O presente trabalho teve como foco um problema que vem se tornando cada vez mais grave nos dias de hoje, já que, com o crescimento das zonas urbanas, algumas residências estão se aproximando dos transformadores de potência. Transformadores são máquinas cuja função é aumentar ou diminuir a diferença de potencial de uma rede elétrica, porém, eles geram um nível significativo de ruído, podendo ser prejudicial à saúde. Esse ruído é causado pela magnetostricção pelas chapas de Fe-Si as quais compõem o núcleo. Assim, o intuito desse trabalho foi estudar duas ligas de Fe-Ti, para averiguar seu comportamento magnetostrictivo. Além disso, é importante estudar as propriedades magnéticas dessas ligas, a fim de se analisar quais são as perdas magnéticas associadas a essas ligas. Portanto, também foram feitas medidas de indução magnética, permeabilidade magnética e resistividade elétrica. Também foram feitas análises de microscopia eletrônica de varredura, para análise do tamanho e morfologia dos grãos, difratometria de raios X, para identificação das fases presentes e difração de elétrons retroespalhados, para identificação da textura. Todas essas medidas foram feitas para as ligas de Fe-Ti e uma liga de Fe-Si comercial de grão orientado e os resultados foram comparados entre si. As ligas de Fe-Ti apresentaram valores de magnetostricção menores do que a liga de Fe-Si, porém, apresentaram valores bem maiores quando analisadas as perdas magnéticas. Com isso, foi possível fazer uma primeira análise das ligas de Fe-Ti, criando diversas possibilidades para estudos futuros. / The present research had as focus a problem that is becoming very serious nowadays, since with the growth of urban areas, some households are approaching the power transformers. Power transformers are machines whose function is to increase or decrease the potential difference of a power grid, but they generate a significant level of noise, which can be harmful to human health. This noise is created by the magnetostriction of the Fe-Si plates that compose the core. Thus, the purpose of this research was to study two Fe-Ti alloys to investigate their magnetostrictive behavior. In addition, it is important to study the magnetic properties of these alloys in order to analyze the magnetic losses associated with these samples. Therefore, measurements of magnetic induction, magnetic permeability and electrical resistivity were also made. Were made analyzes of scanning electron microscopy to investigate the size and morphology of the grains, X-ray diffraction, to identify the phases that are present in the material and electron backscatter diffraction to identify the texture.These characterizations were made for the Fe-Ti alloys and a commercial grain-oriented Fe-Si alloy. The results were compared to each other. Fe-Ti alloys showed lower magnetostriction values than the Fe-Si alloy, however they presented highest values of magnetic losses. As a result, it was possible to make a first analysis of the Fe-Ti alloys properties, creating several possibilities for future studies in order to improve Fe-Ti alloys properties.

O presente trabalho teve como foco um problema que vem se tornando cada vez mais grave nos dias de hoje, já que, com o crescimento das zonas urbanas, algumas residências estão se aproximando dos transformadores de potência. Transformadores são máquinas cuja função é aumentar ou diminuir a diferença de potencial de uma rede elétrica, porém, eles geram um nível significativo de ruído, podendo ser prejudicial à saúde. Esse ruído é causado pela magnetostricção pelas chapas de Fe-Si as quais compõem o núcleo. Assim, o intuito desse trabalho foi estudar duas ligas de Fe-Ti, para averiguar seu comportamento magnetostrictivo. Além disso, é importante estudar as propriedades magnéticas dessas ligas, a fim de se analisar quais são as perdas magnéticas associadas a essas ligas. Portanto, também foram feitas medidas de indução magnética, permeabilidade magnética e resistividade elétrica. Também foram feitas análises de microscopia eletrônica de varredura, para análise do tamanho e morfologia dos grãos, difratometria de raios X, para identificação das fases presentes e difração de elétrons retroespalhados, para identificação da textura. Todas essas medidas foram feitas para as ligas de Fe-Ti e uma liga de Fe-Si comercial de grão orientado e os resultados foram comparados entre si. As ligas de Fe-Ti apresentaram valores de magnetostricção menores do que a liga de Fe-Si, porém, apresentaram valores bem maiores quando analisadas as perdas magnéticas. Com isso, foi possível fazer uma primeira análise das ligas de Fe-Ti, criando diversas possibilidades para estudos futuros. / The present research had as focus a problem that is becoming very serious nowadays, since with the growth of urban areas, some households are approaching the power transformers. Power transformers are machines whose function is to increase or decrease the potential difference of a power grid, but they generate a significant level of noise, which can be harmful to human health. This noise is created by the magnetostriction of the Fe-Si plates that compose the core. Thus, the purpose of this research was to study two Fe-Ti alloys to investigate their magnetostrictive behavior. In addition, it is important to study the magnetic properties of these alloys in order to analyze the magnetic losses associated with these samples. Therefore, measurements of magnetic induction, magnetic permeability and electrical resistivity were also made. Were made analyzes of scanning electron microscopy to investigate the size and morphology of the grains, X-ray diffraction, to identify the phases that are present in the material and electron backscatter diffraction to identify the texture.These characterizations were made for the Fe-Ti alloys and a commercial grain-oriented Fe-Si alloy. The results were compared to each other. Fe-Ti alloys showed lower magnetostriction values than the Fe-Si alloy, however they presented highest values of magnetic losses. As a result, it was possible to make a first analysis of the Fe-Ti alloys properties, creating several possibilities for future studies in order to improve Fe-Ti alloys properties.

Les alliages Fe-Si sont connus pour combiner d’excellentes propriétés magnétiques avec de bonnes propriétés électriques (forte résistivité électrique). Dans ce contexte nous avons recherché à élaborer des matériaux à forte teneur en Si, souvent difficiles à obtenir et mettre en forme industriellement. Des alliages magnétiques doux de type Fe-Si avec une teneur élevée en Si (4,5%, 6,5%, 10% et 15% massique) ont été obtenus avec succès à l’état nanocristallin par broyage mécanique et recuit. La formation des alliages a été étudiée par diffraction X, spectroscopie Mössbauer et analyses thermomagnétiques. La stabilité thermique de la poudre a été analysée par DSC. Des mesures d’aimantation ont été réalisées pour caractériser les performances magnétiques. La durée de broyage nécessaire pour la formation de l’alliage a été déterminée pour chaque teneur en Si. Pour les faibles temps de broyage, le recuit conduit à la formation du composé Fe3Si. Après la formation de l’alliage par le broyage mécanique, l’effet du recuit est seulement de réduire les tensions internes du second ordre, induites dans la poudre par le broyage. L’addition de Si conduit à la diminution de la température de Curie de 770 °C pour le Fe pur, à 725 °C pour une teneur de 4,5% massique de Si et à 550 °C pour 15% massique de Si. Pour les temps faibles de broyage, l’écart entre l’aimantation de la poudre avant et après recuit est dû à la formation du composé Fe3Si pendant le recuit, lequel a une aimantation plus faible que la solution solide de Feα(Si). Pour les longs temps de broyage, le recuit à 400 °C pour 4 heures n’a pas d’effet sur la valeur de l’aimantation à saturation. En augmentant la teneur en Si, l’aimantation à saturation de l’alliage Fe-Si décroit.Les alliages Ni3Fe (aussi connus comme Permalloys) présentent de meilleures propriétés magnétiques, mais ils ont une résistivité inférieure à celles des Fe-Si. Une voie attractive semble la combinaison des propriétés des 2 classes de matériaux doux en formant un composite. Les alliages Fe-Si précédemment obtenus ont été utilisés pour l’élaboration des poudres composites de type Permalloy/Fe-Si par la mécanosynthèse. Le broyage mécanique conduit à la formation des particules composites avec un aspect stratifié. Quatre heures de broyage de l’alliage Fe-Si avec du Ni3Fe ne conduisent pas à la formation des nouvelles phases, mais la formation d’un alliage ternaire Ni-Fe-Si résulte d’un recuit ultérieur à 900 °C. L’aimantation à saturation du composite augmente avec la croissance de la teneur le d’alliage Fe-Si, mais le temps de broyage ne semble avoir aucun effet sur cela.Une étude préliminaire a été réalisée sur l’élaboration des compacts composites de type Ni3Fe/Fe-Si par frittage flash, dans le but de préserver l’état nanocristallin par de basses températures de frittage. L’influence de la température de frittage et de la durée de maintien sur la structure, et les propriétés physiques des compacts est discutée. Des températures allant jusqu'à 750 °C pour une durée de maintien minimale ou un palier de 2 minutes maximum à 700 °C ne conduisent pas à la diffusion des éléments des alliages. L'augmentation de la température ou de la durée de frittage conduit à des cristallites plus grandes, mais qui restent dans le domaine nano pour les températures étudiées. La densité des compactes augmente avec la température et le palier. En outre, la résistivité diminue en augmentant ces 2 paramètres. L'effet de la teneur en Fe-Si est de diminuer la densité et en même temps d'augmenter la résistivité des compacts. La perméabilité magnétique est réduite avec l'augmentation de la température et de la durée de frittage, ainsi que lors de la diminution du contenu de Ni3Fe. Une température élevée et un long temps de maintien à la température de frittage conduisent à l’augmentation des pertes magnétiques. Le champ coercitif est également influencé par les paramètres de frittage, via l'effet qu'ils ont sur la taille des cristallites. / Fe-Si alloys are known for combining excellent magnetic properties with good electric characteristics (high resistivity). In this context we sought to develop materials with a relatively high Si content, often difficult to obtain and shape industrially.In this thesis, soft magnetic Fe-Si alloys with high Si content (4.5, 6.5, 10 and 15 wt. %) were successfully obtained in nanocrystalline state by mechanical alloying and annealing. The formation of the alloy was studied by X-ray and neutron diffraction, Mossbauer spectroscopy and thermomagnetic analysis. DSC technique was used in order to study the powder’s thermal stability. Magnetisation measurements were also made in order to characterise their magnetic performances. The milling duration necessary for the formation of the alloy was determined for each Si content. For low milling times, annealing leads to the formation of the Fe3Si compound. Once the alloy is formed by mechanical milling, the effect of the annealing is only to reduce the second order stress induced in the powder by the milling process. Si addition leads to the decrease of the alloy’s Curie temperature from 770 °C for pure Fe to 725 °C for a 4.5 wt. % Si and down to 550 °C if the Si content increases to 15 wt. %. For low milling times, a gap between the magnetisation of the as-milled alloy and of the milled and subsequently annealed one is due to the formation of the Fe3Si compound during annealing which has a lower magnetisation than that of the αFe (Si) solid solution. For longer milling durations, annealing at 400 °C for 4 hours has no effect on the saturation magnetisation value. By increasing the Si content, the Fe-Si alloy’s saturation magnetisation decreases.Fe-Ni alloys whose composition is close to Ni3Fe (commonly known as Permalloys) have better magnetic properties, but a resistivity well inferior to that of Fe-Si alloys. Therefore, a combination of the properties of these 2 alloy classes of soft magnetic materials into a composite seems to be an attractive route. The previously obtained Fe-Si alloys were used for the preparation of Permalloy/Fe-Si composite powders by mechanical milling. Milling leads to the formation of composite powder particles with a stratified aspect. Milling of the Fe-Si and Ni3Fe alloys for 4 hours does not lead to the formation of new phases, but a subsequent annealing at 900 °C results in the formation of a Ni-Fe-Si alloy. Saturation magnetisation of the composite increases with increasing of the Fe-Si content, but milling duration seems to have no effect on it.A preliminary study was made on the elaboration of Ni3Fe/Fe-Si composite compacts obtained by spark plasma sintering, aiming to preserve the nanocrystalline state by lower sintering temperatures. The influence of the sintering temperature and temperature holding duration on the structure, density, resistivity and magnetic properties of the compacts is discussed. Temperatures of up to 750 °C for minimal holding duration or a maintain at the temperature of 700 °C for a duration of up to 2 minutes does not lead to a diffusion of the alloys’ elements. Increasing of the sintering temperature or duration leads to larger crystallite sizes, but they remain in the nano domain for the studied temperatures. The compacts’ density increases with temperature and sintering duration. Resistivity, on the other hand decreases when increasing the aforementioned parameters. The effect of the Fe-Si content is to decrease the density and at the same time increase the compacts’ resistivity. Magnetic permeability is reduced with increasing sintering temperature and duration, as well as when decreasing of the Ni3Fe content. High temperature and long maintaining duration leads to an increase of magnetic losses. Coercive field is also influenced by sintering parameters by the effect they have on the crystallite size.

Made available in DSpace on 2015-05-08T15:00:07Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2402829 bytes, checksum: 4c7bc88fa5d8c55de36b5f29f4cc3cf0 (MD5) Previous issue date: 2011-03-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents a study about microstructural evolution of nanocrystalline alloys Fe50%Si processed by milling of high energy. With the objective of pondering the process variables, a factorial design 2² with central point was used. The design formed the basis for the development of a semi-empirical model of Fe-Si phase formation, which subsequently led to the response surface equation. The samples were characterized by Xray Fluorescence, Scanning Electron Microscope (SEM), Mössbauer Spectroscopy and Xray diffraction. The structural changes are discussed based on obtained diffractograms and using the Rietveld refinement in the TOPAS software. By calculating the effects of variables, it's shown that for the characteristics of the used system, the milling time has a preponderant effect on the other ones. The study on the evolution of the performed lattice parameter shows the occurrence of two distinct phases with increasing rotation speed and milling time. This evolution demonstrates a transition from disordered to ordered phase of Fe-Si intermetallic compound. / Este trabalho apresenta um estudo sobre a evolução microestrutural de ligas nanocristalinas de Fe50%Si formadas através do processo de moagem de alta energia. Com o objetivo de ponderar sobre as variáveis do processo, um planejamento fatorial 2² com ponto central foi utilizado. O planejamento serviu de base para o desenvolvimento de um modelo semi-empírico de formação da fase Fe-Si que, posteriormente, originou a superfície de resposta da equação. As amostras foram caracterizadas por Fluorescência de Raios-X, Microscopia Eletrônica de Varredura, Espectros de Mössbauer e Difração de Raios-X. As mudanças estruturais são discutidas com base nos difratogramas obtidos e utilizando o refinamento de Rietveld no programa TOPAS. Através do cálculo dos efeitos das variáveis, mostra-se que para as características do sistema utilizado, o tempo de moagem possui um efeito preponderante sobre as demais. O estudo sobre a evolução do parâmetro de rede realizado mostra a ocorrência de duas fases distintas com o incremento da velocidade de rotação e do tempo de moagem. Tal evolução revela a passagem da fase desordenada para ordenada do composto intermetálico Fe-Si.

The magnetic Fe–6.5 wt% Si powder was produced by gas atomization and its microstructure was also investigated. The secondary dendritic arm spacing (SDAS) is related to the droplet size, λ = 0.29 · D⁰·⁵, and the numerical solidification model was applied to the system, giving rise to the correlation of microstructure to the solidification process of the droplet. It is found that the solid fraction at the end of recalescence is strongly dependent on the undercooling achieved before nucleation; the chances for the smaller droplets to form the grain-refined microstructures are less than the larger ones. Furthermore, the SDAS is strongly influenced by the cooling rate of post-recalescence solidification, and the relationship can be expressed as follows, λ = 74.2 · (T)⁻⁰·³⁴⁷. Then, the growth of the SDAS is driven by the solute diffusion of the interdendritic liquids, leading to a coarsening phenomenon, shown in a cubic root law of local solidification time, λ = 10.73 · (tf)⁰·²⁹⁶.

Cette étude vise à établir des corrélations entre les propriétés structurales et les propriétés magnétiques des revêtements Fe-Si élaborés par pulvérisation cathodique magnétron. L'alliage Fe-Si (coté riche en Fe) a été choisi en raison de son caractère ferromagnétique doux et de son fort potentiel applicatif dans différents systèmes magnétiques. Dans cet esprit, nous avons réalisé une série d'alliages avec différentes teneurs en Si. Les résultats obtenus nous ont permis de mettre en évidence les propriétés magnétiques extrêmement douces de l'alliage Fe-Si à 25 at.% de Si. L'étude des domaines magnétiques de cet alliage par effet Kerr a révélé une configuration en domaines parfaitement parallèle sur une surface importante de l'échantillon. Ce type de configuration est particulièrement recherché dans les couches minces magnétiques utilisées dans les capteurs à magnéto-impédance géante. C'est pourquoi nous avons étendu notre étude des propriétés de cet alliage par la réalisation de capteurs à magnéto-impédance en structure sandwich. La caractérisation de ces capteurs a clairement montré l'influence de l'orientation des domaines magnétiques ainsi que l'épaisseur totale des capteurs sur la variation d'impédance de ces derniers.

The objective of this work is to assess the potential of Mn-Fe-Si-P for magnetic heat pump applications. Mn-Fe-Si-P is a first order transition magnetocaloric material made from safe and abundantly available constituents. A significant magnetocaloric effect occurs at the transition temperature of the material. The transition temperature can be tuned by changing the atom ratios to a region near room temperature. Mn-Fe-Si-P in magnetic heat pumps is investigated by determining the material's properties, 1D system modeling and experiments in a magnetic heat pump prototype. We characterize six samples of Mn-Fe-Si-P, based on their heat capacity and magnetization. The reversible component of the adiabatic temperature change is found from the entropy diagram and compared to cyclic adiabatic temperature change measurements. Five of the six samples are selected to be formed into epoxy xed crushed particulate beds, which can be installed into a magnetic heat pump prototype. A system model is constructed to understand the losses of the magnetic heat pump prototype. Several experiments are performed with Gd with rejection temperatures around room temperature. Including dead volume and casing losses improves the modeling outcomes to match the experimental results closer. Experiments with Mn-Fe-Si-P are performed. Five materials are formed into modular beds that can be combined into two layer configurations. Six experimental configurations are tested, one single layer regenerator test with a passive lead second layer, and five experiments using two layers with varying transition temperature spacing between the materials. The best performance of the beds was found at close spacing at suitable rejection temperatures. It was found that at far spacing, the performance of stronger materials would produce a lower temperature span than that of weaker materials at close spacing. The experiments provide results that are used to validate the system modeling approach using the material data obtained of the Mn-Fe-Si-P samples. We integrate material properties into a system model. A framework is proposed to take into account the hysteresis. This framework shows an improvement of the predicted trend for a single layer case. The proximity of simulation and experimental multi-layering results are dependent on the rejection temperature. At the higher end of the rejection temperature the modeling results over-predict the temperature span around the active region. At lower rejection temperatures the simulation under-predicts the experimental temperature span. The inclusion of experimental pressure drop improved the trends found at higher rejection temperatures. A further improvement was found varying the interstitial heat transfer term. Modeling future research should focus on characterizing the thermo-hydraulic closure relationships for crushed particulate epoxy xed beds, and improvements to the heat loss model. Mn-Fe-Si-P is able to produce a temperature span, when a suitable set of Mn-Fe- Si-P materials are selected based on minimal hysteresis, making it a viable material for magnetic heat pump applications. The performance of Mn-Fe-Si-P is further improved by layering materials with a closely spaced transition temperature. Future research should focus on increasing the production of Mn-Fe-Si-P materials with low hysteresis, and improving the regenerator matrix geometry and stability. / Graduate

The aim of this work was to improve the heat capacity estimation of a material for usage within a CALPHAD-type assessment. An algorithm is derived that estimates the trend of heat capacity with temperature based on zero Kelvin properties and the thermal expansion coefficient at the Debye temperature. The algorithm predicts not only the trend of heat capacity but also the temperature trend of the volume and the bulk modulus, which can be also included in new thermodynamic databases. The algorithm is used to assess thermophysical properties of the intermetallic phases eta (Fe2Al5), epsilon~(Fe5Al8) and tau4 (FeAl3Si2). The heat capacity of the intermetallic phases zeta, eta, theta and epsilon of the Al-Fe system and of tau4 of the Al-Fe-Si system was measured using DSC. For the phases zeta, eta, and theta, a non-linearly increasing heat capacity approaching the melting temperature was observed. In addition, the heat capacity of three bcc-based Al-Fe samples including the B2-->A2 transition were determined. The Al-rich section of the Al-Fe phase diagram was studied using DTA and quenching experiments. The homogeneity ranges of the intermetallic phases were determined using SEM/WDS measurements. Based on own and literature values, a thermodynamic description of the Al-Fe system was assessed including the modelling of A2/B2 ordering and the homogeneity range of all intermetallic phases. In addition, thermodynamic parameters of the Al-Fe-Si, Al-Fe-Mg, and the Fe-Mg-Si system were assessed to obtain a thermodynamic description of the Al-rich side of the Al-Fe-Si-Mg system, which can be used to study phase transitions of typical A356-aluminium alloys.

Copper based alloys play important role in high heat flux applications, particularly in rocket technology, the liner of the combustion chamber, and also in other heat transfer vessels. In these applications, one needs excellent high-temperature strength without sacrificing the thermal conductivity significantly. However, it is a challenging and difficult task to significantly improve the balance between strength and conductivities (electrical and thermal) of Cu-based alloys. In general, microstructural attributes, responsible for increasing mechanical strength of the alloy, also affect the transport properties by creating scattering centers. Hence, delicate optimization is needed for developing balanced alloy system for better performance. A substantial amount of research efforts has therefore been focused on devising methodologies to synthesize copper based alloys with a good combination of strength and conductivity. The present thesis deals with the development of a newer class of high strength high conductivity copper base alloy through tuning of phase transformation and careful additions of ternary and quaternary alloying elements and ultimately by microstructural engineering. In this thesis, we report the development of novel high strength high conductivity Cu-based alloy series in the Cu-Fe-Si system through rapid solidification process using suction casting apparatus. We have also optimized the alloys by altering and fine tuning the alloy compositions in order to achieve balanced and optimum properties. The strength of copper can be increased by various strengthening mechanisms. In general, precipitation hardening, dispersion strengthening and solid solution strengthening are the three most effective mechanisms for improving the strength of copper. Among these, solid solution strengthening has the most detrimental effect on the transport properties due to the presence of solute atoms which act as prominent scattering centres. Precipitation hardened copper alloys are often unable to retain strength at high temperatures, due to the coarsening of the precipitates. Currently, efforts are being made to develop newer dispersion strengthened copper alloys. These alloys contain a fine dispersion of nanometer sized oxides or other intermetallic compounds in the copper matrix. Dispersion strengthened copper alloys show impressive mechanical strength as well as thermal stability. In this thesis, we have explored the possibility of obtaining structurally ordered intermetallic dispersions through exploiting immiscibility of solutes in copper based alloys. The immiscibility promotes precipitation and decrease the solid solubility of solute elements in the matrix which in turn minimizes the scattering process and thus offers the possibility of improved transport properties. These ordered and coherent dispersion of intermetallic particles in the continuous copper matrix, dispersed during solidification, are believed to be the main contributor to the improvement of mechanical strength of the alloy. Crystallographically ordered structure and the coherency strain associated with the intermetallic particles in the copper matrix, together contribute to the mechanical strength through the mechanism of order hardening and coherency strengthening. These also, promote a low interfacial energy between precipitates and matrix in the alloy. This low interfacial energy reduces the driving force for coarsening process and thus helps in retaining the mechanical strength at elevated temperatures. Releasing of coherency strain at the precipitate-matrix interface with increasing temperature also yields a dramatic effect on the enhancement of thermal conductivity at high service temperatures. In the current study, we have selected three alloy compositions in the Cu-Fe-Si system at the higher end of copper. These are Cu-20Fe-5Si (at%), Cu-2.5Fe-2.5Si (at%) and Cu-1.0Fe-1.0Si (at%) respectively. We have systematically increased the concentration of copper, and altered the ratio of Fe and Si in order to achieve the better combination of properties (mechanical and transport) through fine tuning the microstructure. The present sets of alloys have been chill cast by the suction casting technique. This rapid solidification process, associated with moderate undercooling, is capable of accessing the submerged metastable miscibility gap of the Cu-Fe binary system. The higher quenching rate moves the system far away from equilibrium and hence, the solidification process occurs at the non-equilibrium regime. Rapid solidification of a copper rich Fe-Cu melt promotes the precipitation of the γFe from copper solid solution due to the immiscibility of Fe and Cu. In this scenario, the addition of a small quantity of silicon as a ternary element leads to its partition to both copper and iron rich phases. However, the larger chemical affinity between Fe and Si, leads to the formation of an ordered structure. However, the FCC crystal field of the copper matrix tends to promote an FCC based novel L12 ordered structure of the Fe3Si intermetallic particles instead of the ordered DO3 structure of Fe3Si composition normally observed in the bulk alloy. This nano meter sized L12 ordered particles maintain a cube-on-cube orientation relationship with the surrounding copper matrix and are associated with large coherency strain. A good lattice matching between these L12 ordered particles and copper matrix will promote a low interfacial energy and thus, a low driving force for particle coarsening. The present thesis is divided into eight chapters. The first chapter introduces the present work and the organization of the thesis. In the second chapter, current status in the development of the copper alloys and the general principle of alloy developments has been described. This includes both experimental and theoretical developments that can be used for developing high strength Cu based alloys. Chapter three, titled as „experimental procedure‟, describes the detailed description of materials and experimental techniques, adopted for the current studies. There are three chapters that deal with the main results of the thesis. Chapter eight, describes the suggestion for future work. The fourth chapter, titled as „Chill cast Cu75Fe20Si5 alloy: Microstructural Evolution and Properties‟, explores the detailed microstructural evolution of the Cu75Fe20Si5 alloy. This chapter also discusses the microstructure-property correlations. The microstructure of the alloy exhibits a multi-scale hierarchical structure during rapid solidification. The solidified microstructure contains Fe-rich globules with DO3 ordered structure, embedded in the continuous Cu-rich matrix. The continuous copper matrix also contains nanometer sized (average diameter 12 nm) coherent particles that exhibit Ashby-Brown strain contrast. Characterization of these phases has been carried out by a combination of X-ray diffraction, electron probe microanalysis and transmission electron microscopy coupled with energy dispersive spectroscopy. This multi-scale complex copper alloy (Cu75Fe20Si5 ) has achieved a remarkable yield and ultimate tensile strength at both room temperature and elevated temperatures in comparison to other copper based alloys. The yield strength and ultimate tensile strength at room temperature are 516±17 MPa and 635±14 MPa respectively whereas yield strength and ultimate tensile strength at 6000C turn out to be 95±11 MPa and 105±12 MPa respectively. In spite of achieving good mechanical strength, this alloy suffers from deterioration of electrical and thermal conductivity due to the presence of high volume fraction of the second phase and alloying elements. The room temperature electrical resistivity of this alloy shows that it is 10 times higher than that of pure copper (alloy resistivity = 1.70E-05 Ohm-cm at 250C and pure Copper- 1.68 × 10-6 Ohm-cm at 200C ). The thermal conductivity of this alloy turns out to be 88 W/m.K at 500C and 161 W/m.K at 6000C respectively which is much smaller in comparison to pure copper ( pure copper ≈ 401 W/m.K at 50 to 6000C). Attempts have been made to overcome the lowering of the transport properties by careful alteration of alloy compositions and fine tuning the microstructure. A new alloy with composition Cu-2.5Fe-2.5Si (at %) has been synthesized in order to achieve better transport properties without significantly sacrificing the mechanical strength. In this new alloy, we have reduced the volume fraction of the second phase (Fe-rich DO3 ordered globules) by lowering the addition of the alloying elements. We have also tried to alter the Fe to Si ratio in such a way that we can retain nanometer sized coherent particles in the matrix that provides strengthening. We arrived at a Fe and Si atom ratio of 1:1. The study of this alloy is presented in chapter five titled as „Chill cast Cu95Fe2.5Si2.5 alloy: Microstructural Evolution and Properties‟. Microstructural characterization indicates that the alloy contains only the nano meter sized coherent L12 ordered particles in the copper matrix. These particles show the Ashby-Brown strain contrast and are rich in iron and silicon. The absence of the high volume fraction of DO3 ordered Fe-rich globular phase and the smaller addition of the alloying elements ensure an improvement in the transport properties. The average resistivity value of this alloy at 250C is 3.5053 × 10-6 (Ohm-cm). This value represents a dramatic improvement in electrical properties in comparison to the Cu75Fe20Si5 alloy (Cu75Fe20Si5 alloy: 1.70E-05 Ohm-cm at 250C). The result is even better when we consider the temperature dependent thermal conductivity of the Cu95Fe2.5Si2.5 alloy. The thermal conductivity of this alloy turns out to be 236 W/m.K at 500C and 313 W/m.K at 6000C respectively. Though the thermal conductivity at room temperature is lower than pure copper, the gap reduces with increasing temperature (pure copper ≈ 401 W/m.K at 50 to 6000C and Cu75Fe20Si5 alloy: 88 W/m.K at 500C and 161 W/m.K at 6000C). This trend of temperature dependent thermal conductivity has made this alloy as one of the potential candidates for high-temperature applications. In situ heating experiment using transmission electron microscope (up to 4500C) and the heat treatment analysis at 6000C confirm that these L12 ordered particles are structurally stable at high temperatures and believed to be the main contributor to high mechanical strength in the alloy through the mechanism of order hardening and coherency strengthening. Coherent nature of the interface between the ordered particles and copper matrix also promotes low interfacial energy in the alloy and thus offers resistance to coarsening at elevated temperatures. Along with the attractive transport properties, this alloy also exhibits its success of retaining mechanical strength at both ambient and high temperatures as compared to the earlier alloy. The room temperature yield strength and ultimate tensile strength of this alloy are recorded as 580±18 MPa and 690±16 MPa respectively whereas the yield strength and ultimate tensile strength at 6000C of this alloy obtained as 128±8 MPa and 150±10 MPa respectively. Thus newly modified alloy exhibits an excellent balance between mechanical strength and conductivity (electrical and thermal) and can be regarded as a promising alloy for high strength high heat flux applications. The possibilities of the Cu95Fe2.5Si2.5 alloy as a potential candidate for high strength high conductivity application has provided the motivation for further optimization of the composition of this class of alloy. Mechanical strength and transport properties of a precipitation strengthened alloy always depends on the structure, shape, volume fractions and the number densities of the precipitate particles. Electrical and thermal conductivity are also sensitive to the presence of third elements and the number densities of the precipitates in the alloy. Thus, optimization of the volume fraction and the number density of the precipitates can yield a better alloy. With this objective, we have further increased the concentration of copper while keeping the Fe and Si atom ratio fixed at 1:1. Chapter six, titled as „Chill cast Cu98Fe1.0Si1.0 alloy: Microstructural Evolution and Properties‟ describes the microstructural evolution and microstructure-property correlation of this new alloy. Characterization analysis (X-ray diffraction, electron probe microanalysis and transmission electron microscopy) confirms that the microstructure of this alloy contains similar kind of nanometer sized L12 ordered particles with lower number density as compared to Cu95Fe2.5Si2.5 alloy (Relative planar number density of the particles: Cu98Fe1.0Si1.0 = 0.13 and Cu95Fe2.5Si2.5 = 0.20). This nano sized coherently ordered particles show the similar Ashby-Brown strain contrast and are rich in iron and silicon similar to the Cu95Fe2.5Si2.5 alloy. This dilute alloy exhibits slight improvement in transport properties in comparison to the earlier Cu95Fe2.5Si2.5 alloy. The electrical resistivity of this alloy at 250C is 3.438E-6 Ohm-cm (Cu95Fe2.5Si2.5 = 3.5053 × 10-6 Ohm-cm at 250C). The thermal conductivity values of this alloy are 243 W/m.K and 338 W/m.K at 500C and 6000C respectively (Cu95Fe2.5Si2.5 = 236 W/m.K at 500C and 313 W/m.K at 6000C). This increase in transport properties is associated with further compositional dilution and the presence of lower number density of the ordered particles in the copper matrix. The mechanism of strengthening is similar to the earlier alloys. The only difference lies in the fact that this present alloy contains lower number density of the L12 ordered particles in the copper matrix. This lower number density is responsible for the loss in mechanical strength of this alloy. The room temperature yield strength and the ultimate tensile strength of this present alloy are 467±16 MPa and 558±12 MPa whereas yield strength and ultimate tensile strength at 6000C are recorded as 102±13 MPa and 110±12 MPa respectively. Though the alloy exhibits some loss in mechanical strength, the values are still attractive in comparison to other commercially available copper based alloys. Both the alloy Cu98Fe1.0Si1.0 and Cu95Fe2.5Si2.5 demonstrate an excellent balance of mechanical strength and transport properties and have the potential to become a high strength and high conductivity materials for high temperature applications. Chapter seven is entitled as „Comparison between the alloy systems‟. In this chapter, we have presented a comparison of our new alloys with other commercially available Cu-base alloys. The thesis ends with a chapter titled as “Suggestions for future work”. We have included a descriptive note for possible future extension of our current work in this chapter.

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