystalline Ni and Ni-1.2wt.%P. However, the grain boundary characteristics were not studied in great detail. Mehta et al. (1995) also investigated the presence of lattice defects and noted that electrodeposited nanocrystalline Ni and Niwt.%P had very few dislocations and thus, the dislocation density in these materials is considered to be very low. The presence of other lattice defects in these materials, for

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example, stacking faults, still remains to be analyzed. Since the addition of Fe to Ni in the fcc phase is known to decrease the stacking fault energy [Charnock and Nutting (1967)], stacking faults are likely to be present. Although the presence of stacking faults has been reported in electrodeposited nanocrystalline Ni and Ni-Fe alloys [Yang et al. (2009), Wu et al.

(2006), Ebrahimi and Li (2003)], an in-depth analysis on both a qualitative and quantitative basis is still necessary. In general, additional information on grain boundary characteristics and lattice defects can lead to a better understanding of their possible effects on various material properties and behaviour.

The indentation behaviour of these electrodeposited nanocrystalline materials, e.g., hardness and Young’s modulus, has been studied in some detail. The hardness of nanocrystalline Ni was studied by El-Sherik et al. (1992) who observed a noticeable deviation from the regular Hall-Petch relationship. Nanocrystalline Ni-Fe alloys were also studied by Cheung et al. (1995) who showed a similar deviation from the regular Hall-Petch relationship but also noted that these changes were not due to any solid solution strengthening effect. Rather, the deviation from the Hall-Petch relationship is considered to be a result of a change in the dominant deformation mechanism [e.g., Chokshi et al. (1989)].

In the case of Young’s modulus, only electrodeposited nanocrystalline Ni-2.5wt.%P has been studied systematically with grain size [Zhou et al. (2003a), Zhou et al. (2003b)]. There remains the opportunity to also systematically study the Young’s modulus of electrodeposited nanocrystalline Ni and Ni-Fe alloys. In addition to the effect of grain size, alloying may also be taken into consideration along with other potentially influential factors such as anisotropy. Additionally, the potential effects of internal stresses on the properties of

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these nanocrystalline materials should also be considered. For example, it has been suggested by Cammarata and Eby (1991) that the strains which result from decreasing grain size and the corresponding effect of interface stresses can produce higher order elastic effects.

That is, very fine grained materials may have slightly reduced Young’s moduli if internal stresses are present.

Electrodeposition processes typically produce materials that have some degree of internal stress [Schlesinger and Paunovic (2000)]. These internal stresses may be analyzed on both a microscopic and a macroscopic scale. For example, there are several studies which consider the effect of grain size on microstrain (or microstress) for electrodeposited Ni [Mishra et al. (2004), Wang (1997)] and Ni-Fe alloys [Li and Ebrahimi (2003)]. Generally speaking, microstrain results from atomic scale imperfections, e.g., dislocations, vacancies, grain boundaries, voids, inclusions [Macherauch and Kloos (1987)], which induce XRD line

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nanocrystalline materials, various studies have shown that microstrain clearly increases with decreasing grain size, especially at less than 20 nm [e.g., Li and Ebrahimi (2003), Qin and Szpunar (2005)]. The nature of this microstrain has been related to observable local strains at grain boundaries using HR-TEM, e.g., for nanocrystalline Pd produced the inert gas condensation technique [Wunderlich et al. (1990)]. A systematic study over a broad grain size range to determine microstrain from XRD line broadening combined with an investigation on the grain boundary characteristics of electrodeposited nanocrystalline materials has yet to be carried out. In addition to this, an investigation on the presence of

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lattice defects, such as stacking faults, may also reveal a contribution to microstrain induced XRD line broadening.

Macroscopic stresses (macrostresses) are manifested over large distances or many grains. These macrostresses cause shifting of the characteristic XRD lines [Macherauch and Kloos (1987)], or a detectable uniform strain [Cullity and Stock (2001)]. There are only a few studies which provide some insight into a microstructural relationship with macrostress.

For example, in an investigation on electrodeposited nanocrystalline Ni-Fe, Czerwinski (1996) established a relationship between grain size and macrostress in Fe-Ni alloy deposits (15wt.%Ni): a decrease in grain size is accompanied by a rise in macrostresses. The technique used in this study was the “bent-strip” method [Kouymdjiev (1985)] which is inexpensive and thus, quite popular in practice. However, this method has often produced results which are inconsistent from study to study. On the other hand, XRD methods are rarely used in practice, yet they offer the advantage of being substantially more accurate and consistent. El-Sherik et al. (2005) used an XRD method to measure the macrostress in electrodeposited nanocrystalline Ni (10 nm) and compared it with a large grain electrodeposited Ni (5000 nm) to find that the nanocrystalline electrodeposit had a macrostress which was about six times greater. A systematic study with grain size that covers a broad range using an XRD technique can provide a more accurate and consistent means to experimentally decipher a structure-macrostress relationship.

1.2. Research Objectives The research objectives of this study are based on the current opportunities for the

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contribution of knowledge in this field. The synthesis technique employed in this study to produce the nanocrystalline Ni and Ni-Fe alloys was electrodeposition of foils in a continuous process. Using these materials, an intensive investigation into certain

characteristics and properties was carried out. The main research objectives were:

1. To synthesize a series of nanocrystalline Ni and Ni-Fe alloy foils spanning a broad grain size range using a novel continuous electrodeposition technique [Palumbo et al.


2. To characterize the series of nanocrystalline Ni and Ni-Fe alloys using X-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HR-TEM), and thermal analysis. In particular, an attempt was made to divulge as much microstructural information as possible using HR-TEM for the purpose of assessing possible relationships with the behaviour of these materials.

3. To determine the hardness and Young’s modulus of these materials using the nanoindentation technique. In particular, Young’s modulus was studied extensively for any effect related to grain size and other possible influencing factors, such as alloying with Fe and anisotropy.

4. To determine the microstrain in these materials and evaluate the relationship with grain size. Additionally, to determine if there are other possible influencing factors, including lattice defects such as stacking faults which can arise from alloying with Fe.

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5. To systematically determine the macrostresses from strain measurements using a twodimensional (2D) XRD technique as a function of grain size and investigate the origins of these macrostresses in electrodeposited nanocrystalline materials.

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1.3. References Aus, M.J., Szpunar, B., A.M. El-Sherik, U. Erb, G. Palumbo and K.T. Aust, Scripta Metall.

Mater. 27 (1992) 1639.

Aus, M.J., “Magnetic Properties of Nanocrystalline Transition Metals”, Ph.D. Thesis, University of Toronto, 1999.

Bozorth, R., “Ferromagnetism”, Wiley-IEEE Press, New York, 1978.

Cammarata, R.C. and R.K. Eby, J. Mater. Res. 6 (1991) 888.

Charnock, W. and J. Nutting, Met. Sci. J. 1 (1967) 123.

Cheung, C.K.S., F. Djuanda, U. Erb and G. Palumbo, Nanostruct. Mat., 5 (1995) 513.

Chokshi, A.H., A.H. Rosen, J. Karch and H. Gleiter, Scripta Metall. Mater. 23 (1989) 1679.

Cullity, B.D. and S.R. Stock, “Elements of X-Ray Diffraction, Third Edition”, Prentice Hall, New Jersey, 2001.

Czerwinski, F., J. Electrochem. Soc. 143 (1996) 3327.

Ebrahimi, F. and H.Q. Li, Rev. Adv. Mater. Sci. 5 (2003) 134.

El-Sherik, A.M., U. Erb, G. Palumbo and K.T. Aust, Scripta Metall. Mater., 27 (1992) 1185.

El-Sherik, A.M., J. Shirokoff and U. Erb, J. All. Comp. 389 (2005) 140.

Erb, U., K.T. Aust and G. Palumbo, in “Nanostructured Materials: Processing, Properties, and Applications – 2nd Edition”, Ed. C.C. Koch, Noyes | William Andrew, New York, 2007.

Herzer, G., Scripta Metall. Mater. 33 (1995) 1741.

Kouymdiev, C.N., Surf. Technol. 26 (1985) 35, 45, 57.

Krstic, V.D., U. Erb and G. Palumbo, Scripta Metall. Mater. 29 (1993) 1501.

Li, H.Q. and F. Ebrahimi, Mater. Sci. Eng., A347 (2003) 93.

Macherauch, E. and K. H. Kloos, in: Residual Stresses in Science and Technology (E.

Macherauch and V. Hauk, ed.), Informationsgesellschaft, Oberursel, 1987.

Marsh, J.S., “The Alloys of Iron and Nickel: Volume I – Special Purpose Alloys”, McGrawHill Book Company, New York, 1938.

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McCrea, J.L.M., G. Palumbo and U.Erb, in “Nanostructured Metals and Alloys: Processing, Microstructure, Mechanical Properties and Applications”, S.H. Whang, Woodhead Publishing, Philadelphia, 2011.

Mehta, S.C., D.A. Smith and U. Erb, Mat. Sci. Eng. A204 (1995) 227.

Mishra, R., B. Basu and R. Balasubramaniam, Mater. Sci. Eng. A 373 (2004) 370.

Nieman, G.W., J.R. Weertman and R.W. Siegel, J. Mater. Res. 6 (1991) 1012.

Palumbo, G., F. Gonzalez, K. Tomantschger, U. Erb and K.T. Aust, Plat. Surf. Finish. 90 (2003) 36.

Palumbo, G., I. Brooks, J. McCrea, G.D. Hibbard, F. Gonzalez, K. Tomantschger and U. Erb, United States Patent Application No. US 2005/0205425 A1, 2005.

Palumbo, G., S.J. Thorpe and K.T. Aust, Scripta Metall. Mater. 24 (1990) 1347.

Qin, W. and J.A. Szpunar, Phil. Mag. Lett. 85 (2005) 649.

Schlesinger, M. and M. Paunovic, “Modern Electroplating, Fourth Edition”, John Wiley & Sons, New York, 2000.

Van Petegem, S., F. Dalla Torre, D. Segers and H. Van Swygenhoven, Scripta Mater. 48 (2003) 17.

Wang, N., “Microstructures and Mechanical Properties of Nanocrystalline Materials”, Ph.D.

Thesis, University of Toronto, 1997.

Wang, N., Z. Wang, K.T. Aust and U. Erb, Mater. Sci. Eng. A237 (1997) 150.

Wu, X.-L., Y.T. Zhu and E. Ma, Appl. Phys. Lett. 88 (2006) 121905.

Wunderlich, W., Y. Ishida and R. Maurer, Scripta Metall. Mater. 24 (1990) 403.

Yang, F., W. Tian, C. Feng and B. Wang, Acta Metall. Sin. (Engl. Lett.) 22 (2009) 383.

Zhou, Y., U. Erb, K.T. Aust and G. Palumbo, Scripta Mater. 48 (2003) 825.

Zhou, Y., U. Erb, K.T. Aust and G. Palumbo, Z. Metallkd. 94 (2003) 1157.

Zhou, Y., S. Van Petegem, D. Segers, U. Erb, K.T. Aust and G. Palumbo, Mater. Sci. Eng.

A512 (2009) p.39.

Zugic, R., B. Szpunar, V.D. Krstic and U. Erb., Phil. Mag. A 75 (1997) 1041.

CHAPTER 2 Literature Review

2.1. Electrodeposited Nanocrystalline Metals and Alloys 2.1.1. Synthesis There have been many reports describing electrodeposits with ultra-fine structure [Safranek (1986), Schlesinger and Paunovic (2000)]. However, no systematic studies were published before the late 1980’s on the synthesis of nanocrystalline materials using the electrodeposition technique. This was the motivation for McMahon and Erb (1989) to carry out such investigations on electrodeposited nanocrystalline Ni-P alloys which eventually led to the identification of a process (Fig. 2.1) by Erb and El-Sherik (1994) and Erb et al. (1995) to electrodeposit a wide range of nanostructured metals and alloys with control over the grain size for the purpose of optimizing material properties.

Figure 2.1.

Electrochemical processing of nanocrystalline metals and alloys: (1) plating cell, (2) electrolyte, (3) anode, (4) ammeter, (5) power source, (6) cathode, (7) transistored switch, (8) wave generator, (9) oscilloscope, and (10) constant temperature bath [Erb et al. (1995)].

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In this process, deposit growth conditions are controlled to favour massive nucleation regardless of coating thickness. These deposits show no cross-sectional structure transition from fine-grained-equiaxed to coarse-grained columnar structure with strong grain shape anisotropy, as in the case of conventional electrodeposits [Erb et al. (2007)]. In conventional electroplating, the deposit shows considerable gradients (through the thickness) in grain shape and size with most final grain sizes being larger than 1 μm (Fig. 2.2a). If the plating parameters are chosen such that massive crystal nucleation dominates over the crystal growth competition at any given stage in the plating process, a nanocrystalline structure over the entire thickness can be obtained (Fig. 2.2b).

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2.1.2. Structure of Fully Dense Nanocrystals Gleiter (1989) first expressed the basic ideas surrounding the structure of nanomaterials where he recognized that large volume fractions of grain boundaries exist in materials with very small grain sizes. In particular, for grain sizes less than 20 nm, the grain

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