A castable alloy, i.e., one that flows easily to fill the entire mold cavity and also has resistance to hot tearing during solidification, must invariably contain a sufficient amount of a eutectic structure. For this reason, most traditional aluminum casting alloys contain silicon because the aluminum-silicon eutectic imparts to the alloy excellent casting characteristics. However, the solidus temperature in the Al-Si system does not exceed 577°C, and the major alloying elements (i.e., zinc, magnesium, and copper) used with silicon in these alloys further lower the solidus temperature. Also, these elements have high diffusivity in aluminum and so, while they enhance the room temperature strength of the alloy, they are not useful at elevated temperatures. Considering nickel-base super alloys, whose mechanical properties are retained up to temperatures that approach 75% of their melting point, it is conceivable that castable aluminum alloys can be developed on the same basis so that they are useful at temperatures approaching 350 °C. A castable aluminum alloy intended for high temperature applications must contain a eutectic structure that is stable at temperatures higher than 600°C, and must contain second phase precipitate particles that are thermodynamically stable at the service temperature. Transition metal trialuminides with the general chemical formula AlxTMy in which TM is a transition metal, are excellent candidates for both the eutectic structure and the precipitate particles. In this research, the use of transition metals in the constitution of aluminum casting alloys is investigated with emphasis on the morphology, crystallography, and mechanisms of formation of the various phases.

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.

No description available.

A castable alloy, i.e., one that flows easily to fill the entire mold cavity and also has resistance to hot tearing during solidification, must invariably contain a sufficient amount of a eutectic structure. For this reason, most traditional aluminum casting alloys contain silicon because the aluminum-silicon eutectic imparts to the alloy excellent casting characteristics. However, the solidus temperature in the Al-Si system does not exceed 577°C, and the major alloying elements (i.e., zinc, magnesium, and copper) used with silicon in these alloys further lower the solidus temperature. Also, these elements have high diffusivity in aluminum and so, while they enhance the room temperature strength of the alloy, they are not useful at elevated temperatures. Considering nickel-base super alloys, whose mechanical properties are retained up to temperatures that approach 75% of their melting point, it is conceivable that castable aluminum alloys can be developed on the same basis so that they are useful at temperatures approaching 350C. A castable aluminum alloy intended for high temperature applications must contain a eutectic structure that is stable at temperatures higher than 600°C, and must contain second phase precipitate particles that are thermodynamically stable at the service temperature. Transition metal trialuminides with the general chemical formula AlxTMy in which TM is a transition metal, are excellent candidates for both the eutectic structure and the precipitate particles. In this research, the use of transition metals in the constitution of aluminum casting alloys is investigated with emphasis on the morphology, crystallography, and mechanisms of formation of the various phases.

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.

Ce travail porte sur l'étude par simulation en champ de phase du vieillissement isotherme de trois alliages intermétalliques. Nous avons montré que les alliages Ni-Al "inverses", dans lesquels les précipités sont désordonnés et coexistent avec la matrice ordonnée, reproduisent le chemin cinétique des alliages Ni-Al "directs". Une étude statistique de l'alliage Al-Sc a montré que la théorie LSW ne reproduit pas la fonction de distribution de taille de particules sur les stades de coalescence avancés. Nos simulations montrent que la compétition entre les forces motrices chimique, élastique et l'énergie d'interface est à l'origine de la croissance en forme de "papillon" des précipités Al3Sc isolés. Enfin, nous avons adapté le modèle de champ de phase pour l'alliage ternaire Al-Zr-Sc afin de caractériser l'évolution de la composition des précipités Al3(Zr,Sc) durant la coalescence. Nos simulations reproduisent la structure coeur-coquille de ces précipités observée expérimentalement.