Amorphous ribbons of alloy composition Fe75B10Si15 are cast by melt spinning and annealed to partially nanocrystalline states. The magnetic properties are investigated by VSM and MTGA. Structure is examined using XRD and SEM. Results obtained show nanostructured material with excellent soft magnetism in samples annealed at temperatures below the crystallization temperature as well as enhancement of magnetic hardness for annealing at high temperatures. This validates Herzer’s Random Anisotropy model of magnetism in nanostructured materials and provides basis for further inquiry into tweaking alloy compositions and/or manipulating annealing parameters. Also, increase of Curie temperature is noted with respect to increasing annealing temperatures arising from stress relaxation, validating a study on the relationship between the two.

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Ball milling has been shown to produce nanostructures in metal powders through severe repetitive deformation. Ball milling at cryogenic temperatures (cryomilling) is more effective in this capacity due to the low temperature by slowing recovery and minimizing diffusion distances between different components. Nanostructured metals are of interest because of their unique physical and mechanical properties. The result of cryomilling is powder consisting of crystallites on the order of 30 – 50 nm. In order to characterize the properties of this material, it is often necessary to consolidate the powder, which is often difficult without causing significant grain growth. In this work, aluminum-rich and magnesium-rich alloys of varying composition are produced by cryomilling and characterized by x-ray diffraction. A novel consolidation process called high shear powder consolidation (HSPC) is used to densify as-received and as-milled powders with minimal growth. The construction of a cryomill, along with a modification for improving process yield, has provided a platform for the study of nanocrystalline metals. It has been shown that bulk nanocrystalline materials are attainable and that alloy composition influences mechanical properties. / Master of Science

Crack propagation studies in nanocrystalline alpha-iron samples with grain sizes ranging from 6 to 12 nm are reported at temperatures ranging from 100K to 600K using atomistic simulations. For all grain sizes, a combination of intragranular and intergranular fracture is observed. Mechanisms such as grain boundary accommodation, grain boundary triple junction, grain nucleation and grain rotation are observed to dictate the plastic deformation energy release. Intergranular fracture is shown to proceed by the coalescence of nanovoids formed at the grain boundaries ahead of the crack. The simulations also show that at an atomistic scale the fracture resistance and plastic deformation energy release mechanisms increase with increasing temperature. Finally a softening of the material occurs with decreasing grain size. The elastic properties are found to decrease and the fracture resistance to increase with decreasing grain size. / Master of Science

Yes / The tribological performance of a nanocrystalline coating is heavily influenced by its composition, morphology, and microstructural characteristics. This research work describes the effect of heat treatment temperature on the microstructural, morphological, and mechanical behavior of nanocrystalline Ni/TiO2 coatings produced by electrophoresis. The surface morphology and coating cross section were characterized by scanning electron microscopy (SEM). The composition of coatings and the percentage of TiO2 nanoparticles incorporated in the Ni matrix were studied and estimated by using an energy-dispersive spectroscopic (EDS) analysis, while x-ray diffractometry (XRD) was used to investigate the effect of heat treatment temperature on phase structure. The results showed agglomeration of TiO2 nanoparticles on the surface of the coating. The high hardness and wear resistance recorded for the as-deposited coating was attributed to the uniform distribution of TiO2 nanoparticle clusters throughout the cross section of the coating. Heat treatment of the Ni/TiO2 coatings to temperatures above 200 °C led to significant grain growth that changed the surface morphology of the coating and reduced the strengthening effects of the nanoparticles, thus causing a reduction in the hardness and wear resistance of the coatings.

Conventional liquid lubricants are faced with limitations under extreme cyclic operating conditions, such as in applications that require lubrication when changing from atmospheric pressure to ultrahigh vacuum and ambient air to dry nitrogen (e.g., satellite components), and room to elevated (>500°C) temperatures (e.g., aerospace bearings). Alternatively, solid lubricant coatings can be used in conditions where synthetic liquid lubricants and greases are not applicable; however, individual solid lubricant phases usually perform best only for a limited range of operating conditions. Therefore, solid lubricants that can adequately perform over a wider range of environmental conditions are needed, especially during thermal cycling with temperatures exceeding 500°C. One potential material class investigated in this dissertation is lubricious oxides, because unlike other solid lubricant coatings they are typically thermodynamically stable in air and at elevated temperatures. While past studies have been focused on binary metal oxide coatings, such as ZnO, there have been very few ternary oxide and no reported quaternary oxide investigations. The premise behind the addition of the third and fourth refractory metals Ti and Zr is to increase the number of hard and wear resistant phases while maintaining solid lubrication with ZnO. Therefore, the major focus of this dissertation is to investigate the processing-structure-tribological property relations of composite ZnO, TiO2 and ZrO2 phases that form ternary (ZnTi)xOy and quaternary (ZnTiZr)xOy nanocrystalline coatings. The coatings were processed by atomic layer deposition (ALD) using a selective variation of ALD parameters. The growth structure and chemical composition of as-deposited and ex situ annealed ternary and quaternary oxide coatings were studied by combined x-ray diffraction/focused ion beam microscopy/cross-sectional transmission electron microscopy, and x-ray photoelectron spectroscopy/Auger electron spectroscopy, respectively. It was determined that the structure varied from purely nanocrystalline (ternary oxides) to composite amorphous/nanocrystalline (quaternary oxides) depending on ALD parameters and annealing temperatures. In particular, the ZnTiO3 ilmenite phase with (104) textured nanocolumnar grains, exhibiting high stacking fault/partial dislocation densities >1012/cm2, was responsible for the excellent tribological behavior. Steady-state sliding friction coefficients down to 0.12 in humid air and 0.2 in dry nitrogen were measured along with sliding and fretting wear factors in the range of 10-6 to 10-7 mm3/N·m, even after ex situ annealing to 550°C. Additionally, the quaternary oxide phase Zn(Ti,Zr)O3 in solid solution exhibited a low fretting wear rate of 1x10-6 mm3/N·m. In contrast, certain phases, such as Zn2TiO4 cubic spinel, that form at annealing temperatures >550°C were responsible for high friction and wear. Mechanistic studies using the above techniques revealed low friction and wear-reducing surfaces and subsurfaces were due to different velocity accommodation modes (VAM). In the case of the ternary system, sliding-induced plastic deformation was possible when ZnTiO3 (104) stacking faults, bordered by partial dislocations, serve as a pathway for the dislocations to glide parallel to the sliding direction and hence achieve low friction and wear via an intrafilm shear VAM. It was evident that the individual nanocolumnar ZnTiO3 grains were plastically sheared as opposed to being fractured during wear. Conversely for the quaternary system, an interfacial sliding VAM between the counterface and a mechanically mixed layer (tribofilm) composed of the refined coating and counterface material, that also served as a source for the formation of cylindrical rolls, was responsible for wear reduction. Therefore, these lubricious oxides are a potential candidate for solid lubrication at high temperatures (up to 550 °C) and in space environments.

High dielectric constant materials are of technological importance as they lead to the miniaturization of the electronic devices. For instance, in the case of memory devices based on capacitive components, such as static and dynamic random access memories, the dielectric constant will ultimately decide the level of miniaturization. In this context, the observation of anomalously high dielectric constant (>10) in the double perovskite Sr2SbMnO6 (SSM) over wide frequency (100 Hz1 MHz) and (190373 K) temperature range has attracted a great deal of attention. However, unfortunately their dielectric losses were also high which limit their use for possible capacitor and related applications. The dielectric loss however was known to decrease with decreasing crystallite size in electroceramics. Therefore, the present work has been focused on the synthesis of nanocrystalline SSM powders by moltensalt route. The characterization of the ceramics fabricated from these powders for their microstructural and dielectric properties. A cubic phase of SSM powder was obtained by calcining the as synthesized powders at 900°C/10h by using sulphate flux. The crystallite size was ~ 60 nm. The activation energy associated with the particle growth was found to be 95 ± 5 kJmol-1 . The ceramic sintered at 1075°C/16h exhibited high dielectric constant (>10at 1 kHz) with low loss (0.72 at 1 kHz) at room temperature. The results are interpreted in terms of a twolayer model with conducting grains partitioned from each other by poorly conducting grain boundaries. Using this model, we attributed the two electrical responses in impedance and modulus formalisms to the grain and grain boundary effects, respectively, while the detected Debyelike relaxation and large dielectric constant were explained in terms of MaxwellWagner relaxation.

In this thesis, novel bulk Al-based alloys with high content of Al have been produced by powder metallurgy methods from amorphous and partially amorphous materials. Different processing routes, i.e. mechanical alloying of elemental powder mixtures, controlled pulverization of melt-spun glassy ribbons and gas atomization, have been employed for the production of the Al-based powders. Among the different processing routes, gas atomization is the best choice for the production of Al-based amorphous and partially amorphous powders as precursors for the subsequent consolidation step because it allows the production of large quantities of powders with homogeneous properties (e.g. structure and thermal stability) along with a uniform size distribution of particles. Amorphous and nanocrystalline powders have to be consolidated to achieve dense bulk specimens. However, consolidation of these phases is not an easy task and special care has to be taken with respect to accurate control of the consolidation parameters in order to achieve dense bulk specimens without inducing undesirable microstructural transformations (e.g. crystallization and grain coarsening) or insufficient particle bonding. Consequently, the effect of temperature on viscosity as well as on phase formation has been studied in detail in order to select the proper consolidation parameters. Following their characterization, the Al-based powders have been consolidated into bulk specimens by hot pressing (HP), hot extrusion and spark plasma sintering (SPS) and their microstructure and mechanical properties have been extensively investigated. Consolidation into highly-dense bulk samples cannot be achieved without extended crystallization of the glassy precursors. Nevertheless, partial or full crystallization during consolidation leads to remarkable mechanical properties. For example, HP Al84Gd6Ni7Co3 samples display a remarkably high strength of about 1500 MPa, which is three times larger than the conventional high-strength Al-based alloys, along with a limited but distinct plastic deformability (3.5 – 4%). Lower strength (930 MPa) but remarkably larger plastic strain exceeding 25 % has been achieved for the Al87Ni8La5 gas-atomized powders consolidated by SPS above their crystallization temperature. Similarly, HP Al90.4Y4.4Ni4.3Co0.9 bulk samples display high compression strength ranging between 820 and 925 MPa combined with plastic strain in the range 14 – 30%. Finally, preliminary tensile tests for the Al90.4Y4.3Ni4.4Co0.9 alloy reveal promising tensile properties comparable to commercial high-strength Al-based alloys. The mechanical behavior of the consolidated specimens is strictly linked with their microstructure. High strength and reduced plasticity are observed when a residual amorphous phase is present. On the other hand, reduced strength but enhanced plastic deformation is a result of the complete crystallization of the glass and of the formation of a partially or fully interconnected network of deformable fcc Al. These results indicate that the combined devitrification and consolidation of glassy precursors is a particularly suitable method for the production of Al-based materials characterized by high strength combined with considerable plastic strain. Through this method, the mechanical properties of the consolidated samples can be varied within a wide range of strength and ductility depending on the microstructure and the consolidation techniques used. This might open a new route for the development of innovative high-performance Al-based materials for transport applications.

Nanocrystalline cerium oxide thin films on metal and semiconductor substrates have been fabricated with a novel electrodeposition approach - anodic oxidation. X-ray diffraction analysis indicated that as-produced cerium oxide films are characteristic face-centered cubic fluorite structure with 5 ~ 20 nm crystal sizes. X-ray photoelectron spectroscopy study probes the non-stoichiometry property of as-produced films. Raman spectroscopy and Scanning Electron Microscopy have been applied to analyze the films as well. Deposition mode, current density, reaction temperature and pH have also been investigated and the deposition condition has been optimized for preferred oriented film formation: galvanostatic deposition with current density of -0.06 mA/cm2, T > 50oC and 7 < pH < 10. Generally, potentiostatic deposition results in random structured cerium oxide films. Sintering of potentiostatic deposited cerium oxide films leads to crystal growth and reach nearly full density at 1100oC. It is demonstrated that in-air heating favors the 1:2 stoichiometry of CeO2. Nanocrystalline cerium oxide powders (4 ~ 10 nm) have been produced with anodic electrochemical synthesis. X-ray diffraction and Raman spectroscopy were employed to investigate lattice expansion phenomenon related to the nanoscale cerium oxide particles. The pH of reaction solution plays an important role in electrochemical synthesis of cerium oxide films and powder. Cyclic voltammetry and rotation disk electrode voltammetry have been used to study the reaction mechanisms. The results indicate that the film deposition and powder formation follow different reaction schemes. Ce(III)-L complexation is a reversible process, Ce3+ at medium basic pH region (7~10) is electrochemically oxidized to and then CeO2 film is deposited on the substrate. CE mechanism is suggested to be involved in the formation of films, free Ce3+ species is coordinated with OH- at high basic pH region (>10) to Ce2O3 immediately prior to electrochemically oxidation Ce2O3 to CeO2. CeO2 / montmorillonite nanocomposites were electrochemically produced. X-ray diffraction and Raman spectroscopy illustrate the retaining of FCC structure for cerium oxide. Fourier Transform Infrared Spectroscopy and Differential Scanning Calorimetry of composites indicate the insertion of montmorillonite platelets into the structural matrix of cerium oxide. Sintering study of the nanocomposites demonstrates that low concentration of montmorillonite platelet coordination into cerium oxide matrix increases crystal growth rate whereas high concentration of montmoillonite in nanocomposites retards the increase of crystallite size during the densification process.