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CHAPTER 7 Conclusions This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process.

Nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into various characteristics and properties was carried out. The main objective was to expand upon the knowledge and understanding around this class of materials. The

following are the key findings:

1. In this study, the grain boundary characteristics for electrodeposited nanocrystalline Ni and Ni-Fe was revealed. HR-TEM analysis showed evidence of local strain at different types of grain boundaries, including high and low angle grain boundaries, and twin boundaries. The nature of these local strains at grain boundaries can be explained based on the interpretation of a non-equilibrium grain boundary that results when the conditions of compatibility are not satisfied. Such grain boundaries have long-range elastic fields since local deformation is necessary for joining of the crystals. Due to the presence of a high volume fraction of intercrystal defects in nanocrystalline materials it is conceivable that the occurrence of these local strains is

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2. The electrodeposited nanocrystalline Ni and Ni-Fe alloys were not exposed to any external loads, and thus, defects such as dislocations and or deformation stacking faults are present in negligible quantities. However, the presence of twin faults of the

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growth type, or “growth faults”, which arise during the synthesis process have been identified. The pure Ni samples were found to have a lower occurrence of growth

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increased presence of growth faults. The addition of soluble alloying elements in fcc metals, such as Fe in Ni, is known to decrease the stacking fault energy. As a result, an increased presence of growth faults can be expected with increasing Fe concentration.

3. In general, a decrease in grain size is accompanied by an increase in the total enthalpy (stored energy) in electrodeposited nanocrystalline Ni and Ni-Fe alloys. Decreasing grain size and/or increasing Fe was also accompanied by an increase in the peak temperature and thus, thermal stability. The heat released during anisothermal annealing is considered to be almost exclusively a result of reducing intercrystal defects (i.e., grain boundaries and triple junctions). In the case of the nanocrystalline Ni-Fe alloys, a contribution to the heat release was expected from ordering of the lattice structure.

4. The hardness of the nanocrystalline Ni and Ni-Fe alloys was found to follow the regular Hall-Petch behaviour down to about a grain size of 20 nm. Increasing Fe concentration and solid-solution strengthening was not considered to be a contributing factor. Below grain sizes of 20 nm, a deviation from the Hall-Petch behaviour was observed. This deviation was owed to the operation of deformation mechanisms other than dislocation slip.

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5. There was no significant influence of grain size on the Young’s modulus of nanocrystalline Ni and Ni-Fe alloys with grain sizes greater than 20 nm. At grain sizes less than 20 nm there was as slight reduction in the Young’s modulus values when compared to their conventional polycrystalline counterparts. Although the general trend with grain size was consistent with the composite model predictions, there was notable variability. The effect of texture was analyzed and found to influence the measured Young’s modulus values over the entire grain size range.

Moreover, at less than 20 nm, there is likely a combined grain size and texture effect.

6. The microstrain for the nanocrystalline Ni and Ni-Fe alloys increased with decreasing grain size in a manner that is consistent with an inverse relationship. This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. When lattice defects are considered to be negligible, microstrain induced XRD line broadening is predominantly a result of increasing intercrystal defects, i.e., grain boundaries. However, when the presence of lattice defects is significant, e.g., the high density of growth faults seen in the nanocrystalline Ni-Fe alloys, there is an observable contribution to microstrain induced XRD line

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7. The macrostresses in the nanocrystalline Ni and Ni-Fe alloys were found to be relatively low down to about 20 nm. At grain sizes less than 20 nm, there was a sharp increase in the compressive macrostress which also followed an inverse grain size relationship. The onset of this increasing macrostress was owed to the corresponding

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increase in intercrystal defects or number of interfaces in the solid. Such interfaces are associated with stresses. When the volume fraction of intercrystal defects is high, the interface stresses become significant and must be equilibrated by a homogeneous bulk stress or analogously macrostress. Thus, the origin of the macrostresses in these materials and dependence on grain size is considered to be a result of the interactions at interfaces and their elastic deformations necessary to maintain a coherent network

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8. The origins of microstrain are linked to the macroscopic strains (macrostresses) since they can both result in XRD line shifts. That is, the interface stresses present in these materials induce local strains or elastic deformations near grain boundaries that are homogeneous. For the foils, the macrostresses are bi-axial and result only in the planar directions. Since the macrostresses are compressive, there is a net contraction in the planar directions or decrease in the interplanar spacing. Microstrain in nanocrystalline materials result in XRD line broadening and a superimposed XRD line shift which is consistent with an increased interplanar spacing. This change in lattice parameter is detected in a direction which is normal to the plane. It follows that the material is undergoing an elastic response that obeys the principle behind the

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CHAPTER 8 Recommendations for Future Work

1. There remains the opportunity to further characterize these materials using various advanced techniques. For example, the presence of porosity in these materials can be better evaluated using positron annihilation spectroscopy. It would also be of significant interest to determine the grain boundary characteristics of these materials with respect to angles of misorientation in order to determine fractions of random and special grain boundaries based on the coincidence site lattice model (CSL). This information could possibly be used to further advance the understanding behind the influence of intercrystal defects (i.e., grain boundaries) on internal stresses and other characteristics.

2. The presence of internal stresses in nanocrystalline materials is intrinsic given that there is a significant volume fraction of intercrystal defects. As seen in this study, there is a tendency for grain boundaries to be in a non-equilibrium state resulting in local strains or elastic deformations. A possibility may exist to minimize the occurrence of these local strains by introducing a grain boundary relaxation process via a heat treatment that does not induce grain growth. Further investigation on this possibility is warranted.

3. The effect of internal stresses on various properties requires further study. In particular, magnetic properties are known to be affected by internal stresses. For example, the inability to obtain coercivity values for the nanocrystalline Ni-Fe alloys

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that are comparable with large grained polycrystalline materials could be due in part by the presence of internal stresses. Confirmation of this would be crucial in directing additional work to potentially improve this magnetic property in electrodeposited nanocrystalline materials.

4. Given the macrostress values in the planar directions, one can decipher the principle stresses using Mohr’s circle calculations. Employing this analysis can also help to resolve the reasons for the curvature which has been observed in some of the samples produced in this study. Furthermore, the a directional dependence on the way in which the foil is produced may provide a link to the observed curvature and present an opportunity for optimization with the objective of obtaining flat foils.

5. In this study, the nanocrystalline Ni-Fe alloys were limited to one grain size per Fe concentration. It would be interesting to produce samples with the same Fe concentration and varying grain sizes to eliminate alloying effects and to isolate structural effects.

6. This study was limited to nanocrystalline Ni and Ni-Fe alloys. It would be beneficial to evaluate other fcc phase materials to determine if the conclusions made in this study can be applied to other systems. In addition to this, an analysis of bcc and/or hcp phase materials using the same techniques would also be of particular interest.

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investigation on internal stresses with grain sizes that are less than 10 nm and perhaps amorphous material is recommended. A study on the amorphous material is particularly interesting in that a transition from a state of stress to a minimal stress could be expected due to the loss of crystallinity.

APPENDICES Appendix A: SEM Images

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0.2 0.1 0.0

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0.20 0.15 0.10 0.05 0.00

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0.10 0.05 0.00

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0.2 0.1 0.0

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Figure B.6. TEM images of sample no. 6 (Ni-7.3wt.%Fe, 32 nm): (a) BF image, (b) DF image, (c) SAD pattern, and (d) grain size distribution.

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0.2 0.1 0.0

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Figure B.7. TEM images of sample no. 7 (Ni-16wt.%Fe, 12 nm): (a) BF image, (b) DF image, (c) SAD pattern, and (d) grain size distribution.

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0.2 Figure B.8. TEM images of sample no. 8 (Ni-23wt.%Fe, 10 nm): (a) BF image, (b) DF image, (c) SAD pattern, and (d) grain size distribution.

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0.3 0.2 0.1 0.0

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Figure B.9. TEM images of sample no. 9 (Ni-32wt.%Fe, 10 nm): (a) BF image, (b) DF image, (c) SAD pattern, and (d) grain size distribution.

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Appendix C: XRD Patterns Figure C.1. XRD pattern (Cu-Kα) for (calculated) Ni powder standard.

Figure C.2. XRD pattern (Cu-Kα) for sample no. 1 (Ni, ~255 nm).

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Figure C.7. XRD pattern (Cu-Kα) for sample no. 6 (Ni-7.3wt.%Fe, 32 nm).

Figure C.8. XRD pattern (Cu-Kα) for sample no. 7 (Ni-16wt.%Fe, 12 nm).

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Figure C.9. XRD pattern (Cu-Kα) for sample no. 8 (Ni-23wt.%Fe, 10 nm).

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