Powder metallurgy (PM) steel is produced by near net shape manufacturing, which is used to fabricate alloy steels for many purposes. Designing new powder metal steels that can form a significant fraction of martensite relies on hardenability calculations developed for wrought steels. These proven tools are built upon assumptions for wrought steels that do not hold true for PM steels. One assumption is that the alloying elements are homogenized throughout the material. In admixed powder blends that are industrially sintered this is not the case. Using prealloyed powder is a solution to this issue, yet it places restrictions on alloy design and compressibility. There are tools available to computationally optimize diffusion problems, yet the complexity during the sintering of PM steel is such that a robust model has yet been produced. It is intuitive that with smaller particles of Fe sintering time can be reduced. A direct experimental investigation linking Fe-powders’ sizes and hardenability on Fe-C-Cr-Mn-Mo-Ni PM steel was subject to microstructure analysis and mechanical properties (Jominy test) for comparative analysis. Another assumption that is made for wrought steel is a consistent density of 7.87g/cm3. This is not the case for PM steel as the press and sinter method produces pores, decreasing the density. This directly affects the thermal conductivity and phase transformation of the steel. In an effort to understand how these differences affect Grossmann’s predictions of hardenability, a direct experimental investigation linking the density to hardenability was launched on prealloyed FL-4605 and FL-4605+2%Cu. Specifically the Jominy test was completed on a range of densities, as well as compared to software predictions. The chemical variations in admixed and sintered PM steel produce a unique system where one TTT diagram cannot predict the entire final microstructure. PM steel such as this is observed in industry, and can be created through incorporating larger Fe-particles such that less alloying constituents have a chance to fully alloy these regions. Since the large particles will not have the chance to be alloyed, they will not have the ability to form martensite. Since the regions between large particles will be alloyed, martensite will form, creating a hard matrix surrounding softer particles. This structure is characteristic of a metal matrix composite (MMC), and therefore should be treated as such. There are methods of MMC design that involve numerical methods of predicting strength and toughness. These methods, along with experimental data (tensile and Charpy testing) of Fe-C-Cr-Mn-Mo-Ni PM steels with ranging volume fractions of pearlitic inclusions were compared. / Thesis / Master of Applied Science (MASc)

Ferroelectric-reinforced metal matrix composites (FR-MMCs) offer the potential to improve damping characteristics of structural materials. Many structural materials are valued based on their stiffness and strength; however, stiff materials typically have limited inherent ability to dampen mechanical or acoustic vibrations. The addition of ferroelectric ceramic particles may also augment the strength of the matrix, creating a multifunctional composite. The damping behavior of two FR-MMC systems has been examined. One involved the incorporation of barium titanate (BaTiO3) particles into a Cu- 10w%Sn (bearing bronze) matrix and the other incorporating them into an electroformed Ni matrix. Here the damping properties of the resulting ferroelectric reinforced metal matrix composites (FR-MMCs) have been investigated versus frequency, temperature (above and below the Curie temperature of the reinforcement), and number of strain cycles. FR-MMCs currently represent a material system capable of exhibiting increased damping ability, as compared to the structural metal matrix alone. Dynamic mechanical analysis and neutron diffraction have shown that much of this added damping ability can be attributed to the ferroelectric/ferroelastic nature of the reinforcement. / Ph. D.

A systematic and scientific approach has been applied to the study of codeformation processing. A series of composites having mechanically-dissimilar phases were developed in which the high temperature flow behavior of the reinforcement material could be varied independent of the matrix. This was accomplished through the use of a series of intermetallic matrix composites (IMCs) as discontinuous reinforcements in an otherwise conventional metal matrix composite. The IMCs are produced using an in-situ reaction synthesis technique, called the XD™ process. The temperature of the exothermic synthesis reaction, called the adiabatic temperature, has been calculated and shown to increase with increasing volume percentage of TiB2 reinforcement. Further, this temperature has been shown to effect the size and spacing of the TiB2, microstructural features which are often used in discontinuous composite strength models. Study of the high temperature flow behavior of the components of the metal/IMC composite is critical to the development of an understanding of codeformation. A series of compression tests performed at 1000° to 1200°C and strain-rates of 10-3 and 10-4 sec-1. Peak flow stresses were used to evaluate the influence of material properties and process conditions. These data were incorporated into phenomenologically-based constitutive equations that have been used to predict the flow behavior. It has been determined that plastic deformation of the IMCs occurs readily, and is largely TiB2 independent, at temperatures approaching the melting point of the intermetallic matrices. / Ph. D.

Magnetic alloys that are amorphous exhibit soft magnetic properties; hence they play an essential role in electronic and electrical systems and devices. They are used in applications that include electrical power generation and transmission, electronic motors, solenoids, relays, magnetic shielding, and electromagnets. This work was an attempt to investigate the solid-state formation of Co-C amorphous alloys, their thermal stability and magnetic properties. Amorphous Co-C alloys with compositions of 2 to 40 at.% C were successfully synthesized from elemental Co and C (graphite) using mechanical alloying, a solid-state powder processing technique. All alloy compositions were milled for up 40 hours. After 20h of milling some of the alloys (≤ 20 at.% C) had partially amorphized, while the higher concentrations had completely amorphized. After 40h of milling, complete amorphization was observed in all alloys, except for the 2 and 5 at.% C alloys. The thermal analyses of the milled powders showed very interesting results. DSC results indicated that alloys with compositions through 20 at.% C crystalized in two steps; the lower temperature event precipitated metastable cobalt carbide from the amorphous phase, followed by the eventual transformation to fcc cobalt and graphite from both the remaining amorphous and the metastable carbide at the higher temperature. Two types of carbides were observed - Co3C in the 2 and 5 at.% C alloys, and Co2C in the higher carbon alloys through 20 at.% C. For compositions above 20 at.% C, only one step crystallization was observed, that of the decomposition of the amorphous phase to amorphous carbon and cobalt – primarily fcc phase. Activation energy calculations show that the low temperature carbide precipitation was controlled by carbon diffusion, while the high temperature decomposition reaction forming cobalt and amorphous carbon was controlled by cobalt diffusion. Room temperature magnetic measurements of the milled powders were made using vibrating sample magnetometer (VSM). High saturation magnetization (Ms) and very low coercivity (Hc) are desired for efficient performance of soft magnets. But in this study, Ms decreased with both carbon composition and milling time. It decreased from 195 Am2/kg for the un-milled pure Co to between 178 and 44 Am2/kg for the alloys, with the worst being the 40 at.% C sample milled for 40h. The Ms drop as function of composition made sense, as its related to the volume fraction of cobalt in the alloy. However, the Ms drop as a function of milling time is unclear. In the case of Hc, its value did drop from 12.7 kA/m for the un-milled pure Co to between 7.5 and 1.3 kA/m when the C content is less than 15 at.%. These gains are not significant enough to favor the use of these alloys as soft magnets. Amorphous metal alloys tend to have strengths that are much higher than their crystalline counterparts, and they have hardness values comparable to those of particulate ceramic materials used to reinforce metal matrices. The Co-C amorphous alloy with 40 at.% C that had been milled for 40h (the most stable of all the samples) was used to reinforce cobalt matrix by powder processing methods that included spark plasma sintering (SPS) at temperatures below those of crystallization. Volume fraction ranged from 1 to 20 % reinforcement. The densities of these composites were between 81 and 85 % of theoretical values, hence there were substantial porosities. Despite this the matrix strengthening of the cobalt matrix, as assessed by Vickers microhardness tests, was significant. Hardness increased from 210 HV for unreinforced matrix to 537 HV for the 20 vol.% amorphous. The primary contributor to the strengthening appears to be boundary strengthening by the particles whose average size of about 4 microns is comparable to the grain size of the matrices of the composites. The hardness data fits the Hall Petch-like relationship based on particle spacing. Having a reinforcement particle with a chemistry similar to that of the matrix as is the case in this study, has the potential to improve interfacial bonding and also minimize the difference between the components' coefficient of thermal expansions, which are major issues with the use of ceramics to reinforce metal matrices. The microstructures of the composites indicated good bonding at their interfaces. / Doctor of Philosophy / Magnetic alloys that are amorphous (have no long-range atomic order) exhibit soft magnetic material properties (easily magnetized and demagnetized); hence they play an essential role in electronic and electrical applications. This work investigated the solid-state formation of Cobalt-Carbon (Co-C) amorphous alloys, their thermal stability and magnetic properties. Amorphous Co-C alloys with compositions of 2 to 40 atomic weight % of C were successfully synthesized from elemental Co and C (as graphite) using a mechanical alloying technique (high-energy milling to alloy materials by impact). All alloy compositions were milled for up 40 hours. After 20h of milling some of the alloys (≤ 20 atomic weight % of C) had partially become amorphous, while the higher concentrations had completely become amorphous. After 40h of milling, complete amorphization was observed in all alloy compositions, except for the 2 and 5 atomic weight % of C alloys (2-5 atomic weight % of C). Thermal analyses (Differential Scanning Calorimetry, DSC) of the milled powders showed that alloys with compositions through 20 atomic weight % of C crystalized via a low temperature precipitation of a metastable cobalt carbide from the amorphous phase, followed by a high temperature transformation to a face centered cubic (fcc) cobalt and graphite phase from both the remaining amorphous and the metastable carbide. Activation energy calculations showed that the low temperature carbide precipitation was controlled by carbon diffusion, while the high temperature decomposition reaction forming cobalt and amorphous carbon was controlled by cobalt diffusion. High saturation magnetization (Ms) and very low coercivity (Hc) are desired for efficient performance of soft magnets. Thus, room temperature magnetic measurements of the milled powders were made using vibrating sample magnetometer (VSM). But in this study, Ms decreased with both carbon composition and milling time. The Ms drop as function of composition made sense, as its related to the volume fraction of cobalt in the alloy. However, the Ms drop as a function of milling time is unclear. In the case of Hc, its value did drop from 12.7 kA/m for the un-milled pure Co to between 7.5 and 1.3 kA/m when the C content is less than 15 atomic weight %. These gains are not significant enough to favor the use of these alloys as soft magnets. Amorphous metal alloys tend to have strengths that are much higher than their crystalline counterparts, and they have hardness values comparable to those of particulate ceramic materials used to reinforce metal matrices. The Co-C amorphous alloy with 40 atomic weight % of C that had been milled for 40h was used to reinforce cobalt matrix by powder processing methods (including spark plasma sintering (SPS) at temperatures below those of crystallization). The densities of these composites were between 81 and 85 % of theoretical values and hence there was substantial porosity. Despite this the matrix strengthening of the cobalt matrix, as assessed by Vickers microhardness tests, was significant. The primary contributor to the strengthening appeared to be boundary strengthening by the particles whose average size of about 4 microns was comparable to the grain size of the matrices of the composites. Having a reinforcement particle with a chemistry similar to that of the matrix has the potential to improve interfacial bonding and also minimize the difference between the components' coefficient of thermal expansions, which are major issues with the use of ceramics to reinforce metal matrices. The microstructures of the composites indicated good bonding at their interfaces.

Thermal spray coatings offer new alternatives in the production of electronic power modules that use alumina ceramic as an isolation layer. Current processes use direct bond copper (DBC) soldered to a nickel plated copper heat spreader. A coefficient of thermal expansion (CTE) mismatch exists between copper and alumina and leads to reliability issues that arise due to product failure during thermal cycling and lifetime operation. The substitution of an AlSiC metal matrix composite (MMC) heat spreader baseplate addresses the problem of CTE mismatch and will reduce the number of product failures related to cracking and delamination caused by this pronounced mismatch in the thermal expansion coefficient.. The substitution of an AlSiC (MMC) heat spreader baseplate also allows the production process to be achieved with a fewer number of metallization layers. Thermal spray can apply alumina ceramic coatings directly to the AlSiC (MMC) baseplates. A reduction in process steps will lead to a reduction in manufacturing costs, the main driving objective in Microelectronics Industries. Thermal spray coatings have a major problem since they have a porous microstructure which can trap undesired moisture. The moisture basically causes the coatings to have a lower dielectric breakdown voltage and a higher leakage current at normal operating voltages. This problem can be eliminated by manufacturing the electronic power modules in a controlled environment and packaging the devices in a hermetically sealed package. This thesis analyzes the data obtained from direct-voltage dielectric breakdown and direct-voltage leakage current tests conducted on coupons manufactured using the thermal plasma spray coating process and the thermal high-velocity oxyfuel (HVOF) coating process. ASTM specifications defining appropriate testing procedures are used in testing the dielectric strength of these coupons. Issues relating to the dielectric strength and dielectric leakage current are evaluated and validated at the Microelectronics Laboratory at Virginia Polytechnic Institute & State University. The objective to conduct this research study using plasma and HVOF alumina coatings as dielectric isolation layers is to support the Microelectronics Industries in developing a product with increased reliability at a lower manufacturing cost. / Master of Science

Silicon carbide and nanostructured ferritic alloy (SiC-NFA) composites have the potential to maintain the outstanding high temperature corrosion and irradiation resistance and enhance the mechanical integrity for nuclear cladding. However, the formation of detrimental silicide phases due to reaction between SiC and NFA remains a major challenge. By introducing a carbon interfacial barrier on NFA (C@NFA), SiC-C@NFA composites are investigated to reduce the reaction between SiC and NFA. In a similar way, the effect of chromium carbide (Cr3C2) interfacial barrier on SiC (Cr3C2@SiC) is also presented for Cr3C2@SiC-NFA composites. Both the coatings were successful in suppressing silicide formation. However, despite the presence of coatings, SiC was fully consumed during spark plasma sintering process. TEM and EBSD investigations revealed that spark plasma sintered SiC-C@NFA and Cr3C2@SiC-NFA formed varying amounts of different carbides such as (Fe,Cr)7C3, (Ti,W)C and graphite phases in their microstructure. Detailed microstructural examinations after long term thermal treatment at 1000oC on the microstructure of Cr3C2@SiC-NFA showed precipitation of new (Fe,Cr)7C3, (Ti,W)C carbides and also the growth of existing and new carbides. The results were successfully explained using ThermoCalc precipitation and coarsening simulations respectively. The oxidation resistance of 5, 15 and 25 vol% SiC@NFA and Cr3C2@SiC-NFA composites at 500-1000oC temperature under air+45%water vapor containing atmosphere is investigated. Oxidation temperature effects on surface morphologies, scale characteristics, and cross-sectional microstructures were investigated and analyzed using XRD and SEM. SiC-C@NFA showed reduced weight gain but also showed considerable internal oxidation. Cr3C2@SiC-NFA composites showed a reduction in weight gain with the increasing volume fraction of Cr3C2@SiC (5, 15 and 25) without any indication of internal oxidation in the microstructure. 25 vol% SiC-C@NFA and 25 vol% Cr3C2@SiC-NFA showed over 90% and 97% increase in oxidation resistance (in terms of weight gain) as compared to NFA. The results were explained using the fundamental understanding of the oxidation process and ThermoCalc/DICTRA simulations. Finally, the irradiation performance of SiC-C@NFA and Cr3C2@SiC-NFA composites was assessed in comparison with NFA using state-of-the-art TEM equipped with in-situ ion irradiation capability. Kr++ ions with 1 MeV energy was used for irradiation experiments. The effect of ion irradiation was recorded after particular dose levels (0-10 dpa) at 300oC and 450oC temperatures. NFA sample showed heavy dislocation damage at both 300oC and 450oC increasing gradually with dose levels (0-10 dpa). Cr3C2@SiC-NFA showed similar behavior as NFA at 300oC. However, at 450oC, Cr3C2@SiC-NFA showed remarkably low dislocation loop density and loop size as compared to NFA. At 300oC, microstructures of NFA and Cr3C2@SiC-NFA show predominantly 1/2<111> type dislocation loops. At 450oC, NFA showed predominantly <100> type loops, however, Cr3C2@SiC-NFA composite was still predominant in ½<111> loops. The possible reasons for this interesting behavior were discussed based on the large surface sink effects and enhanced interstitial-vacancy recombination at higher temperatures. The molecular dynamics simulations did not show considerable difference in formation energies of ½<111> and <100> loops for NFA and Cr3C2@SiC-NFA composites. The additional Si element in the SiC-NFA sample could have been an important factor in determining the dominant loop types. SiC-C@NFA composites showed heavy dislocation damage during irradiation at 300oC. At 450oC, SiC-C@NFA showed high dislocation damage in thicker regions. Thinner regions near the edge of TEM samples were largely free from dislocation loops. The precipitation and growth of new (Ti,W)C carbides were observed at 450oC with increasing irradiation dose. (Fe,Cr)7C3 precipitates were largely free from any dislocation damage. Some Kr bubbles were observed inside (Fe,Cr)7C3 precipitates and at the interface between α-ferrite matrix and carbides ((Fe,Cr)7C3, (Ti,W)C). The results were discussed using the fundamental understanding of irradiation and ThermoCalc simulations. / Doctor of Philosophy / With the United Nations describing climate change as 'the most systematic threat to humankind', there is a serious need to control the world's carbon emissions. The ever increasing global energy needs can be fulfilled by the development of clean energy technologies. Nuclear power is an attractive option as it can produce low cost electricity on a large scale with greenhouse gas emissions per kilowatt-hour equivalent to wind, hydropower and solar. The problem with nuclear power is its vulnerability to potentially disastrous accidents. Traditionally, fuel claddings, rods which encase nuclear fuel (e.g. UO2), are made using zirconium based alloys. Under 'loss of coolant accident (LOCA) scenarios' zirconium reacts with high temperature steam to produce large amounts of hydrogen which can explode. The risks associated with accidents can be greatly reduced by the development of new accident tolerant materials. Nanostructured ferritic alloys (NFA) and silicon carbide (SiC) are long considered are leading candidates for replacing zirconium alloys for fuel cladding applications. In this dissertation, a novel composite of SiC and NFA was fabricated using spark plasma sintering (SPS) technology. Chromium carbide (Cr3C2) and carbon (C) coatings were employed on SiC and NFA powder particles respectively to act as reaction barrier between SiC and NFA. Microstructural evolution after spark plasma sintering was studied using advanced characterization tools such as scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) techniques. The results revealed that the Cr3C2 and C coatings successfully suppressed the formation of detrimental reaction products such as iron silicide. However, some reaction products such as (Fe,Cr)7C3 and (Ti,W)C carbides and graphite retained in the microstructure. This novel composite material was subjected to high temperature oxidation under a water vapor environment to study its performance under the simulated reactor environment. The degradation of the material due to high temperature irradiation was studied using state-of-the-art TEM equipped with in-situ ion irradiation capabilities. The results revealed excellent oxidation and irradiation resistance in SiC-NFA composites as compared to NFA. The results were discussed based on fundamental theories and thermodynamic simulations using ThermoCalc software. The findings of this dissertation imply a great potential for SiC-NFA based composites for future reactor material designs.

The in-situ synthesis of piezoelectric-reinforced metal matrix composites has been attempted with a variety of target matrix and reinforcement materials using reaction synthesis and high energy ball milling. Zinc oxide (ZnO) and barium titanate (BaTiO₃) have been successfully synthesized within copper and iron matrices in a range of volume percentages using reaction synthesis. The microstructures of these composites have been analyzed and found to partially consist of an interpenetrating microstructure. After considering experimental findings and thermodynamic issues involved with synthesis, ideal reaction system parameters have been identified that promote the creation of a composite with ideal microstructure and formulated composition. Reactive high energy ball milling has been used to create copper matrix composites reinforced with zinc oxide and copper matrix composites reinforced with lead titanate (PbTiO₃). The microstructures and compositions of each volume percentage formulation of the composite powders have been analyzed. In this work, several promising piezoelectric-reinforced metal matrix composite systems have been identified as having potential to be synthesized in an in-situ manner. / Master of Science

Composite materials are multifunctional materials having unique mechanical and physical properties that can be tailored to meet the requirements of a particular application. Aluminium based Metal Matrix Composites (MMC) always draw the attention of researchers due to its unique characteristics such as better strength to weight ratio, low wear rate and lower thermal expansion coefficient. There are various methods for manufacturing of MMC that can be grouped into two major categories: (a) Solid sate method such as powder metallurgy, co-extrusion and (b) Liquid state method such as stir casting. All of these methods for production of composites have their own advantages and disadvantages. Porosity, and poor wettabilty of dispersoids with matrix are few common problems in solid state route. Formations of undesirable phases, and segregation of dispersoids are common problems in liquid state processing route. Friction Stir Processing (FSP) technique, a derivative technique of Friction Stir Welding (FSW) has emerged as a major solid state technique to produce composites. However, there are several challenges associated with it. Most of the past work has been on limited volume of material. Researchers have tried to combine FSP technique with powder metallurgy technique to overcome aforementioned challenges associated with these techniques. Where on one hand, powder metallurgy ensures the uniform dispersion of dispersoids in the matrix, on the other hand FSP on sintered billet removes the pores and other defects. The combination of these two techniques leads to a more controlled and uniform properties. However, at the same time, it can be noted that the combination of these processes is tedious and time consuming. In this study, an attempt is made to achieve bulk dispersion of a second phase into an aluminium matrix using FSP technique. A 5 mm thickness composite is attempted in this work. To achieve this objective proper and uniform mixing of the particles is required. To achieve this, new tools and processing steps are to be designed and analyzed for a better understanding of material flow around the tool pin and the effect of different tool pin geometries on the material flow. Keeping this objective, a detailed study is carried out on material flow during FSW process using aluminium as base metal. A marker material technique is employed to understand the material flow. A strip of copper is selected as the marker material. Material flow can be qualitatively predicted during the process by observing the distribution of marker material in the weld nugget. Three different kinds of tools, each with an additional feature are designed for this purpose (a) Plain frustum shape pin (b) threaded frustum shape pin and, (c) Triflute pin . The material flow due to the plain pin tool can be considered as primary flow during the FSP. Three different kinds of flow zones are observed in the weld nugget in the case of plain tool. It is found that higher numbers of geometrical features (threads and flutes) not only enhance the material flow but also lead to the additional flow currents and more thorough and uniform mixing. A closer study of the weld nugget revealed that the copper marker particles and the matrix were diffusion bonded. Based on the reaction time available and temperature in the weld nugget a diffusion layer thickness of 4 nm is expected between copper and aluminium. However, the diffusion layer thickness was found to be 3.5 μm, which is nearly three orders of magnitude higher. This can be attributed to the enhancement of diffusion due to simultaneous application of strain and temperature. As copper is soluble in the aluminium, an insoluble marker material tin was used for study of flow in the weld nugget. However, the effect of insolubility and lower melting point had some unexpected effect on the processing loads. The normal load during steady state tool traverse in conventional butt-welding is found to be around 2.7 KN while it attains an average value of 14.7 KN when a thin strip of tin is sandwiched between these plates. However, a drop in the torque of around 13.12 NM is observed when tin was sandwiched between the plates as compared to the case when no insert was present. On closer examination of the flow behavior, it is seen that the tin melted, squeezed out and formed a lubricious layer between the tool and the work piece. This reduced the torque significantly and a concomitant drop in temperature was observed. The interaction between the tool and the colder aluminium work piece would thus result in much larger normal and transverse load Based on the expected and unexpected results of flow pattern in the weld nugget, a new FSP tool and processing steps were developed to manufacture MMC. Tungsten, which is the highest melting point metal is chosen as the dispersing phase. Further, as tungsten has high melting point, the kinetics of intermetallics formation would be low for the given FSP processing time at the processing temperature. This would lead to tungsten acting as a more ductile strengthening particle, which is expected to should give some unique characteristics to the MMC. Tungsten powder with an average diameter of 414 nm was dispersed in aluminum matrix with three different proportions after optimizing all the process parameters. It is noted that the mechanical properties are significantly influenced as the tungsten content in the matrix increases. In practice, MMC shows relatively low ductility compared to the parent metal. However in this case the composite exhibited even better ductility than the as received aluminium plates (rolled sheets). The composite showed around 129 MPa of yield strength along with 21% ductility when tungsten content is 3.8 at.%. It is also found that the reaction between aluminum and tungsten occurs during the processing and form the Al12W intermetallic phase. Though the formation of this intermetallic phase was unlikely due to the low temperature and short time available during the process, the reaction kinetics between aluminium and tungsten would have been enhanced due to the simultaneous application of strain and temperature. Given that the metal-metal, tungsten-aluminium composite produced by FSP had unique properties and also formed intermetallics, a study on incorporation of a highly insoluble material, graphite was carried out. Further graphite with its own unique properties and very low wettability with aluminium could possibly impart completely different properties to the system. Past work on graphite aluminium composites produced by other methods did not show promise. As FSP imposes high strains at relatively high flow stresses on the processed material, it was seen that the graphite got sheared to form multi-layer graphene composites with the aluminium. The graphene sheets are formed by mechanical exfoliation of graphite particles during its incorporation in the matrix. The formation of graphene was confirmed after separating the graphite from the processed zone and TEM studies of the composite. It is seen that most of the graphite got converted into multilayer graphene. This aluminium-graphene composite exhibited enhanced ductility and UTS. As received aluminium plates exhibited only 11% ductility and around 100 MPa of UTS while this composite exhibited around 26 % ductility and 147 MPa of UTS. However, there is only a slight improvement in yield strength of this composite.

Microwave tubes are the choice of a wide range of high power and millimeter wave applications in radar, electronic warfare and communication systems. Advances in these devices are due to device innovation, improved modeling, and development of advanced materials. In a microwave tube, electron emitter is the source of electron beam and it is one of the vital components determining the life & performance of the device. High power, high frequency microwave tubes require electron emitters with high emission density. The present thesis aims at developing the materials for high emission density electron emitters. It is aimed to improve the emission density of thermionic cathodes for use in conventional microwave tubes and to develop cold emitters like ferroelectric cathodes for high power microwave devices. The work reported in the present thesis is a modest attempt of the author towards this aim. The thesis is organized in six chapters. Chapter 1 gives a brief introduction of thermionic and ferroelectric emitters. Different types of electron emission mechanisms and a brief background of thermionic and ferroelectricemitters are discussed in this chapter. The genesis of the problem taken up and its importance as well as the plan and scope of the work is also given in this chapter. In Chapter 2, the basic experimental techniques used in the present work are discussed. Preparation of mixed metal matrix and M- type dispenser cathodes and their characterization techniques has been discussed in this chapter. Subsequently, ferroelectric materials preparation and characterization for their material properties and electron emission has been discussed. A brief introduction to FEM software ANSYS, used for thermal analysis of dispenser cathodes and electrostatic field analysis of ferroelectric cathodes, has been given at the end of this chapter. Thermal analysis, development process, emission characterization, work function distribution, of W-Ir mixed metal matrix (MM type) cathodes and a simple innovative technique to estimate the barium evaporation rate from the emission data of the dispenser cathodes is presented in Chapter 3. Under normal microwave tube operating conditions, the cathode of the electron gun has to be heated up to 1050°C to obtain stable thermionic electron emission. Thermal analysis is a first step in the development process of cathodes, optimizing its structure for improved performance with respect to its operating power, warm-up time and efficiency. Thermal analysis of a dispenser cathode in electron gun environment using the FEM software ANSYS and its experimental validation are presented. Development of porous W-Ir mixed metal matrix material required for dispenser cathode applications has been discussed. Determination of pore size, pore density and pore uniformity has been carried out. The performance of the cathodes made with these pellets is at par with the results reported in the literature. The surface of mixed metal pellet is an inherently two-phase structure consisting of tungsten solid solution phase and W-Ir ε phase causing more spread in the spatial distribution of work function. W-Ir mixed metal matrix cathodes have been realized and their work function distribution has been determined form the measured I-V characteristics. Also in this chapter, a novel technique for estimation of barium evaporation rate for dispenser cathodes from their I-V characteristics is presented. Results of life test carried out on these cathodes are given at the end of the chapter. In Chapter 4, work carried out on enhancing the emission properties of mixed metal matrix cathodes by suitably modifying the impregnant mix is discussed. W-Ir MM type cathodes discussed in the previous chapter give a emission current density of ~ 7.5A/cm2 with a work function of 1.99 eV. Thesevalues are very close to that of B-type cathode. In this chapter, it is explored to suitably dope the 5BaO:3CaO:2Al2O3 impregnant mix to reduce the work function of W-Ir cathodes. Lithium and Scandium oxides have been added to the 5:3:2 imp regnant mix. Lithium oxide doped impregnated MM type cathodes have given more than 30 A/cm2 current density at 1050oC. For scandium oxide doped MM type cathodes current density has increased to 15 A/cm2 at the same temperature. In Chapter 5, Electron emission from the ferroelectric cathodes has been discussed. FEM simulation of Ferroelectric cathodes to study the electrical excitation effects on emission. Triple point electric field in FE Cathodes is very large and can lead to field emission from the metallic grid at triple points. FEM simulation has been carried out to find out the effect of grid thickness on triple junction electric field using ANSYS software. From FEM modeling it is also seen that if a dielectric layer of lower dielectric constant (εr≤10) is placed between the grid and the ferroelectric material the triple junction electric field increases three fold. Use of dielectric layer can also reduce the secondary electron coefficient (δ) and surface plasma generation. Lanthanum doped PZT has been chosen for the study and these materials have been tested in diode configuration for emission characterization in demountable vacuum systems. Repeatable electron emission has been achieved for all the three compositions of PLZT (x/65/35) material (x = 7, 8, 9). However, it has been observed that when the ferroelectric is subjected to repetitive unipolar electrical excitation, fatigue is set in and cathode material is cracking. To study the effect of domain switching on the residual stress in the ferroelectric material, XRD studies have been carried out. Shift in XRD peaks for fresh and emission tested samples has been used to calculate the residual stress developed in the samples. Details of High current switch realized using ferroelectric cathodes have been discussed. Chapter 6 gives the Summary of the work done and suggestions for further research on W-Ir mixed metal matrix cathodes and ferroelectric cathodes.

In this thesis, a novel in-situ method for incorporating nanoscale ceramic particles into metal has been developed. The ceramic phase is introduced as an organic-polymer precursor that pyrolyzes in-situ to produce a ceramic phase within the metal melt. The environment used to shield the melt from burning also protects the organic precursor from oxidation. The evolution of volatiles (predominantly hydrogen) as well as the mechanical stirring causes the polymer particles to fragment into nanoscale dispersions of a ceramic phase. These “Polymer-based In-situ Process-Metal Matrix Composites” (PIP-MMCs) are likely to have great generality, because many different kinds of organic precursors are commercially available, for producing oxides, carbides, nitrides, and borides. Also, the process would permit the addition of large volume fractions of a ceramic phase, enabling nanostructural design, and production of MMCs with a wide range of mechanical properties, meant especially for high temperature applications. An important and noteworthy feature of the present process, which distinguishes it from other methods, is that all the constituents of the ceramic phase are built into the organic molecules of the precursor (e.g., polysilazanes contain silicon, carbon, and nitrogen); therefore, a reaction between the polymer and the host metal is not required to produce the dispersion of the refractory phase. The polymer precursor powder, with a mean particle size of 31.5 µm, was added equivalent to 5 and 10 weight % of the melt (pure magnesium) by a liquid metal stir-casting technique. SEM and OM microstructural observations show that in the cast structure the pyrolysis products are present in the dendrite boundary region in the form of rod/platelets having a thickness of 100 to 200 nm. After extrusion the particles are broken down into fine particles, having a size that is comparable to the thickness of the platelets, in the 100 to 200 nm range, and are distributed more uniformly. In addition, limited TEM studies revealed the formation of even finer particles of 10-50 nm. X-ray diffraction analysis shows the presence of a small quantity of an intermetallic phase (Mg2Si) in the matrix, which is unintended in this process. There was a significant improvement in mechanical properties of the PIP-MMCs compared to the pure Mg. These composites showed higher macro-and micro-hardness. The composite exhibited better compressive strength at both room temperature and at elevated temperatures. The increase in the density of PIP-composites is less than 1% of Mg. Five weight percent of the precursor produced a two-fold increase in the room-temperature yield strength and reduced the steady state creep rate at 723 K by one to two orders of magnitude. PIP-MMCs showed higher damping capacity and modulus compared to pure Mg, with the damping capacity increasing by about 1.6 times and the dynamic modulus by 11%-16%. PIP-composites showed an increase in the sliding wear resistance by more than 25% compared to pure Mg.