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«STRUCTURE AND PROPERTIES OF ELECTRODEPOSITED NANOCRYSTALLINE NI AND NI-FE ALLOY CONTINUOUS FOILS by Jason Derek Giallonardo A thesis submitted in ...»

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3.3. Compositional Analysis 3.3.1. Energy Dispersive X-Ray Spectroscopy (EDS) Energy dispersive X-ray spectroscopy (EDS) is an analytic technique commonly used for the elemental analysis or chemical characterization of various types of materials. The EDS technique detects X-rays emitted from the sample while it is being bombarded by an electron beam that is supplied by a scanning electron microscope (SEM). For the purposes of this study, the technique was used to analyze the Fe concentration in the Ni-Fe alloy deposits.

The equipment employed was a JEOL JXA 840A Scanning Electron Microscope (SEM) equipped with an energy dispersive X-Ray spectrometer (EDS). The SEM was operated at 20 kV to develop an image at approximately 500 X magnification which is adequate for an accurate EDS measurement. The calibration of the EDS was checked using a standard sample prior to carrying out the analysis on the samples. A minimum of five readings were taken at different locations to determine the average elemental composition for each of the samples in wt.%.

3.3.2. LECO Carbon/Sulfur Analyzer S and C are the two main impurities known to be present in electrodeposited nanocrystalline Ni and Ni-Fe by virtue of the process outlined in El-Sherik and Erb (1995), Erb and El-Sherik (1994), Erb et al. (1995). S, in particular, is known to affect various properties including impact strength [Dini et al. (1975)] and corrosion [Marcus and Talah

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(1989)]. These impurities can be introduced from the various organic additives used in the electrolyte, such as the proprietary additives Nanovate™ A24, B16 and C77.

The majority of metals and their alloys will combust in oxygen when heated to a high enough temperature. The S in the sample is oxidized to sulfur dioxide (SO2) and the C to carbon dioxide (CO2). These gases can be then be measured by infrared (IR) detectors to quantify the amount of S and C in the sample. Complete systems for this type of analysis are made, for example, by LECO (St. Joseph, MI, USA). In this study, S and C were analyzed using a LECO Carbon/Sulfur analyzer (model No. CS-244-784-000). Prior to analysis, the starting weight in a crucible is recorded and then the “accelerator” is added. In this analysis, W was used as the “accelerator”. Samples of approximately 0.5 g were analyzed in duplicate to determine the S and C concentrations in wt.%.

3.4. Microstructural Characterization 3.4.1. Scanning Electron Microscopy (SEM) A Hitachi S-4500 field emission scanning electron microscope (SEM) operated at 15 kV was employed to image the surface of the samples produced in this study. An analysis of the surface quality and morphological characteristics was performed to assess the effectiveness of the electrodeposition technique and the overall quality of the samples that were prepared.

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3.4.2. Transmission Electron Microscopy 3.4.2.1. Conventional In this study, a Hitachi H-800 Transmission Electron Microscope (TEM) operated at 200 kV was used to produce bright-field (BF) and dark-field (DF) images as well as selected area diffraction patterns. The dark-field (DF) images were used specifically to determine the grain size of the sample by counting at least 200 grain diameters and determining the equivalent-circle diameters. Selected area diffraction (SAD) patterns were indexed using the

following equation:

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where, L is the cameral length, d the interplanar spacing, λ the electron wavelength at 200 kV acceleration voltage and R is the radius of the diffraction ring.

3.4.2.2. High-Resolution A Titan II FEI 80-300 high-resolution transmission electron microscope (HR-TEM) operated at 200 kV was used for defect analysis in several electrodeposits. A series of at least twenty images were examined to determine grain boundary characteristics and to identify any lattice defects. These features were analyzed by generating Fast Fourier Transform (FFT) patterns and inverse FFT (IFFT) images using the image processing software by Gatan – Digital Micrograph™ v.3.11.0.

3.2.2.4. Sample Preparation All TEM samples were prepared from the 50 µm foils produced using the electrodeposition technique described earlier. 3 mm discs were stamped out of the 50 μm

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foils using a Gatan sample punch. A Struers Tenupol-3 jet polishing system was used to thin the foils. The jet polishing solution employed was a mixture of 90-80% methanol and 10perchloric acid (by volume). During the jet polishing process, the solution was kept at a temperature between -40 and -50oC using liquid nitrogen. The general jet polishing parameter working ranges are listed in Table 3.1.

Table 3.1.

Parameter working ranges for jet polishing using the Struers Tenupol-3.

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3.4.3. X-Ray Diffraction (XRD) X-ray diffraction (XRD) was used extensively as a tool to characterize the microstructure of all materials produced in the current study. Various methods were applied to determine lattice parameters, interplanar spacing, crystallographic texture, grain size, microstrain and growth fault probabilities in these materials.

In this study, diffraction patterns were generated primarily using an automated Siemens/Bruker D5000 diffractometer. The system is equipped with a high power line focus Cu-K (λ = 0.1542 nm) source operating at 50 kV/35mA. A solid-state Si/Li Kevex detector was used for removal of Kβ lines. The diffraction patterns were collected on a θ/2θ BraggBrentano reflection geometry with fixed slits. A step scan mode was used for data

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acquisition with step size of 0.02o ( 2 ) and counting time of 2.5 s per step. The 2θ range used was 30 to 130o. Data processing was carried out with Bruker AXS software Eva v.8.0.

Additional diffraction patterns were produced using a Rigaku MiniFlex equipped with a CoK (λ = 0.1790 nm) source operated at 40kV/15mA and a step size of 0.02o 2 per minute over a range of 40 to 120o. Data processing was carried out with MDI Jade 5.0.26 (SP1) software. The 50 µm foil samples were cut into 2 cm x 2 cm squares which is approximately the sample holder size.

3.4.3.1. Basic Equations The Bragg law for constructive interference in XRD is given by the following equation,

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where, dhkl is the interplanar spacing and θ is the diffraction angle, λ is the X-ray wavelength, and n is the order of reflection which may be any integer (1, 2, 3, …). The interplanar spacing, dhkl, is a function of the Miller indices (h, k, and l) as well as the lattice parameters.

In the case of crystal structures having cubic symmetry,

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where, a is the lattice parameter (unit cell edge length).

3.4.3.2. Texture The degree of preferred orientation may be determined by comparing relative intensities of particular types of planes in the material with those in a random structure. In

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the case of a fibre texture, the orientation index can be obtained from the conventional X-ray diffractometer with θ-2θ geometrical arrangement. The degree of preferred orientation can be quantitatively described in terms of the orientation index, I hkl. The orientation index, I hkl, which is defined as the fractional intensity of a particular set of planes to its fractional intensity in the same material with a random structure [Willson and Rogers (1964)],

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diffraction lines (i.e., hkl = 111, 200, 222, 311, etc for fcc materials), and the subscripts e and r are for the electrodeposit and the random structure, respectively. If the orientation index for a line is equal to 1, then the structure is random. If the orientation index does not equal 1, then a texture exists.

3.4.3.3. Line Broadening Line broadening in XRD for polycrystalline materials is commonly from a contribution of grain size, microstrain, and instrumental effects [Cullity and Stock (2001)].

These contributions can be effectively separated by considering the lines to be broadened according to either a Lorentzian or Gaussian shape. For Lorentzian shaped lines,

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where, Bm is the experimentally measured full-width at half-maximum (FWHM), Bsize is the FWHM due to grain size, Bstrain is the FWHM due to microstrain, and Binst is the FWHM

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performing XRD using identical conditions on either a well annealed (large grain size) or powder sample of the same substance.

3.4.3.4. Grain Size Another use for the diffraction pattern is the estimation of crystal size [Cullity and Stock (2001)]. When the size of the crystals is less than about 100 nm there is sufficient diffraction line broadening to allow for the determination of the crystal size using the Scherrer formula,

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where, Bsize = FWHM of the broadened diffraction line on the 2θ scale in radians and d = the diameter of the crystals, or grain size. In this study, the grain size of each of the samples was determined by analyzing the (111) and the (200) lines of XRD patterns generated using the Rigaku MiniFlex (Co-K) diffractometer. The software used to analyze the grain size provides an accurate estimation of grain size by taking into account the instrumental line broadening. The instrumental line broadening was determined by generating an XRD pattern of a polycrystalline nickel powder and measuring the FWHM for each of the resulting lines.

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3.4.3.5. Microstrain In addition to line broadening due to small crystals, non-uniform microstrain and stacking faults also cause X-ray diffraction lines to broaden [Cullity and Stock (2001)].

Microstrain analysis was performed on the (XRD) patterns generated using the Siemens/Bruker D5000 (Cu-K) diffractometer for each of the deposits produced. In this case, line broadening due to the instrument is accounted for by using the fundamental parameters approach (FPA). The full-width at half-maximum (FWHM) of multiple broadened XRD lines were determined by employing the double-Voigt method [Balzar (1999)] using the Bruker AXS profile fitting software Topas™ v.2.1. This method separates the effects of line broadening due to grain size and microstrain while maximizing accuracy by considering both Gauss and Cauchy distribution functions. The line broadening due to microstrain is calculated based on the relationship obtained by differentiating Bragg’s law with respect to the interplanar spacing, d, to yield the relation [Wilson (1962)],

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where, Bstrain is the line broadening (FWHM) due to microstrain,   d / d is the measured microstrain which represents the variation in interplanar spacing, and θ is one half of the line’s 2θ peak value in radians.

3.4.3.6. Growth Fault Probabilities Growth fault probabilities, , were determined using the XRD patterns and employing the method described by Cohen and Wagner (1962). The method analyzes the peak asymmetry by determining the displacement of the center of gravity of a peak from the peak maximum which is then used to calculate the probability, , of finding growth faults,

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where, C.G. is the displacement of the center of gravity for the (111) and (200) peaks in degrees, and 111 and  200 are one half of the 2θ values for the (111) and (200) peaks in degrees, respectively. Combining (3-15) and (3-16) we arrive at,

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The XRD patterns generated were employed for this analysis to determine the growth fault probabilities for each of the samples produced in this study. C.G. was determined using a non-linear least squares fitting program.

3.5. Thermal Analysis Differential scanning calorimetry (DSC) is a technique that measures the temperature and heat flow associated with transitions in materials as a function of time and temperature.

It provides qualitative and quantitative information about the chemical and physical changes that involve endothermic or exothermic processes. In the case of nanocrystalline materials, the total enthalpy or total heat release ( H total ) corresponding to grain growth in the form of an exothermic event allows for determination of the peak temperature, T p, which can be used as a relative measure of the thermal stability.

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In this study, a TA Instruments Q20 conventional DSC was used to measure the heat flow from the samples produced in this study. 3 mm discs were stamped out of the 50 μm foils using a Gatan sample punch. In order to obtain a sufficient heat flow signal, about 7-12 mg of sample was required. Several 3mm discs were stacked on top of each other and sealed in hermetic Al containers. The temperature range was approximately 50 to 500oC. All samples were subjected to a heating rate of 5oC/min. The exothermic event corresponding to grain growth was integrated using TA Universal Analysis 2000 Version 4.7A software to determine the total heat release ( H total ) and peak temperature, T p.

3.6. Nanoindentation There have been many advances in the development of a nanoindentation technique for determining the mechanical properties of materials on the submicron level [Pethica et al.

(1983), Oliver et al. (1986), Doerner and Nix (1986)]. The technique is based on developing load-displacement data that is eventually analyzed allowing for the determination of mechanical properties such as the hardness, H, and Young’s modulus, E. The technique was further refined by Oliver and Pharr (1992), Fischer-Cripps (2002), Klinger and Rabkin (2003), and Oliver and Pharr (2004) which eventually lead to the development of an ASTM standard (E2546-07 “Standard Practice for Instrumented Indentation Testing”) that was released after the initiation of the work in this thesis.

The method employed in this thesis is the same described by Oliver and Pharr (1992) and is based on the Berkovich indenter (three-sided pyramid). Fig. 3.2 presents a cross

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Figure 3.2.

Schematic representation of a section through an indentation identifying the parameters used in the analysis [Oliver and Pharr (1992)].

Figure 3.3.



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