Metal Matrix Composites (MMCs) are used as structural materials because of their ability to have a combination of high strength and good ductility. A common problem with MMCs utiliz-ing vastly different materials is the difficulty in forming a strong matrix/reinforcement interface without suffering extensive dissolution, debonding, or chemical reactions between the compo-nents. In this work, a nickel base amorphous particulate reinforced crystalline nickel matrix composite is processed. The reinforcement, an equimolar NiW amorphous powder, was synthe-sized using the mechanical alloying process. The amorphous and crystalline nickel powders were blended in varying volume fractions and then consolidated using hot-isostatic pressing (HIP). This work reveals that the amorphous NiW reinforcement provides strength and hardness to the ductile Ni matrix while simultaneously maintaining a strong interfacial bond due to the similar chemistry of the two components. The strengthening achieved in the composite is attrib-uted to the particulate/matrix boundary strengthening. / Master of Science

A mathematical model has been created based on J.D. Eshelby's equivalent inclusion method that can predict the elastic modulus and damping capability in the form of Joule heat for any piezoelectric ceramic-reinforced metal matrix composite system. Specifically, barium titanate (BaTiO₃), lead titanate (PbTiO₃), and zinc oxide (ZnO) piezoelectric ceramics have been modeled as dispersed particles shaped as spheres, prolate spheroids, and discs within a host of common structural metallic matrices including 304 stainless steel, mild steel, aluminum, brass, copper, lead, magnesium, nickel, Ni-20wt%Cr, tin, titanium, Ti-6Al-4V(at%), and tungsten. Composite systems that were predicted to exhibit the greatest level of damping capacity include copper, aluminum, and magnesium matrices reinforced with PbTiO₃, BaTiO₃, and ZnO, in descending order of damping magnitude. In general, higher-conducting, lower-stiffness metallic matrices coupled with more-piezoelectric, higher-stiffness ceramic reinforcement resulted in the greatest level of predicted damping capability and enhanced composite elastic modulus. Additionally, a Ni-20wt%Cr-30v%BaTiO₃ composite has been created using mechanical alloying processing. Specifically, pure constituent powders were combined stoichiometrically in a SPEX milling vial utilizing a charge ratio of 4:1 and subsequently milled for 24 hours. Separate composite powder samples were then annealed in a hydrogen tube furnace at 400°C, 500°C, and 600°C for one and five hours at each temperature. X-ray diffraction was performed on the as-milled and the annealed powders revealing that each was composed of the starting constituents in the appropriate proportions. Representative powders were mounted and polished using common metallographic procedures and microstructures were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. All of the powders exhibited a good dispersion of BaTiO₃ particles ranging in diameter from 1μm to about 25nm with no noticeable difference between the as-milled and the annealed powders. / Master of Science

Ferroelectric and piezoelectric ceramic reinforced metal matrix composites are new materials being explored for vibration damping purposes. The high damping ability of ferroelectric and piezoelectric ceramics such as barium titanate (BaTiO3) and lead zirconate titanate (PZT) is due to the anelastic response of ferroelastic domain walls to applied external stress. In piezoelectric ceramics, vibration energy can also be dissipated through the direct piezoelectric effect if the appropriate electric circuit is connected across the ceramic. In this work we have examined the vibration damping behavior of BaTiO3, nickel-barium titanate (Ni-BaTiO3) composites and nickel-lead zirconate titanate (Ni-PZT) composites. BaTiO3 ceramics were fabricated by a combination of uniaxial pressing and cold isostatic pressing followed by sintering in air. Low frequency (0.1Hz-10Hz) damping capacity of BaTiO3, tanδ has been measured in three-point bend configuration on a dynamic mechanical analyzer. Tanδ has been found to increase with temperature up to the Curie temperature (Tc) of BaTiO3, after which there was a drop in damping capacity values due to the disappearance of ferroelectric domains above Tc. Furthermore within the frequency range tested, tanδ has been found to decrease with increasing vibration frequency. We also observed that tanδ decays with the number of vibration cycles (N). The decrease in tanδ with N, however, is fully recovered if BaTiO3 is heated above the Tc. Ni-BaTiO3 composite composed of a layer of BaTiO3 ceramic sandwiched between two layers of Ni were fabricated using a combination of electroless plating and electroforming. The damping behavior of the composite was analyzed in terms of the damping mechanisms below Tc and the damping mechanisms above Tc of BaTiO3. Below Tc, vibration damping ability of the composite was highly influenced by ferroelastic damping in the BaTiO3 component. Above the Curie temperature, the damping capacity was influence more by the inherent damping mechanisms in the nickel matrix. The damping mechanisms in Ni-PZT composites were evaluated at a low vibration frequency of 1Hz. In these composites we identified ferroelastic domain wall motion as the main damping mechanism active below the Tc of PZT. Using a poled PZT ceramic enhanced the damping capacity of the composite because of favorable ferroelastic domain orientation in the direction of applied stress. Based on our experimental results, we found no evidence of a direct piezoelectric damping mechanism in the Ni-PZT composites. / Ph. D.

Reinforcing metals with compositionally similar amorphous particles has been found to create composites with good interfacial bonding. It is conceivable that significant additional strengthening in amorphous reinforced composites can be realized by creating high-aspect ratio reinforcements; attritor milling holds promise in this regard. In this work, mechanical alloying was used to produce equimolar Ni-W powder that became a composite of amorphous Ni-W with undissolved W crystallites. A mixture of nickel powder and ten volume percent amorphous Ni-W powder was blended by attritor milling for either one or three hours, compacted by combustion-driven compaction and sintered for up to fifty hours at 600ºC. Prolonged times at elevated temperatures led to crystallization of the amorphous reinforcement particles and dissolution of tungsten into the matrix. Vickers macrohardness tests on the sintered composites yielded lower-than-expected values. Microscopy after hardness testing revealed sliding of particles at their boundaries, indicating poor bonding between them. It is believed that the sintering process was compromised by contamination from organic vapor present in the tube furnace used. While attritor milling effected smaller reinforcement particles, the small increase in aspect ratio would likely have been insufficient to cause significant strengthening by shear load transfer. / Master of Science

Metal matrix composites offer unique opportunities for achieving multi-functionality in materials. In an attempt to investigate the possibility of enhancing damping characteristics of structural metals, copper was reinforced with tetragonal ferroelectric BaTiO3 particulates (Cu-BaTiO3 composites) using powder metallurgy techniques. The effect of particulate size and three processing conditions, sintering atmosphere, cooling rate and, uniaxial compaction pressure on the tetragonality and hence the ferroelectric properties of barium titanate powder were investigated using differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The results show that sintering atmosphere and cooling rates have little effect on the tetragonality of barium titanate powder. Tetragonality of barium titanate powder decreased gradually with decreasing particle size. The decrease in tetragonality with decreasing particle size, however, was only severe in the very fine powders. Although no direct relationship was found between uniaxial compaction pressure and tetragonality, uniaxial pressure may also decrease the tetragonality of barium titanate. Three Cu-BaTiO3 composites, D1, D2 and D3 reinforced with 40vol% barium titanate particles of average sizes 209μm, 66μm and 2μm were respectively fabricated. The retention of the ferroelectric tetragonal phase of barium titanate after composite processing was confirmed by DSC. Composite microstructures observed using optical and scanning electron microscopy revealed uniform dispersions of barium titanate particles in D1 and D2. In D3, the barium titanate formed a chain-like structure because of extensive agglomeration of the fine reinforcement particles. Damping characteristics of the composites were evaluated between 25oC and 165oC at a frequency of 1Hz using dynamic mechanical analysis (DMA). The relative damping capacities (tanδ) in the composites were higher than the unreinforced metal. The damping capacity of composites D1 and D2 was also found to be dependent on temperature. Damping capacity was high from room temperature up to the Curie point of barium titanate, after which there was a slight drop in damping values probably due to a loss in ferroelectric properties. The small drop in damping values recorded in excess of the Curie temperature is an indication that ferroelectricity contributes little to the overall damping capacity of the Cu-BaTiO3 composites. This results from either a reduced ferroelectric damping in barium titanate particles or, poor stress transfer from matrix to reinforcement because of the weak and porous copper-barium titanate interface. / Master of Science

Particulate reinforced Al alloy composites (AlMCs) are recognized as important structural materials due to their lightweight, high modulus and strength and high wear resistance. In order to understand the effect of matrix, reinforcement and secondary processing techniques on the microstructure development and mechanical properties of AlMCs produced by powder metallurgy routes, Al alloy composites reinforced with three types of reinforcements by different secondary processing techniques have been produced and examined. Fabrication of Al or 6061Al alloy based composites reinforced with nano-sized SiC particles (~500nm), micro-sized (<25µm) quasicrystalline alloy particles (hereinafter referred to as “NQX”) and micro-sized Nb particles (~130µm) has been carried out by powder metallurgy routes followed by extrusion or cold rolling. After extrusion, a homogeneous distribution of secondary particles has been obtained with rare interfacial reaction products. The 6061Al/SiC composites exhibit superior mechanical properties than either monolithic alloys or composites reinforced with micro-sized particles with retained ductility while the 6061Al/NQX and 6061Al/Nb composites show limited improvement in tensile strength mainly due to their reinforcement size and poor interfacial bonding. After cold rolling, the evolution in microstructure, texture and strength has been analysed. A typical near β fibre texture with highest intensities near Copper and Brass orientations has been developed for 6061Al/NQX and 6061Al/Nb composites. For 6061Al/SiC composites, a randomized texture with very small grains has achieved due to the presence of the non-deformable SiC particles. Mechanical property tests including microhardness, three-point bending tests and tensile tests have been carried out on cold rolled samples and the results exhibit some level of improvement when compared with as-extruded samples due to work hardening. Finally, the work moves on to the general discussion based on the previous result chapters. The microstructural development related to reinforcement, matrix and interfacial areas during extrusion and cold rolling has been summarised and the correlation between microstructure and mechanical properties has been discussed. The thesis provides a thorough understanding of AlMCs produced by powder metallurgy routes in terms of matrix, reinforcement and processing techniques. It can provide reference to the future development of AlMCs for high strength applications.

Metal matrix composites (MMCs) of Al - 4 wt.% Cu reinforced with different volumetric fractions of B4C particles were produced by hot pressing. The effect of aging temperature on the age hardening response of the composites was studied and compared with the characteristics exhibited by the matrix alloy. Reinforcement addition was found to considerably affect the age hardening behavior. Detailed transmission electron microscopy and differential scanning calorimetry observations were made to understand the aging response of the composites. The low strain rate and high strain rate deformation behavior of the MMCs were determined utilizing low velocity transverse rupture tests and true armor-piercing steel projectiles, respectively. Increasing the volume fraction of B4C led to a decrease in flexural strength. The flexural strength vs. strain rate plots showed a slight increase in strength followed by a decrease for all samples. The mechanical performance of the composites and the unreinforced alloy were greatly improved by heat treatment. The MMCs were found to be inferior to monolithic ceramics when used as facing plates in armors.

The aim of this study was to produce partially reinforced aluminum metal matrix composite components by insertion casting technique and to determine the effects of silicon content, fiber vol% and infiltration temperature on the mechanical properties of inserts, which were the local reinforcement parts of the components. Silicon content of alloys was selected as 7 wt% and 10 wt%. The reinforcement material, i.e. Saffil fiber preforms, had three different fiber vol% of 20, 25 and 30 vol% respectively. The infiltration temperatures of composite specimens were fixed as 750 &deg / C and 800 &deg / C. In the first part of the thesis, physical and mechanical properties of composite specimens were determined according to the parameters of silicon content of the matrix alloy, infiltration temperature and vol% of the reinforcement phase. X-ray diffraction examination of fibers resulted as the fibers mainly composed of deltaalumina fibers and scanning electron microscopy analyses showed that fibers had planar isotropic condition for infiltration. Microstructural examination of composite specimens showed that appropriate fiber/matrix interface was created together with small amount of micro-porosities. Bending tests of the composites showed that as fiber vol% increases flexural strength of the composite increases. The highest strength obtained was 880.52 MPa from AlSi10Mg0.8 matrix alloy reinforced with 30 vol% Saffil fibers and infiltrated at 750 &deg / C. Hardness values were also increased by addition of Saffil fibers and the highest value was obtained as 191 HB from vertical to the fiber orientation of AlSi10Mg0.8 matrix alloy reinforced with 30 vol% Saffil fibers. Density measurement revealed that microporosities existed in the microstructure and the highest difference between the theoretical values and experimental values were observed in the composites of 30 vol% Saffil fiber reinforced ones for both AlSi7Mg0.8 and AlSi10Mg0.8 matrix alloys. In the second part of the experiments, insertion casting operation was performed. At casting temperature of 750 &deg / C, a good interface/component interface was obtained. Image analyses were also showed that there had been no significant fiber damage between the insert and the component.

Ceramic metal matrix composites (CMMCs) are materials generally created by mixing of hard ceramic particles in a metal matrix. They were expected to combine the ductility and toughness of the metal with the high strength and elastic modulus of the ceramic. MMCs have potential applications in automotive, aeronautical and aerospace industries. Hence, a simple and economical method for fabricating MMCs is an area of intense research. In MMCs, damage evolution starts preferentially at particle matrix interface or at particle clusters in the matrix. This is due to the different physical and mechanical properties of the particle and matrix. Higher local particle volume content leads to higher stress triaxiality making it a preferential site for damage nucleation. Problems with lowering of ductility, fatigue, fracture and impact resistance, agglomeration of ceramic phase and issues related to the predictability of properties of MMCs have been the major issues that have limited their use. In order to overcome some of these shortcomings, the use of nano particles has been attracting increasing attention. The reason is their capability in improving the mechanical and physical properties of traditional MMCs. The dispersion of a nanoscale ceramic phase is needed in order to overcome the problems related to fatigue, fracture toughness, and creep behaviour at high temperatures. However, manufacturing costs, preparation of nano composites and environmental concerns have to be addressed. Agglomeration of nano particles, when produced by the melt stir casting route, the primary route to produce MMCs, is a serious issue that limits the use of nano-particles to produce MMCs with good properties. To avoid agglomeration of the ceramic phase MMCs/nano MMCs have been produced through the powder metallurgy route. Agglomeration is avoided as this is a solid state process. Secondary processing, such as extrusion and rolling are often needed to fully consolidate materials produced in this manner. A high extrusion ratio is often required to get MMCs without porosity. A new method of making nano-ceramic MMC using a polymer derived ceramics (PDC) has been reported. A polymer derived ceramic is a material that converts itself into a ceramic when heated above a particular temperature. In the PDC method a polymer precursor is dispersed in the metal and then converted in-situ to a ceramic phase. A feature of this process 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 or air is not required to produce the ceramic phase. The polymer can be introduced through casting or powder metallurgy route. In the casting route, the polymer powder is directly added to molten metal and pyrolyzed in-situ to create castings of metal-matrix composites. These composites have shown better properties at elevated temperatures but the problem of agglomeration of particles due to Van der Waal's forces and porosity still remains. In the powder method, the organic precursor was milled with copper powder and then plasma sprayed to produce a metal matrix composite. It is reported that these composites retains its mechanical strength close to the melting point of the copper. However, getting a nano sized distribution is difficult through this route as the plasma spray route is a melting and solidification method. Solid state processing by powder metallurgy is possibly a better method to produce well dispersed nano-MMCs. However, powder metallurgy routes are much more expensive and only parts of limited sizes can be produced by this method. Another solid state process Friction Stir Processing (FSP) has successfully evolved as an alternative technique to fabricating metal matrix composites. FSP is based on the principles of Friction Stir Welding (FSW). In FSW, a rotating tool with a pin and a shoulder is inserted into the material to be joined, and traversed along the line of the joint. The friction between the tool and the work piece result in localized heating that softens and plasticizes the material. During production of MMCs using FSP method, the material undergoes intense plastic deformation resulting in mixing of ceramic particles and the metal. FSP also results in significant grain refinement of the metal and has also been used to homogenize the microstructure. FSP technology has also been used to fabricate surface/bulk composites of Al-SiC, friction stir surfacing of cast aluminum silicon alloy with boron carbide and molybdenum disulphide powders and to produce ultra-fine grained Cu-SiC composites. A major problem in the FSP of MMCs is severe tool wear that results from abrasion with hard ceramic particles. The progressive wear of the tool has been reported to increase the likelihood of void or defect development. This change in geometry has been reported in the friction stir welding of several MMCs. The problems concerning the tool life has become a serious issue in the application of FSP for producing MMCs. In the present work the advantages of the PDC method and FSP have been combined to produce polymer derived nano ceramic MMCs. This method mainly consists of three steps. In the first step, a polymer, which pyrolysis to form a PDC at temperatures lower than the melting point of the metal, is dispersed in the metal by FSP. This step is different from the melt route where the PDC forms at temperatures above the melting point of the metal. In the second step, external pyrolysis of the polymer dispersed material is carried out. Since this is a solid state process at stresses much higher than the shear or fracture of the polymer is expected to get evenly and finely distribution in the metal. This is done by heating the polymer dispersed material to a temperature above the pyrolysation temperature of the ceramic but lower than the melting point of the metal matrix. It should be mentioned that some pyrolysis of the polymer is possible during the FSP process itself. In the third step FSP is carried out on the pyrolised material for removing porosity that would form due to gas evolution during pyrolysis and to get a more uniform dispersion of polymer derived ceramic particles in the matrix. This method will produce nano-scale metal matrix composites with a relatively high volume fraction of the ceramic phase. This method can be extended to big sheets or a particular region in a sheet with no or low wear of tools. The material selected for the present study were pure Copper (99.9%) and Nickel Aluminum Bronze (NAB) copper alloy. The polymer precursor was poly (urea methyl vinyl) silazane, which is available commercially as CERASET. The polymer consists of silicon, carbon, nitrogen, oxygen and hydrogen atoms. The liquid precursor was thermally cross-linked into a rigid polymer, which was milled into a powder. This powder, having angular shaped particles of an average size of 10 µm, was used as the reinforcement. The polysilazanes convert into a highly refractory and amorphous ceramic upon pyrolysis and is known as polymer-derived silicon carbonitride which consists principally of silicon, carbon and nitrogen. The in-situ process is feasible because copper melts above the temperature at which the organic phase begins to pyrolise. The polysilazanes pyrolise in the temperature range of 973 to 1273 K, which lie below the melting temperature of copper, 1356K.The precursor has a density of approximately 1 gcm-3 in the organic phase and approximately 2 gcm-3 in the ceramic state. In the present work, we seek to introduce approximately 20 vol% of the ceramic phase into copper. The microstructure and mechanical properties of the developed copper-based in-situ polymer derived nano MMCs have been characterized in detail to understand the distribution of particles. The microstructure of the as received, processed as well as the FSP composite material was characterized using Optical Microscope (OM), Scanning Electron Microscope (SEM), Electron Probe Micro Analyzer (EPMA) and Transmission Electron Microscope (TEM). OM and SEM microstructural observations show that PDC particles are distributed uniformly with a bimodal (submicron+micron) distribution. In addition, TEM micrographs reveal the formation of very fine PDC particles of diameter 10-30 nm. X-ray diffraction and Thermo-gravimetric analysis confirms the presence of ceramic phase (Si3N4/SiC) in the matrix. Significant improvement in mechanical properties of the FSP PD-MMCs has been observed. This in-situ formed Cu/PDC composites show five times increase in micro-hardness (260Hv - 2.5GPa) compared to processed copper base metal and in-situ NAB/PDC composite shows two times increase in micro-hardness (325Hv- 3.2GPa) compared to NAB matrix. The Cu-PDC composites exhibited better tensile strength at room temperature. In-situ formed Cu-PDC composite’s yield strength increased from 110MPa to 235MPa as compared to processed base metal, where as ultimate tensile strength increases from 246MPa to 312MPa compared to processed base metal at room temperature. This strengthening could be attributed to the presence of in-situ formed hard phases and the concomitant changes in the microstructure of the matrix material such as reduction in grain size and contribution from Orowan strengthening. In the present work, we have observed tool wear by observing tool after each FSP pass and apart from producing a significantly harder material with higher elastic modulus, possibly for the first time, the issue of tool wear has been overcome. This is due to the fact that the composite is made by the polymer route and that the ceramic fractures easily till it reaches the nano-size. Wear studies of this composite was carried out in a pin-on-disc machine by sliding a pin made from the composite against an alumina disc. The wear rate of the FSP PD-MMC composites increased from 1.63×10-5 to 5.72×10-6 mm3/Nm. Improved wear resistance could be attributed to the presence of the in-situ formed hard nano-phase.

High shear mixing offers a promising solution for particle dispersion in a liquid with intensive turbulence and high shear rate, and has been widely used in the chemical, food and pharmaceutical industries. However, a practical high shear mixing process has not yet been adapted to solve the particle agglomeration in metallurgy due to the high service temperature and reactive environment of liquid metal. In this study, the effect of high shear mixing using the newly designed rotor-stator high shear device have been investigated with both Al and Mg matrix composites reinforced with SiC particles through casting. The microstructural observation of high shear treated Al and Mg composites show improved particle distribution uniformity in the as-cast state. Increased mechanical properties and reduced volume fraction of porosity are also obtained in the composite samples processed with high shear. With the melt conditioning procedure developed for twin roll casting process, two distinct solutions has been provided for thin gauge Mg strip casting with advanced microstructure and defect control. The melt conditioning treatment activates the MgO as heterogeneous nuclei of α-Mg through dispersion from continuous films to discrete particles. Thus enhanced heterogeneous nucleation in the twin roll casting process not only refines the α-Mg grain size but also eliminates the centre line segregation through equiaxed grain growth and localized solute distribution. The grain refinement of the α-Mg through SiC addition has also been studied through EBSD and crystallographic approaches. Two reproducible and distinct crystallographic orientation relationships between α-SiC (6H) and α-Mg have been determined: [1010]SiC//[2113]Mg, (0006)SiC//(1011)Mg, (1216)SiC//(2202)Mg and [0110]SiC//[1100]Mg, (0006)SiC// (0002)Mg, (2110)SiC//(1120)Mg.