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A large number of advanced structural materials are based on metallic materials where alloying additions play a key role in imparting the required properties. Most of the commercially important aluminium alloys are classified by the nature of the alloying additions. Among them the 2219, 2618, 5086, and 7075 are important class of lightweight alloys that plays critical role in modern engineering application. However, despite having a series of commercially useful aluminum alloys for commercial applications the increasing need of improved performance requires newer development in particular for applications that require high strength at elevated temperatures and performance at extreme environments. Precipitations of the intermetallic compounds containing copper during thermal treatments play a very important role in developing high strength aluminium alloys. Although,these precipitates are stable at fairly high temperatures, the rapid coarsening of these second phase precipitates (e.g. Al2Cu), leads to loss of strength at elevated temperature. Several approaches are explored to overcome this problem. One of them is to utilize non-equilibrium solidification route, which can increase solid solubility and hence increasing the precipitate density. Nonequilibrium processing can also alter the selection pathway of the competitive phases and evolution of the microstructure. Recently, non–equilibrium solidification by suction casting technique is becoming increasingly popular for casting of metallic materials of any shape. In this technique solidification is effected by sucking the molten alloy into water cooled copper mold using a suction force resulting from the differences between the melting chamber in Argon gas pressure and casting chamber under vacuum. The present thesis aims to develop a set of newer alloys with small amount of alloying additions primarily based on nickel that can retain reasonable strength at high temperature by utilizing the non-equilibrium solidification route. In addition to Ni (≤ 0.10at.%), the thesis present results of the effect of minor addition of Sc and Zr as ternary and quaternary additions. Following a short review in chapter 2, Chapter 3 presents the experimental techniques adopted for both preparation of alloys and their characterization. Chapter 4 deals with the results of alloying of aluminum with minor amount of nickel. The Ni in the range of 0.05-0.20at% was used to develop a high temperature template, containing a set of hardening intermetallic compounds to increase the strength of the host matrix. The microstructural investigations of the suction cast alloys reveal a characteristic feathery microstructure. At higher magnification the microstructure reveals the presence of fine dispersions of a second phase. Both x-ray and transmission electron microscopy confirms the phase in the dispersions to be primarily crystalline Al9Ni2 phase having a monoclinic crystal structure. This phase does not exist in equilibrium phase diagram. Only at higher concentration one can observe equilibrium Al3Ni (Orthorhombic) particles. The size of the particle ranges from 50-200nm. Beyond~0.5at%Ni, the microstructure changes to normal cellular type solidification morphology with interdendritic space decorated by the eutectic network of Al-Al3Ni having a rod eutectic morphology. A careful observation of alloys with small amount of Ni reveals that the feathery structure is associated with the thin cells, which have grown by continuously splitting the tip yielding a fractal like dendritic morphology. The dispersoids form at the intercellular regions. We have presented clear evidence of their origin from the interdendritic liquid, which most likely underwent Rayleigh instability. The random distribution reflects the nature of the dendritic growth. We have argued that these inter-dendritic liquid droplets, which are enriched with Ni, get undercooled. The metastable Al9Ni2 phase nucleates and grows in this liquid. In order to confirm this scenario, we have carried out a phase field simulation for dendritic growth of aluminium solid solution in the alloy melt both under the condition of constraint growth and free growth. The observed distribution of the dispersoid is well reflected in the phase field simulation. The chapter also report the response of effect of direct ageing of suction cast alloy as one expect an extension solid solubility of Ni in Al. A small increase in hardness could be observed during ageing treatment. In order to determine the thermal stability of the intermetallic particles, the samples of the suction cast alloys were exposed at 200°C for 200h and 500°C for 100h respectively. No change in the microstructure could be observed excepting a slight coarsening indicating the dispersed particles are thermally stable. After exposure at two different temperatures the maximum retained hardness was measured to be 350MPa. We have also attempted to correlate the hardness with coarsening behavior of particles. The feathery morphology of the cast structures and fine dispersion of the intermetallic phase is expected to improve the tensile strength of the alloy. The tensile yield strength of cast alloys was determined to be 150MPa ± 20 for Al- 0.09at%Ni alloy. We have tried to estimate the expected strength of the alloy from quantitative microstructural parameters using possible hardening mechanism. The estimates are in good agreement to the observed values. The chapter 5 reports attempts to develop thermally stable precipitation strengthened aluminum alloys by retaining the dispersion template developed earlier alloyed with Ni. Then, the binary alloys were added with extremely low diffusivity element Zr. The element Zr is traditionally added in the aluminium alloys as grain refiner and as a powerful agent for inhibiting recrystallization especially for high strength aluminium alloys. However, in this work we have alloyed Zr for imparting precipitation hardening. An amount of 0.15at%Zr was added to the suction cast alloys of Al-0.05, 0.09 and 0.20at%Ni. The first two alloys exhibit the formation of metastable phase Al9Ni2 during solidification stage. Increase the concentration of the alloy to Al-0.20at% Ni with 0.15at%Zr additions exhibits combination of both stable Al3Ni and Al9Ni2 metastable phases. Microstructures of these alloys show columnar cells of ~200μm with dispersions of spherical nodules of Al9Ni2 and Al3Ni with varying size ranges from 200-500nm. Particle size distribution of Zr containing aluminium alloys with 0.05at% Ni is 595nm ± 20 while the alloy having the 0.09 at% Ni has the optimum size of 290nm. Further increase of Zr composition above 0.20at % led to columnar to equiaxed transition. The as cast alloys containing Zr does not show the improvement with limited yield strength of the order of 150MPa. The equivalent hardness of the samples has been measured to be about 370-420MPa. Heat-treated alloys however show the presence of Al3Zr (L12) precipitates with ~20nm size that are coherent with the matrix. Binary suction cast Al-0.15at%Zr alloy after ageing exhibits tensile yield strength of ~200MPa. With ternary aluminium alloy with minor additions Ni and Zr, The strength increases to ~300MPa. Additionally, the alloy continue retain a maximum hardness of 870-920MPa even after long hours of aging. The Zr containing alloys were proved to be stable. When the tests were carried out on a nominally alloyed sample of Al-0.09at%Ni-0.15at%Zr peak aged and exposed to 250°C for 200h, the yield strength under compression tests was found to be 280MPa. The chapter 6 of the thesis discusses the role of Sc with the ternary Al-alloys with Ni and Zr. Addition of small quantities 0.1 and 0.2at%Sc substantially reduces the inter-particle distance of precipitates by increasing volume fraction and number of nano-sized particles. It has been observed and presented in this thesis that the Sc addition provides the highest incremental strengthening per atom percent of any alloying element. Chill-suction cast samples show equiaxed cells in the samples with dispersions of particles inside and some segregated particles at the cell boundaries. To achieve a further increase in the number density of precipitates we processed the suction cast alloys with additional heat treatment at 375 and 450°C. All the suction cast alloys with varying Ni content and keeping the Sc and Zr constant at 0.10 and 0.15at% respectively exhibit formation of Al9Ni2 phase. The alloy Al-0.20at%Ni-0.10at%Sc-0.15at%Zr also contain stable phase of Al3Ni with an eutectic morphology. The DSC experiments in the dynamic mode with heating rate of 10°C min-1 exhibit two distinct exothermic peaks due to precipitates from solution at 375 and 450°C. The TEM analysis using STEMEDX has further confirmed the existence of nano-sized particles 30-50 nm of both phases of Al3Sc and Al3 (Sc, Zr). The tensile yield strength of the as cast alloy show 200MPa while after precipitation treatment, we observe improved yield strength 350-450MPa. Thermal stability of the alloys were tested after peak aged condition and exposed to 200°C for 250h. The results show that the yield strength is unaffected implying the coarsening resistance of the precipitate particles. Overall the thesis establishes that with minimum alloying additions, it is possible to design alloys that are expected to perform for high temperature applications by the formation of set of dispersions of Al9Ni2 (monoclinic) and precipitates of ordered cubic phases of (L12) structure of Al3Zr, Al3Sc and Al3 (Sc, Zr) with required number density of particles.

Zr cladding alloys have been used for many years as the first safety barrier layer of a nuclear reactor. However, the recent Fukushima accidents and industrial demands to increase the burnup of fuels have led to increasing interest in a detailed mechanistic understanding of aqueous corrosion and hydrogen pickup and the performance at high temperatures. As part of an international MUZIC-2 programme (Mechanistic Understanding of Zr Corrosion and Hydrogen pickup), I have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a series of zirconium alloys at different stages of corrosion and hydrogen pickup. Samples from both autoclave and in-reactor conditions were available to compare, I have focussed on RXA (recrystallised 580°C) Zr-1.0Nb and annealed (720°C) Zr-1.0Nb alloys. A set of samples from different exposures times were chosen to represent early, pre-transition and post-transition samples in order to compare the microstructure and microchemistry of the oxides, the metal-oxide interface and the metal. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM (ASTAR mapping) were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave with different corrosion rates, with more uneven metal-oxide interface, more parallel cracks and less organised oxide grains in the fast corroding samples. Comparing with autoclave samples, the in-reactor samples have shorter, less well-aligned monoclinic grains and more tetragonal grains. The rapidly oxidising annealed Zr-1.0Nb alloy also have much higher tetragonal grain fraction comparing with the slow corrosion rate RXA Zr-1.0Nb alloys. Porosity in the oxide is predicted to have a major influence on the overall rate of corrosion and hydrogen pickup, and there is much more porosity in the annealed Zr-1.0Nb alloy than found in either the RXA alloy or the similar alloy exposed to neutron irradiation. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Electron Energy Loss Spectroscopy (EELS) analysis of second phase particles can reveal the main and the minor element distributions respectively. The annealed Zr-1.0Nb alloys have Î2-Zr SPPs with nano crystalline structure and much larger size. Although they does not relate with the higher density of cracks in the oxide, the large SPP size can connect together all the small cracks that are generated by the huge amount of tetragonal to monoclinic phase transformation during corrosion and provides pathway for corrosion and hydrogen pickup. Two kinds of SPPs are found in the RXA Zr-1.0Nb alloys, one is Î2-Nb and another one is Zr-Nb-Fe Laves phase. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of Î2-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in Î2-Nb SPPs in the oxide revealed the fully oxidised Nb⁵⁺ state in the SPPs deep into the oxide, but Nb²⁺ in the crystalline SPPs near the metal-oxide interface. EELS, TKD and ASTAR mapping have also revealed the presence of suboxide layers with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen- rich zone combining with the part where porosity is not interconnected is the protective oxide that is rate limiting in the key in the transport processes involved in corrosion and hydrogen pickup.

No description available.

The thesis introduces a novel alloy system based on submicron distributions of intermetallic phases realised through eutectic solidification in the ternary system Ni-Al-Zr. Various compositions in this system comprising of intermetallic phases distributed in different eutectic structures show ultra-high strength at temperatures upto 700°C combined with reasonable tensile plasticity, exceptional oxidation resistance and high temperature structural stability. Intermetallics have long been used in high temperature alloys systems such as in the classical Ni-base superalloys that derive their strength from nanoscale dispersions of the aluminide, Ni3Al(γ’) in a matrix of disordered fcc Ni (γ), alloyed with expensive, high density refractory elements such as Re and Ru. The high temperature applications of intermetallics derive from their strength retention to high temperatures, creep resistance enabled by low diffusion rates, and attractive oxidation resistance based on high concentration of elements such as Al that forms stable oxides. Several decades of effort on the development of new generation of intermetallic alloys through the 80’s and 90’s have gone unrewarded, with the exception of TiAl based alloys that are now used in recent generation aircraft engines. The promise of intermetallics as high temperature candidate materials is limited by their poor ductility or toughness arising from several intrinsic properties such as low grain boundary cohesive strength (in the case of Ni3Al) or an insufficient number of slip systems (as in NiAl) or extrinsic effects such as embrittlement by hydrogen (Fe3Al) that derive fundamentally from the existence of directionality in bonding. However, low ductility or toughness can often be alleviated by limiting the length scale for slip. We have therefore examined the possibility of combining intermetallics in the form of eutectic structures, potentially limiting slip lengths within each intermetallic constituent. Eutectic structures in binary systems limit the choice of intermetallic combinations so that finding such combinations with engineering potential is difficult. On the other hand combinations of three elements or more would enable a significantly larger set of permutations of eutectic intermetallics, provided the constituent binary phase diagrams contain either eutectic or peritectic reactions involving intermetallic phases, as well as intermediate intermetallic phases. The ternary Ni-Al-Zr system met our criterion in several ways. The Ni-Al binary phase diagram shows a peritectic reaction from liquid and NiAl (Pm 3m, B2 with a lattice parameter of 0.288nm) to form Ni3Al (Pm 3m, L12 with a lattice parameter of 0.356 nm), intermetallics that have been extensively investigated in earlier literature. The Ni-Zr system shows a peritectic reaction between liquid and the Ni7Zr2 (C12/m1 with a lattice parameters a=0.469nm, b=0.823nm, c=1.219nm) phase to form the intermetallic Ni5Zr (F 43m with a lattice parameter of 0.670nm). Further the NiAl and Ni7Zr2 are both intermediate phases and should therefore form a mono-variant eutectic on the composition line joining these two phases in the ternary system. We note that Zr participates in many glass forming systems. In the Ni-Zr system, for example, glass forming ability has been associated with the structure of the liquid phase and associated low diffusivity. As a consequence, a fine scale eutectic structure may be expected. Zr has also been reported to strengthen and ductilise Ni3Al and NiAl. Finally, both Al and Zr form stable oxides and might promote oxidation resistance. After introducing the thesis in Chapter 1, the experimental details are outlined in the Chapter 2. The experimental results and subsequent discussions are presented in three subsequent chapters. Chapter 3 reports the microstructural aspects of as cast alloys in this ternary system Previous literature and our analysis of phase equilibria in the Ni-Al-Zr system based on Thermo-Calc, suggested that solidification from the liquid to form the Ni3Al + Ni5Zr, Ni3Al + Ni7Zr2 and NiAl+ Ni7Zr2 eutectics is possible. We obtained eutectic structures involving combinations of these intermetallic phases along a constant zirconium section at 11 at. %. The alloy A (Ni-77 at.%, Zr-11at.% and rest Al) contains eutectic structures containing the Ni3Al and Ni5Zr phases in two morphologies, a planar, lamellar structure and a more irregular form. The alloys B (Ni-74 at.%, Zr-11at.% and rest Al) and C (Ni-71 at.%, Zr-11at.% and rest Al) contain two different eutectic structures that combine the Ni3Al and Ni7Zr2 phases, and the NiAl and Ni7Zr2 phases. These phases were identified by a combination of X-ray diffraction, transmission electron microscopy coupled with energy dispersive spectroscopy and electron probe microanalysis. The volume fraction of each eutectic constituent is different in the two compositions in that alloy B(Ni-74 at.%, Zr-11at.% and rest Al) contains significantly higher volume fractions of the eutectic containing the Ni3Al and Ni7Zr2 phases than the alloy C (Ni-71 at.%, Zr¬11at.% and rest Al). In order to understand effect of individual phases we have melted several other alloys (alloy D to I) bounding these eutectic alloys (7-25 at.% Al, 5-15 at.% Zr and rest Ni) that form primary solidification phases of the intermetallic structures that constitute the eutectics. Chapter 4 discusses the mechanical behaviour of the fully eutectic alloys alloys as well as alloys with a combination of primary phases along with a eutectic. Mechanical behaviour was assessed in vacuum arc melted and suction cast material. The compressive strength of eutectic and off-eutectic compositions has been evaluated as a function of temperature. Very high strength levels of around 2 GPa could be achieved accompanied by reasonable room temperature tensile plasticity in the range 3-4%. The introduction of the respective primary phases of NiAl, Ni3Al, Ni5Zr and Ni7Zr2 results in decrease of strength. We have explored the origins of strength and tensile plasticity in alloys through micro and pico indentation (hardness) measurements and an examination of slip lines and crack initiation on pre-polished surface of the tensile tested samples as well as by transmission electron microscopy. Chapter 5 explores the oxidation resistance of these alloys in isothermal tests. The oxidation resistance of alloys compares well with recently developed cast single crystal alloys. Clearly, the oxide scale is extremely adherent and no spalling occurs. Electron microprobe analysis shows the presence of a fine scale, layered oxide structures and reaction zones within the substrate. The oxidation behaviour has been characterized using TGA, XRD and EPMA. We have attempted to understand the mechanism of oxidation through analysis of rate constants and activation energy coupled with microstructural observations. Chapter 6 presents a summary of the current work and present the scope for further work.

The excellent high temperature microstructure stability, high strength, and oxidation resistance of intermetallics has for long driven the development of intermetallic based alloys. More recent studies demonstrated attractive properties of eutectic intermetallic in the Ni-Al-Zr systems. This thesis deals with study of binary Ni3Al+Ni7Zr2, NiAl+Ni7Zr2 and Ni3Al+NiAl+Ni7Zr2 ternary intermetallic eutectic alloys in this system and includes the identification of compositions that would yield each eutectic structure and their microstructural characterization, mechanical and oxidation behavior. The thesis is divided into six chapters. Chapter 1 reviews the study on high temperature materials development and presents the objectives of work in the current thesis. Various experimental techniques used for alloy preparation (vacuum arc melting and vacuum suction casting), microstructural characterization (optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray Diffraction (XRD), electron probe micro analyzer (EPMA), differential scanning calorimetry (DSC)), compression tests, microhardness tests and thermo gravimetric analysis (TGA) are described in Chapter 2. The specific background of work related to each chapter together with experimental results and discussion are given in next three chapters. Chapter 3 reports the method of identification of the composition for each of the eutectic alloys referred to above. The identification of alloy compositions of binary eutectics Ni3Al+Ni7Zr2 (Ni-13.5Al-11Zr), NiAl+Ni7Zr2 (Ni-19Al-12Zr) and Ni3Al+NiAl+Ni7Zr2 ternary eutectic (Ni-18.4Al-11.6Zr) is carried out with the help of available liquidus projection of Ni-Al-Zr system, and the iterative melting of numerous compositions that were refined to define the critical compositions for each eutectic. The microstructural features of these alloys have been characterized using optical and electron microscopy. Phase identification is confirmed by X ray diffraction, EPMA and TEM. The microstructure of Ni3Al+Ni7Zr2 and Ni3Al+NiAl+Ni7Zr2 ternary eutectic alloy shows similar eutectic morphologies. The eutectic colony consists of lamellar plates at center and intermixed lamellar-rod irregular morphologies towards the boundaries of the colonies. However, the NiAl+Ni7Zr2 eutectic alloy shows a fine, lamellar plate morphology throughout the microstructure. The orientation relationship between eutectic phases is determined using TEM technique for each alloy composition. Onsets of melting and liquidus temperatures have been identified by Differential Scanning Calorimetry. Modified liquidus projections of Ni-Al-Zr system near the Ni3Al+NiAl+Ni7Zr2 ternary eutectic region have been derived from present experimental work. Chapter 4 focuses on understanding the mechanical behaviour of these individual eutectics at room temperature and high temperature. An attempt has been made to correlate the microstructure and mechanical properties of eutectics by measuring room temperature hardness, compressive yield strength at various temperatures, and examination of slip bands, crack initiation and fractography. It is observed that NiAl+Ni7Zr2 eutectic possesses the highest yield strength and hardness followed by ternary eutectic and then the Ni3Al+Ni7Zr2 eutectic. The yield strength of these eutectics decreases rapidly beyond 700oC and this decrease is accompanied by substantial increase in compressive ductility and steady state flow, with little work hardening. Chapter 5 explores the isothermal oxidation behavior at high temperatures of these eutectic alloys. Oxidation kinetics have been measured at various temperatures (900oC, 1000oC, 1050oC and 1100oC) are carried out using the thermo gravimetric analysis technique (TGA). The oxidation behavior has been characterized using TGA, X ray diffraction and EPMA. The Top surface of oxide layer shows compact, NiO layer with a fine grain size. The cross section of oxide samples shows five distinct microstructural and compositional layers at steady state. Attempt has been made to understand the oxidation mechanism, sequence of layer formation in correlation with microstructure and weight gains, rate constants and activation energy analysis. Finally Chapter 6 presents a summary of the current work and suggests for further work.

No description available.

Die Cu – Ag - Mikroverbund – Werkstoffe besitzen das Potenzial die gegensätzlichen Anforderungen an das Leitermaterial für den Einsatz in einem Hochfeldmagneten, wie hohe Festigkeit bei gleichzeitig hoher elektrischen Leitfähigkeit und ausreichender Verformbarkeit, zu erfüllen. Außerdem bieten diese Werkstoffe gegenüber den anderen, die dafür in Frage kommen können, den großen technologischen Vorteil einer konventionellen schmelzmetallurgischen Herstellung. Jedoch wurde bisher dafür eine sehr aufwändige Technologie verwendet, die die Herstellung des Leitermaterials nur im Labormaßstab ermöglicht. Die vorliegende Arbeit befasst sich mit einer Technologie der Herstellung von Leitern, die den Anforderungen für den Einsatz in einem Hochfeldmagneten genügen können und in einem großtechnischen Maßstab verfügbar sind. Der Schwerpunkt der Leiterherstellung aus Cu – Ag - Legierung lag in der Einstellung der geeigneten Mikrostruktur über metallkundliche Mechanismen vor der Drahterzeugung. Hierfür wurden während der einzelnen Prozessschritten die Gefügeentwicklung und für die Anwendung relevante Eigenschaften der Legierungen in binären Cu – Ag – Legierungen und in ternären Cu – Ag -X –Legierungen untersucht. Darüber hinaus wurde der Einfluss der Mikrostruktur und der Zusammensetzung auf die mechanischen und elektrischen Eigenschaften der Drähte ermittelt sowie eine Korrelation zwischen Mikrostruktur und elektrischen Eigenschaften aufgestellt.

Die Cu – Ag - Mikroverbund – Werkstoffe besitzen das Potenzial die gegensätzlichen Anforderungen an das Leitermaterial für den Einsatz in einem Hochfeldmagneten, wie hohe Festigkeit bei gleichzeitig hoher elektrischen Leitfähigkeit und ausreichender Verformbarkeit, zu erfüllen. Außerdem bieten diese Werkstoffe gegenüber den anderen, die dafür in Frage kommen können, den großen technologischen Vorteil einer konventionellen schmelzmetallurgischen Herstellung. Jedoch wurde bisher dafür eine sehr aufwändige Technologie verwendet, die die Herstellung des Leitermaterials nur im Labormaßstab ermöglicht. Die vorliegende Arbeit befasst sich mit einer Technologie der Herstellung von Leitern, die den Anforderungen für den Einsatz in einem Hochfeldmagneten genügen können und in einem großtechnischen Maßstab verfügbar sind. Der Schwerpunkt der Leiterherstellung aus Cu – Ag - Legierung lag in der Einstellung der geeigneten Mikrostruktur über metallkundliche Mechanismen vor der Drahterzeugung. Hierfür wurden während der einzelnen Prozessschritten die Gefügeentwicklung und für die Anwendung relevante Eigenschaften der Legierungen in binären Cu – Ag – Legierungen und in ternären Cu – Ag -X –Legierungen untersucht. Darüber hinaus wurde der Einfluss der Mikrostruktur und der Zusammensetzung auf die mechanischen und elektrischen Eigenschaften der Drähte ermittelt sowie eine Korrelation zwischen Mikrostruktur und elektrischen Eigenschaften aufgestellt.