The anisotropic dimensional change on sintering of a prior cold compacted iron was investigated by dilatometry. Shrinkage is larger along the compaction direction than in the compaction plane. This phenomenon is very pronounced during the heating ramp in alpha phase and in particular below the Curie temperature, while in austenitic field is quite poor. The results of dilatometry tests were elaborated according to the shrinkage kinetics model of classical sintering theory, to calculate an effective diffusion coefficient along the two directions, which resulted higher for direction parallel to the compaction direction than perpendicular to it. In both directions, the effective diffusion coefficient is larger than that reported in the literature for pure iron, corresponding to an equilibrium density of structural defects. It also varies during the isothermal holding time. This discrepancy is attributed to the defectiveness introduced by cold compaction, that increases diffusivity through the activation of dislocation pipe mechanism, which is particularly intense below the Curie. This interpretation may also justify anisotropy of shrinkage due to the inhomogeneous deformation of interparticle contact regions that was measured with ISE method and EBSD analysis. The anisotropic shrinkage was also described through a modified micromechanical model proposed by the continuum mechanics approach, where the porous body is composed by aligned, elongated particles and elliptic pores, whose geometrical parameters were obtained through image analysis of SEM microhgraphs. The dislocation density calculated for different sintering temperatures was comparable to that measured experimentally. The effect of green density on anisotropy of shrinkage was investigated, too. Anisotropy tends to increase with green density, because of the larger plastic deformation introduced in the interparticle regions by the compaction pressure.

The influence of the powder material and the main processing parameters on geometrical features and dilution of the clads is investigated and discussed. Physical and analytical model that allow the explanation of the process and the prediction of the clad geometry and dilution is discussed. Using these models, useful tools for cladding operators and engineers are proposed. The energetic balance of the process is presented. Energetic redistribution in laser cladding process is analysed in detail, and quantification of process efficiency and energy losses is given. The influence of the processing parameters and the chemical/physical properties of the materials is considered throughout the various experiments performed. Some selected properties of the coating produced in different processing conditions are analysed. In specific, the variation of the chemical composition of the clad due to substrate dilution is considered, and its effect on the characteristics of the coatings is discussed.

The standard powder metallurgy process is composed by three main step, the powder production, the compaction and the sintering, and the possible secondary operation that allow to improve the mechanical properties and/or the dimensional and geometrical precision. The present work aims at investigating the influence of processing variables on the dimensional and geometrical precision of parts produced by Powder Metallurgy.

Memristor-based computing architecture with advanced signal processing capabilities investigates analogue applications of memristors, particularly in image processing, combining them with conventional electronic circuitry. The concept of memristor (short for memory resistor) was first theorised in 1971 by Prof. Chua while reasoning on the theoretical grounds of of the symmetry of equations governing the fundamental passive circuit theory. This thesis can be split into two parts as follows: i) Memristor Device, Modelling, Characterisation and Programming and ii) Memristor Circuit Applications. In the first part, an overview of the theory of memristors which gives an introductory background is discussed alongside the device modelling to fit experimental data. Electrical characterisation was carried out on fabricated memristors to validate the fundamental fingerprint of these devices and finally, the different programming techniques, particularly the pulse-based technique which was widely used in this work, was exhaustively treated. In terms of applications, starting from a novel memristor-based light to resistance encoder, a more complex architecture based on adaptive background subtraction for scene interpretation used for the analyses of motion and its association to a particular object in the scene is treated in this work. Throughout this thesis, the intention of an overview on the application of memristors in image processing algorithm is emphasised and the last chapter discusses a neural network architecture based on memristors. The intended neural network architecture was trained to perform colour classification targeting applications based on gesture detection. In essence, Memristor-based computing architecture with advanced signal processing capabilities gives an insight into the advantages to come having an hybrid system of standard CMOS image processing techniques with memristive devices particularly in computation-intensive applications requiring high speed and massive parallel signal processing. Typically the realisation of such powerful and dense networks in an integrated circuit with acceptable size which is not resource hungry by using the commonly encountered elements and conventional CMOS technology is becoming increasingly difficult to achieve. Computing architectures based on memristors present the advantages that could help overcome the limitations of an overall implementation with conventional electronic elements.

Cemented carbides are composite materials formed by high amount of WC bonded by a soft phase, usually Co. They are used in many applications, such as drawing dies, cutting tools and hot rolls due to theirs remarkable properties of high hardness and wear resistance. Mechanical properties are strongly related to microstructure, namely the binder amount and the carbide grain size. Increasing the binder content and the carbide grain size, the hardness decreases ad the fracture toughness increases. In this PhD, the correlations between the mechanical properties of WC-Co and the microstructural characteristics, in parts taken from industrial production, were defined. After that, the influence of the residual microporosity on the mechanical properties was evaluated. Considering the production process, another important modification of the final microstructure of WC-Co occurs due to the liquid cobalt migration phenomenon. Based on this, also the liquid cobalt migration that occurs during sintering was investigated. At the end of the thesis, since a few data are available in literature, Thermal Fatigue and oxidation damage in WC-Co were studied. The main results of this PhD thesis show that the hardness and fracture toughness of WC-Co are defined by the mean binder free path and not by the contiguity since the high standard deviations, the microstructural fineness and also the high carbide grain size scatter. Differently, in case of mechanical strength, also the residual microporosity that depends on the dewaxing stage must be defined. Furthermore, the dewaxing stage acts on the liquid cobalt migration that affects the surface properties and also the final microstructure of the WC-Co part in industrial production. At the end, considering the damages that occur during high temperature applications, the TF and oxidation resistance of WC-Co results affected by the Co content: high cobalt content leads to a better condition of TF damage and s higher oxidation resistance.

Rheocasting process integrated with the high pressure die casting method (Rheo-HPDC) has established itself as a new promising technology to produce high-quality components. However, different types of microstructural segregation induced by the semi-solid process influence the properties of the final component. The semi-solid microstructural features and the new compositions require detailed corrosion studies and verification. The first part of this thesis deals with microstructural and corrosion studies of the conventional and Rheo-HPDC Al-Si alloys. In this part, corrosion properties of two Al–Si alloys containing 2.5 and 4.5 wt % silicon cast by Rheo-HPDC method were examined in the diluted Harrison solution using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The microstructural studies revealed that samples taken from different positions (with respect to the feeding gate) contain different fractions of solid and liquid parts of the initial slurry. It was shown that the Rheo-HPDC Al-Si alloys are prone to the localized form of corrosion inside the eutectic region at the interface of aluminum with silicon phase and intermetallic particles. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and the surface quality. Corrosion properties of the two Al-Si alloys cast by the conventional and Rheo-HPDC process were also evaluated and compared in 0.01, 0.05, 0.1 and 0.6 M NaCl solutions. The conventional HPDC and semi-solid alloys presented similar EIS responses. However, the semi-solid samples with a lower fraction of the eutectic phase showed slightly higher impedance values in the more diluted sodium chloride solutions. Corrosion morphological features, including localized corrosion, trenching and co-operative corrosion rings were comparable for both types of alloys. However, in the anodic polarization test, the semi-solid alloys presented a higher resistance to pitting corrosion. To protect aluminum alloys from corrosion, chromium-based conversion coating has been successfully used for decades, due to its extensive protection. However, rising concerns and new restrictions on the environmental hazards of Cr (VI) compounds have led to intensive efforts to develop alternative coatings. The second and third parts of this thesis address the effort to investigate two alternatives in this field. Cerium-based conversion coatings were deposited on the conventional and Rheo-HPDC Al-Si alloys by immersion in cerium nitrate aqueous solutions. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The results revealed that applying cerium-based conversion coating on Al-Si alloys, is possible and a selective deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. Under the accelerated conditions, the deposition mechanism includes dissolution of the aluminum matrix, selective dissolution of aluminum from the noble intermetallic particles, oxidation of iron from these particles, and the deposition of cerium hydroxide/oxide layer. The results revealed that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating obtained from the conversion solution containing H2O2. The aluminum alloy with a higher amount of silicon showed more active surface during the conversion process which reduces the required concentration of Ce(NO3)3 but also makes it difficult to work with more aggressive solutions. In the third part of this thesis, the possible protective effect of polypyrrole coating on pure aluminum and Rheo-HPDC Al-Si alloys was investigated. Different electropolymerization solutions containing the Py monomer, SDS, DHBDS (Tiron), C6H8O7 and NaNO3 were used. The presence of nitrate anions led to the passivation of the aluminum electrode (both pure and alloy) during the electropolymerization and to the deposition of a thicker/more conductive coating. These facts resulted in longer and more efficient corrosion protection in NaCl solutions. This polypyrrole coating was able to keep the alloys’ surface potential noble for at least 168 hours. Which can be attributed to the anodic protection provided by the reduction of the polymer. It was shown that the presence of silicon phase or intermetallic particles has a positive effect on the electropolymerization of polypyrrole film. Therefore, the coatings deposited on the alloys possess higher thicknesses compared to those deposited on the pure aluminum. In the presence of chloride ions, all coatings suffered from the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole with aluminum. This may question the application of polypyrrole coating in concentrated NaCl solutions. However, it is shown that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and the conductivity and the barrier properties of the coating. Therefore, the application of polypyrrole in corrosion protection is not totally ruled out but needs specific considerations.

The contact fatigue and surface damage of prealloyed (Fe-0.85Mo, Fe-1.5Mo) and diffusion bonded (Ni-free, low-Ni, high-Ni) powder metallurgy (PM) steels were investigated. Materials subjected to contact stress fail due to the nucleation of subsurface cracks (contact fatigue cracks), nucleation of brittle surface cracks, and surface plastic deformation. The occurrence of these contact damage mechanisms was predicted using theoretical models, which were developed by assuming that crack nucleation is preceded either by local plastic deformation (contact fatigue and surface plastic deformation) or local brittleness (brittle surface cracks ) of the metallic matrix. With reference to the mean yield strength of the matrix (mean approach) or the yield strength of soft constituents (local approach), the models predict the theoretical resistance of materials to the formation of damage mechanisms. The models were then verified using experimental evidence from lubricated rolling-sliding contact tests. In addition, the effect of compact density and microstructures of materials on the resistance to contact damage mechanisms was investigated. Density and microstructure were modified by varying green density, alloying elements, sintering temperature and time, and applying strengthening treatments: carburizing and shot peening on prealloyed (homogenous microstructure) and carburizing, sinterhardening and through hardening on diffusion bonded (heterogeneous microstructure) steels. The theoretical resistance to subsurface and surface crack nucleation in prealloyed materials was predicted using the mean approach since the microstructure is homogeneous. But the local approach is applied for diffusion bonded materials (Ni-free and low-Ni); exceptionally, the mean approach was applied for some homogeneous microstructure of Ni-free material sintered at a prolonged time. However, the models have a limitation in predicting the contact damage mechanisms in a high-Ni material. This issue may require further investigation to modify the model. Shot peening provides higher resistance to the nucleation of surface cracks. High compact density, high sintering temperature and time, and sinterhardening improve the resistance to contact damage mechanisms for Ni-free and low-Ni materials.

This Thesys reports the study of a HGMS (High GradientMagnetic Separation) process for the treatment of industrialwastewaters that considers an assisted chemical-physical pre-treatment for the removal of heavy metals through the bound by adsorption with added iron-oxide particulate matter (hematite). The considered filter, constituted by ferromagnetic stainless steel wool and permanent magnets, is studied with a new approach based on a statistical analysis that requires the study of the trajectories of the particles. Experimental activity on a laboratory device has been carried out in order to test the model.

Thesis (D. Min.)--McCormick Theological Seminary, 2001. / Includes bibliographical references.

Indiana University-Purdue University Indianapolis (IUPUI)