Carbon nanotubes (CNT) could serve as potential reinforcement for metal matrix composites for improved mechanical properties. However dispersion of carbon nanotubes (CNT) in the matrix has been a longstanding problem, since they tend to form clusters to minimize their surface area. The aim of this study was to use plasma and cold spraying techniques to synthesize CNT reinforced aluminum composite with improved dispersion and to quantify the degree of CNT dispersion as it influences the mechanical properties. Novel method of spray drying was used to disperse CNTs in Al-12 wt.% Si pre-alloyed powder, which was used as feedstock for plasma and cold spraying. A new method for quantification of CNT distribution was developed. Two parameters for CNT dispersion quantification, namely Dispersion parameter (DP) and Clustering Parameter (CP) have been proposed based on the image analysis and distance between the centers of CNTs. Nanomechanical properties were correlated with the dispersion of CNTs in the microstructure. Coating microstructure evolution has been discussed in terms of splat formation, deformation and damage of CNTs and CNT/matrix interface. Effect of Si and CNT content on the reaction at CNT/matrix interface was thermodynamically and kinetically studied. A pseudo phase diagram was computed which predicts the interfacial carbide for reaction between CNT and Al-Si alloy at processing temperature. Kinetic aspects showed that Al4C3 forms with Al-12 wt.% Si alloy while SiC forms with Al-23wt.% Si alloy. Mechanical properties at nano, micro and macro-scale were evaluated using nanoindentation and nanoscratch, microindentation and bulk tensile testing respectively. Nano and micro-scale mechanical properties (elastic modulus, hardness and yield strength) displayed improvement whereas macro-scale mechanical properties were poor. The inversion of the mechanical properties at different scale length was attributed to the porosity, CNT clustering, CNT-splat adhesion and Al4C3 formation at the CNT/matrix interface. The Dispersion parameter (DP) was more sensitive than Clustering parameter (CP) in measuring degree of CNT distribution in the matrix.

No description available.

No description available.

No description available.

No description available.

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.

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.