During solidification of non-eutectic alloys, non-isothermal phase change causes dendritic growth of solid front with liquid phase entrapped within the dendritic network producing the mushy region. Solidification causes rejection of solute at the solid-liquid interface and within the mushy zone, causing a sharp concentration gradient to build up across the mushy region. At the same time, a temperature gradient is present as a result of externally imposed boundary conditions as well as due to evolution of latent heat, giving rise to the so-called “double-diffusive” or thermo-solutal convection. Depending on the relative density of the solute being rejected in the liquid phase during solidification process, thermal and solutal buoyancy can either aid or oppose each other. Rejection of a heavier solute leads to aiding thermo-solutal convection situation whereas the rejection of lighter solute causes the thermal and solutal buoyancy to oppose each other. If the thermal and solutal buoyancies oppose each other, flow instability arises adjacent to the mush-bulk liquid interface regions. Thus, there may be a wide variety of convection situations present in the solidifying domain for different combinations of solution concentrations and externally imposed boundary conditions. The situation becomes even more complex if the solid phase movement along with the bulk flow is involved in the process, leading to multiphase convection. Detachment of solid phase from the solid/liquid interface can be caused by remelting (solutal and/or thermal) and shearing action of a convecting liquid adjacent to the interface. Depending on the drag of the bulk flow and the density of the solid phase relative to that of the bulk liquid, these detached particles can either float or sediment. The redistribution of the rejected solute by means of diffusion (at a local scale) and thermo-solutal convection (at system level length scales) causes heterogeneous orientation of mixture constituents over the solidifying domain popularly known as macro-segregation. From the point of view of manufacturing, severe form of macro-segregation or heterogeneous species distribution is an undesirable phenomenon and hence, a thorough understanding of the species redistribution by means of diffusion and convection during solidification process is very important. Most of the earlier studies on double diffusive convection during solidification involved fixed dendrites. However, the advection of solid particles during the solidification process can generate major instability in the flow pattern while modifying the solid front growth, and hence the macro-segregation pattern considerably. With this viewpoint in mind, the overall objective of the present work is to address these wide-varieties of single phase and multi phase flow situations and their effect on solid front growth and macro-segregation during directional solidification of non-eutectic binary alloys, numerically as well as experimentally. Different configurations of directional solidification processes involving double diffusive convection have been studied for two different kinds of non-eutectic solutions. While solidification of hypoeutectic solutions leads to aiding type double diffusive convection, the solidification of hyper-eutectic solutions is characterized by opposing type double diffusive convection. Solidification of hypo-eutectic solution generally involves single phase flow, while most of the hyper-eutectic solidification involves movement of solid phase (i.e. multiphase flow). As far as the modeling part is concerned, transport phenomena during solidification with multiphase convection are not common in existing literature. This work is a first attempt to develop a solidification model with multiphase flow based entirely on macroscopic parameters. As a first step, a generalized macroscopic framework has been developed for mathematical modeling of multiphase flow during solidification of binary alloy systems. The complete set of equivalent single-domain governing equations (mass, momentum, energy and species conservation) are coupled with the phase (solid and liquid) velocities. A generalized algorithm has been developed to determine solid detachment and solid advection phenomena, based on two critical parameters, namely: critical solid fraction and critical velocity. While the first of these two parameters (critical solid fraction) represents the strength of the dendritic bond, the second (critical velocity) stands for the intensity of flow to create drag force and solutal remelting at the dendrite roots. A new approach for evaluating liquid/solid fraction by using fixed grid enthalpy updating scheme, that accounts for multiphase flow and, at the same time, handles equilibrium and non equilibrium solidification mechanisms, has been proposed. The newly developed model has been validated with existing literatures as well as with experimental observations performed in the present work. The experimental results were obtained by using PIV as well as laser scattering techniques. Side cooled as well as top cooled configurations are studied. Single phase convection is observed for the case of hypo-eutectic solution, whereas hyper-eutectic solutions involve convection with movement of solid phase. For the case of bottom cooled hyper-eutectic solution, finger-like convection leading to freckle formation is observed. For all the hyper-eutectic cases, solid phase movement is found to alter the convection pattern and final macrosegregation significantly. The numerical results are compared with experimental observations both qualitatively as well as quantitatively.

Este trabalho apresenta o estudo termogravimétrico de determinação da composição de ligas metálicas binárias de prata e de cobre, especificamente ligas Ag:Cu, Cu:Zn e Cu:Sn. Foram testadas duas metodologias distintas de análise. Na primeira, a liga foi dissolvida em HNO3 e, depois, submetida à termogravimetria (Método Indireto). Na outra, os ensaios termogravimétricos foram realizados com o material em pó sem ser submetido a um tratamento químico prévio (Método Direto). Foram feitos testes com amostras preparadas em laboratório e com amostras comerciais. Utilizando a conhecida técnica de determinação gravimétrica automática de Duval, vários testes foram efetuados procurando-se usar esta metodologia para ligas Ag:Cu com teores elevados (acima de 30%) de cobre. Os métodos instrumentais de análise, bem como os procedimentos de preparação de amostras e de amostragem, foram adaptados às condições instrumentais atuais. Vários programas de aquecimento foram testados até o estabelecimento das condições instrumentais mais adequadas. Metodologia idêntica foi utilizada para analisar latão (Cu:Zn). Uma amostra de bronze (Cu90:Sn1 0) foi analisada pelo \"Método Direto\". A metodologia usada possibilitou a determinação da composição sem necessidade de dissolução da liga. Inicialmente, a amostra foi submetida a um aquecimento controlado (10ºC min-1 ) em atmosfera dinâmica de ar comprimido até 1100ºC, realizando-se, em seguida, a troca do ar comprimido por N2, para favorecer a redução do CuO a Cu2O e impedir sua reoxidação. Como o outro óxido formado (SnO2) na oxidação da liga é termicamente estável, foi possível determinar nela a porcentagem de cobre. / This work reports the thermogravimetric study of determination of the composition of binaries silver and copper alloys, specific for, Ag:Cu, Cu:Zn and Cu:Sn alloys. Two different were tested ways of analysis. In one of them, the alloy was attached by HNO3 solution before the thermogravimetric analysis (Indirect Method). In the another method, the thermogravimetry was realized without a chemical treatment of the alloy (Direct Method). Experiments were made with specimens prepared in laboratory and commercial alloys. Using the well known Duval\'s automatic gravimetric determination for binaries systems, several tests were realized to use this method for copper rich silver alloys (above 30% in copper). The instrumental methods of analysis and the proceedings used to prepare the samples were adapted to the new instrumentation used in thermal analysis. Several heating rates were tested till the best heating conditions could be found. The same method was used to make the analysis of brass (Cu:Zn). It was made the determination of the composition of a 10% tin-bronze (Cu:Sn). No chemical pre-treatment was used. The sample was heated in air with a heating rate of 10ºC per minute till 1100ºC, then it was made the change of the furnace gas to N2. This change helps the reduction of the CuO and prevents the re-oxidation of the Cu2O formed. The other oxide formed (SnO2) by alloy oxidation is thermal stable, then it is possible the determination of the percentage of copper in this alloy.

L’aluminium et ses alliages sont très largement exploités en industrie car ils sont légers, peu sensibles à la corrosion et parfois ont une dureté comparable à celle de l’acier. Souvent, le matériau est utilisé brut de fonderie, dans ce cas, la composition, la macro et microstructure de matériau issues de la solidification, définissent son comportement suivant les conditions d’utilisation. Cependant, au cours des procédés d’élaboration sur terre, l’écoulement convectif apparaît à cause de la gravité et modifie les conditions locales de la solidification, la distribution du soluté et influe sur les propriétés de matériau. Afin d’analyser et contrôler ces phénomènes, des études expérimentales et numériques ont été réalisés.Deux fours de type Brdigman ont été construits à l’Université de Miskolc par MTA-ME Materials Science Research Group dans le cadre du ESA-MAP MICAST pour étudier l’effet de l’écoulement convectif lors de la solidification des alliages. Ces fours ont été équipés de systèmes électromagnétiques permettant de générer des champs magnétiques rotatif et glissant .Des modèles multiphases développés à SIMAP/EPM, Grenoble, France, ont été utilisés pour la simulation numérique de la solidification d’alliages binaire et ternaire en présence d’un écoulement convectif créé par des forces électromagnétiques. La modélisation de la solidification d’un alliage Al-Si avec un brassage rotatif a été réalisée pour des configurations 2D et 3D avec un couplage entre un modèle macroscopique de transport et des modèles à l’échelle mésoscopique, l’un basé sur la règle de levier et l’autre sur une moyenne d’ensemble Euler-Euler. De plus, les effets de deux modes de brassage par champs glissants sur la solidification d’un alliage ternaire ont été étudiés dans une géométrie 3D à l’aide d’un modèle macroscopique basé sur la règle de levier. Les résultats des modélisations numériques permettent d’expliquer la ségrégation observée sur les échantillons expérimentaux. / Aluminium and aluminium-based alloys are widely in industry due to the corrosion passivity, lighter weight, yet – in several cases – comparable strength with steel. Often, the material is used “as-cast“, that means that composition, macro- and microstructure of the material emerged during the casting defines its behaviour under different loads. Yet, convective flows generally arise in casting processes performed on-ground because of gravity and modify local solidification conditions, and, consequently, solute distribution and affect properties of material. To understand and to be able to control such phenomena, detailed experimental and numerical work has been needed.Two Bridgman-type furnaces were constructed in the University of Miskolc, Hungary, by MTA-ME Materials Science Research Group in the framework of the ESA funded MICAST project for experimental study of the effect of convective flow in solidification of alloys. These facilities were equipped with electromagnetic systems capable to generate rotating and travelling magnetic fields of various intensities.Multiphase models developed at SIMaP/EPM, Grenoble, France, were applied for numerical study of the solidification of binary and ternary aluminium alloys under electromagnetically generated convective flow. Solidification of a binary Al-Si alloy under RMF stirring was done with Euler-Euler ensemble averaging and lever rule mesoscale models coupled with the macroscale transport both in 2D and 3D geometries. Further, effect of various modes of TMF stirring during solidification of a ternary alloys was studied in 3D geometry with lever rule based macroscopic model. Results of numerical simulations well explain the segregation observed in the experimental samples.

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M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology. / This work is an investigation in the development and characterisation of porous Ti-Sn-SiC and Ti-Nb-SiC composites. Pure Titanium (Ti), Tin (Sn), Niobium (Nb) and Silicon carbide (SiC) powders were used as starting materials. The Ti-Sn-SiC and Ti-Nb-SiC composites were produced by powder metallurgy (PM) press-and-sinter route. The Sn is an α-phase stabilizer while Nb is a β-phase stabilizer in Ti alloys. A systematic study of binary Ti-Sn and Ti-Nb alloys was conducted with the addition of SiC particles. The addition of Sn influences the microstructure of the titanium alloy. With increasing the percentage of Sn content, the density of the samples decreases on the Ti-Sn alloys. An increase in the Sn content from 10 to 25 wt. % content resulted in decreased hardness. The Ti-Sn binary revealed stability of the HCP phase with increasing composition of the Sn content. The porous structures of the Ti-Sn-SiC composites were evenly distributed throughout the materials. The sintered densities increase from 94.69% to 96.38%. XRD analysis detected the HCP crystal lattice structure for the Ti5.4Sn3.8SiC and Ti5.6-Sn3.8-SiC composites. XRD pattern of the Ti5.8-Sn3.8-SiC reveals both the HCP and FCC crystal structures. The HCP phase has lattice parameters a= 2.920 Å; c=4.620 Å with smaller c/a ratio of 1.589. Additionally, FCC lattice parameter a=5.620 Å Fm-3m # 225 was obtained both for Ti5.8Sn3.8SiC and Ti6.0Sn3.8SiC XRD patterns. On the other hand, Optical microscopy analysis of the Ti-Nb alloys revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. HCP and BCC phases have the lattice parameters a = 2.951 Å, c = 4.683 Å and 3.268 Å, respectively. An HCP (α′) phase was detected in the Ti8.5Nb1.5 alloy with lattice parameters a = 5.130 Å, c = 9.840 Å while a BCC phase had a = 3.287 Å. The sintered Ti8Nb2 alloy also had the α′-phase with a = 5.141 Å, c = 9.533 Å and BCC phase with a = 3.280 Å lattice parameters. On the contrary, the Ti7.5Nb2.5 alloy formed the α′-phase of a = 5.141 Å, c = 9.533 Å and BCC with a = 3.280 Å lattice parameters. For the 10 and 15 wt.% Nb alloys, very porous structures were observed. The pores appear spherical and widely distributed. As the Nb content is increased to 20 wt.% (Ti7Nb2SiC) and 25 wt.% (Ti7Nb2.5SiC), porosity was minimized. The sintered densities of the Ti-Sn alloys are decreasing from 95.90% to 92.80% with increased amount of Sn in the Ti, while the sintered densities of Ti-Sn-SiC are increasing from 94.69% to 96.38%. The high porosity, which developed in Ti7Nb1SiC and Ti7Nb2.5SiC, affected the densities of these composites. The sintered densities of Ti-Nb alloys are increasing from 92.08% to 97.65% with increased amount of Nb in the Ti. In terms of hardness Ti7Nb1SiC and Ti7Nb2.5SiC resulted in the lowest while Ti7Nb1.5SiC and Ti7Nb2SiC composites were 511.74 HV and 527.678 HV. The porosity levels were increased by the addition of SiC in the Ti-Sn-SiC and Ti-Nb-SiC composites. The XRD analysis revealed phase transformation on the Ti-Nb alloys and Ti-Nb-SiC composites.

No / Alpha phase sintering, sinter hardening, and mechanical properties of prealloyed Fe-1.5Mo base powder with and without additions of elemental Si, ferrosilicon, and carbon under various process conditions have been investigated. Liquid paraffin, as a new lubricating agent, was found to be useful in reducing segregation, interparticle and die wall frictions, as well as reducing ejection forces and die and tool wear. It was found that addition of Si to the base powder enhanced the sintering process by stabilisation of the ¿-phase and formation of two kinds of liquid phase at ~1045 and ~1180°C, corresponding to the solidus and liquidus temperatures, respectively. This addition increased the tensile strength of the as sintered Fe-1.5Mo from 174 to 445 MPa owing to massive solid solution strengthening effect of Si. An optimum sinter hardenable alloy, of composition Fe-1.5Mo + 3Si + 1.2C, provided a high sintered density of 7.55 g cm-3, tensile and bend strengths of 764 and 1405 MPa, respectively, with 2.5% elongation, after sintering at 1250°C for 1 h under hydrogen or vacuum using moderate cooling rates of ¿ 20 K min-1. Faster cooling rates caused brittleness and very low UTS for the high carbon steel. Full heat treatment improved the UTS by 200 MPa which was useful only for the high carbon steel with high cooling rates ¿ 30 K min-1. Depending on the cooling rate, the as sintered microstructures consisted of mainly fine or coarse pearlite, bainite, martensite, and some retained austenite with hardness in the range 250-720 HV10. Some proeutectoid grain boundary cementites were also present in the as sintered high carbon steel. This work, therefore, has shown that high densities with acceptable microstructures and good mechanical properties are achievable with single stage compaction and single sintering operations by using the optimum process conditions and alloying composition without the need for a post-sintering heat treatment.

The primary focus of the present work is the development of macro-models for numerical simulation of binary alloy solidification processes, consistent with microscopic phase-change considerations, with a particular emphasis on capturing the effects of non-equilibrium species redistribution on overall macrosegregation behaviour. As a first step, a generalised macroscopic framework is developed for mathematical modelling of the process. The complete set of equivalent single-phase governing equations (mass, momentum, energy and species conservation) are solved following a pressure-based Finite Volume Method according to the SIMPLER algorithm. An algorithm is also developed for the prescription of the coupling between temperature and the melt-fraction. Based on the above unified approach of solidification modelling, a macroscopic numerical model is devised that is capable of capturing the interaction between the double-diffusive convective field and a localised fluid flow on account of solutal undercooling during non-equilibrium solidification of binary alloys. Numerical simulations are performed for the case of two-dimensional transient solidification of Pb-Sn alloys, and the simulation results are also compared with the corresponding experimental results quoted in the literature. It is observed that non-equilibrium effects on account of solutal undercooling result in an enhanced macrosegregation. Next, the model is extended to capture the effects of dendritic arm coarsening on the macroscopic transport phenomena occurring during a binary alloy solidification process. The numerical results are first tested against experimental results quoted in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom-cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation can be predicted as compared to that dictated by shrinkage-induced fluid flow alone. For an order-of-magnitude assessment of predictions from the numerical models, a systematic approach is subsequently developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. A characteristic velocity scale inside the mushy region is derived, in terms of the morphological parameters of the two-phase region. A subsequent analysis of the energy equation results in an estimation of the solid layer thickness. It is also shown from scaling principles that non-equilibrium effects result in an enhanced macro-segregation compared to the case of an equilibrium model For the sake of assessment of the scaling analysis, the predictions are validated against computational results corresponding to the simulation of a full set of governing equations, thus confirming the trends suggested by the scale analysis. In order to analytically investigate certain limiting cases of unidirectional alloy solidification, a fully analytical solution technique is established for the solution of unidirectional, conduction-dominated, alloy solidification problems. The results are tested for the problem of solidification of an ammonium chloride-water solution, and are compared with those from existing analytical models as well as with the corresponding results from a fully numerical simulation. The effects of different microscopic models on solidification behaviour are illustrated, and transients in temperature and heat flux distribution are also analysed. An excellent agreement between the present solutions and results from the computational simulation can be observed. The generalised numerical model is subsequently utilised to investigate the effects of laminar double-diffusive Rayleigh-Benard convection on directional solidification of binary fluids, when cooled and solidified from the top. A series of experiments is also performed with ammonium chloride-water solutions of hypoeutectic and hypereutectic composition, so as to facilitate comparisons with numerical predictions. While excellent agreements can be obtained for the first case, the second case results in a peculiar situation, where crystals nucleated on the inner roof of the cavity start descending through the bulk fluid, and finally settle down at the bottom of the cavity in the form of a sedimented solid layer. An eutectic solidification front subsequently progresses from the top surface vertically downwards, and eventually meets the heap of solid crystals collected on the floor of the cavity. However, comparison of experimental observations with corresponding numerical results from the present model is not possible under this situation, since the associated transport process involves a complex combination of a number of closely interconnected physical mechanisms, many of which are yet to be resolved. Subsequent to the development of the mathematical model and experimental arrangements for macroscopic transport processes during an alloy solidification process, some of the important modes of double-diffusive instability are analytically investigated, as a binary alloy of any specified initial composition is directionally solidified from the top. By employing a close-formed solution technique, the critical liquid layer heights corresponding to the onset of direct mode of instability are identified, corresponding two a binary alloy with three different initial compositions. In order to simulate turbulent transport during non-equilibrium solidification processes of binary alloys, a modified k-8 model is subsequently developed. Particular emphasis is given for appropriate modelling of turbulence parameters, so that the model merges with single-phase turbulence closure equations in the pure liquid region in a smooth manner. Laboratory experiments are performed using an ammonium chloride-water solution that is solidified by cooling from the top of a rectangular cavity. A good agreement between numerical and experimental results is observed. Finally, in order to study the effects of three-dimensionality in fluid flow on overall macrosegregation behaviour, the interaction between double-diffusive convection and non-equilibrium solidification of a binary mixture in a cubic enclosure (cooled from a side) is numerically investigated using a three-dimensional transient mathematical model. Investigations are carried out for two separate model systems, one corresponding to a typical metal-ally analogue system and other corresponding to an actual metal-alloy system. As a result of three-dimensional convective flow-patterns, a significant solute macrosegregation is observed in the transverse sections of the cavity, which cannot be captured by two-dimensional simulations.

The present thesis deals with Nb-Si alloy composites in both bulk and multilayer forms. The work has been divided into two parts. First part (chapter 4-6) deals with Nb based silicides binary and ternary alloys with alloying additions like Ga and Al. These alloys are synthesized by vacuum arc melting and suction casting (non-equilibrium processing techniques). The studies on intermetallic coatings of Nb-Si alloys and Nb/Si multilayer synthesized by pulsed laser deposition technique have been presented in the second part (chapter7-8). Nb-Si alloys are one of the candidate materials for the advanced structural and microelectronic applications. There are few issues with these materials like poor oxidation resistance, low fracture toughness and brittleness which need to be solved. Microstructure plays a crucial role to control these properties. The main focus of this work is to understand the process of phase transformation and thereby control the microstructure in both bulk alloys and thin films. We have also investigated in a limited manner mechanical and environmental properties of bulk alloys. This thesis is subdivided into nine chapters. After a brief introduction in the first chapter, a brief overview on Nb-Si phase diagram and literature reviews on Nb-Si based alloys are presented with emphasis on the current work in the second chapter. Literature reviews on the phase formations sequence and stability in Nb-Si alloys thin films and Nb/Si multilayers are also discussed in the same chapter. In the third chapter different experimental techniques, processing parameters and characterization tools like XRD, SEM, TEM etc. are briefly discussed. Special emphasis is given on two non-equilibrium techniques: laser deposition technique to deposit the thin film/multilayer and vacuum suction casting to produce the 3 mm diameter rods of different Nb-Si alloys. The fourth chapter discusses the microstructural aspects of Nb-Si alloys prepared by suction casting and its mechanical behavior. The samples have the compositions hypoeutectic (Nb-10at.%Si and Nb-14at.%Si), eutectic (Nb-18.7at.%Si) and hypereutectic (Nb-22at.% Si and Nb-25at.% Si). SEM microstructural analyses of all the samples clearly show the enhancement in the volume fraction of eutectic and decease in the eutectic spacings in microstructure due to large undercooling. Rod eutectic is observed in most of places with irregular eutectic a few places in all samples. First check of phases has been done by XRD in all samples. Phase confirmation using TEM showed the eutectic between Nbss and Nb3Si phases in all samples. The primary phase for hypoeutectic alloys is Nbss (dendritic structure), Nb3Si phase for eutectic composition and β-Nb5Si3 phase for hypereutectic alloys. Compositional analysis using EDS and EPMA also supported the above results. No signature of eutectoid reaction (Nb3Si→Nb+α-Nb5Si3) is observed. Mechanical properties like hardness, strength, ductility and indentation fracture toughness have been determined for above mention alloy compositions. SEM micrographs showed that silicides fractured by cleavage and Nb phase in a ductile manner during the compression tests carried out at room temperature. We attempt to explain how the above mention mechanical properties change with alloy compositions and processing. Chapter five deals with the effect of Ga addition on the microstructure and mechanical properties of the Nb-Si alloy. The composition selected for this study is Nb-20.2at.%Si-2.7at.%Ga. The results of ternary alloy have been compared with the binary alloy composition Nb-18.7at.%Si. Phase analysis has been carried out using TEM and XRD. Ga addition has suppressed the formation of Nb3Si phase and promoted the formation of β-Nb5Si3 phase. Ga addition also established the eutectic between Nbss and β-Nb5Si3, which is a metastable eutectic. Ga added ternary alloy, on suction casting, yields ultrafine eutectic with nanometer length scale (50-100nm). From the compression tests, it is concluded that the combination of ultrafine eutectic (Nbss-β-Nb5Si3) and primary β-Nb5Si3 in ternary alloy results in a high compressive strength ~2.8±0.1 GPa with 4.3% plasticity. In contrast binary alloy under identical conditions shows the compressive strength ~1.35±0.1 GPa and 0.2% plasticity. Ga addition also enhances the indentation fracture toughness from 9.2±0.05 MPa√m (binary) to 24.11±0.5 MPa√m (ternary). Composite hardness values of the ternary and binary alloys are 1064±20 Hv and 1031±20 Hv respectively. Chapter six deals with Al added Nb-Si ternary alloy. Here we have discussed microstructural and mechanical properties like in chapter 5 along with oxidation behavior for the alloy composition Nb-12.7at.%Si-9at.%Al. SEM micrograph shows the presence of primary dendrites structure with ultra fine lamellar eutectic (50-100nm). Detailed TEM studies confirm the Nbss as primary phase present in form of dendrites. These dendrites contain the plate shape precipitates of δ-Nb11Si4 (body centered orthorhombic structure) phase in Nb matrix (primary dendrites). Eutectic phases are Nbss and β-Nb5Si3. The analysis of the results indicates that Al addition promote the formation of β-Nb5Si3 phase in the eutectic. The results of this ternary composition were also compared with the binary alloy composition Nb-18.7at.%Si. Compression tests have been carried out at room and elevated temperatures to measure the strength of the material. Al added ternary alloy yields the compressive strength value 1.6±0.01 GPa whereas binary alloy yields the compressive strength value 1.1±0.01 GPa. Enhancement in indentation fractured toughness is observed in Al added ternary alloy (20.4±0.5MPa√m) compare to binary alloy (9.2±0.05 MPa√m). Thermal analysis by TGA and DTA were used to see the oxidation behavior of Al added ternary alloy. Chapter seven deals with the deposition characteristics and the TEM studies on the laser deposited Nb-Si thin films. Films were deposited on the NaCl crystals and Si single crystal substrates. The compositions chosen in this case are Nb-25at.%Si, Nb-37.5at.%Si and Nb-66.7at.%Si. These compositions correspond to the equilibrium intermetallic compounds Nb3Si, Nb5Si3 and NbSi2 respectively. In this chapter we have briefly discussed the microstructural and phase evolutions in the intermetallic coatings. The smooth films quenched from the vapor and/or plasma state show amorphous structure. The sequence of crystallization was studied by hot stage TEM experiments as well as by cross sectional TEM in the films deposited at the elevated temperatures (600oC and 700oC) on Si substrates. During the hot stage experiment, crystallization is observed in Nb-25at.%Si film around 850oC with nucleation of metastable cubic Nb3Si phase. Occasionally metastable hexagonal Nb3Si3 phase has also been observed (close to Si substrate) along with cubic Nb3Si phase in the films at elevated temperatures. For Nb-37.5at.%Si film, crystallization is observed at 800oC with the nucleation of grains of metastable hexagonal Nb5Si3 phase. Cross-sectional TEM shows the presence of hexagonal Nb5Si3 phase along with few grains of NbSi3 (equilibrium) phase in the films deposited at elevated temperatures. Hot stage experiment of Nb-66.3at.%Si film showed the onset of crystallization much earlier at 400oC and complete crystallization at 600oC. This crystallization leads to the nucleation of grains of NbSi2 phase. Films of this composition deposited at elevated temperatures showed the presence of NbSi2 and metastable hexagonal Nb5Si3 phases (occasionally). The laser ablated films, besides the film matrix also contain the micron and submicron sized spherical droplets of different sizes. These droplets travel at very high velocities and impinge on the substrate resulting in a very high rate of heat transfer during solidification from liquid state. Therefore in this work we have also studied the microstructural evolution in the droplets for each composition. The phases observed in the droplets embedded in the matrix of Nb-25 at% Si alloy film are the bcc Nb and the cubic Nb3Si (metastable phase). The droplets in the matrix of Nb-37.5 at% Si alloy showed the bcc Nb and tetragonal β-Nb5Si3 phases. The phases observed in the droplets of in the Nb-66.3at.%Si alloy are the bcc Nb, tetragonal β-Nb5Si3 and the hexagonal NbSi2 (metastable phase). Chapter eight describes the synthesis and microstructural characterization using TEM of Nb/Si multilayers. The aim of this work is to check the stability and phase formation sequence in Nb/Si multilayer. Nb/Si multilayers were first annealed at different time intervals at 600oC and at different temperatures (for 2 hours) and then characterized by the cross-sectional transmission electron microscopy. As-deposited Nb layer is crystalline while Si layer is amorphous. Microstructural and compositional evidences suggest the intermixing between the Nb and Si layers at the interfaces. Nb/Si multilayer annealed at 600oC for 1 hour, NbSi2 was identified as the first crystalline nucleating phase. However amorphous silicide layers were also observed between Nb and NbSi2 layers. Metastable hexagonal Nb5Si3 was identified as the next crystalline phase that nucleated from the amorphous silicide layers at the interfaces of Nb and NbSi2 layers. Occasionally few grains of cubic Nb3Si phase were also observed after 8 hours of annealing at 600oC. In the chapter we have compared the results to the other reported works in Nb-Si bulk diffusion couples and also thin film couples. The final chapter summarizes the major conclusions of the present work and scope of future work.

No description available.