| dc.description.abstract | 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. | en_US |
