Design and development of high temperature nickel-based superalloys

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2017
Eriş, Rasim
Remarkable high temperature mechanical properties of nickel-based superalloys are related with the arrangement of ternary alloying elements in L12-type ordered γ'-Ni3Al intermetallics. Therefore, in this thesis, high temperature site occupancy preference and energetic-structural characteristics of atomic short range ordering of ternary alloying X elements (X = Co, Cr, Hf, Mo, Nb, Pt, Re, Ru, Ta and W, respectively) in Ni75Al21.875X3.125 alloy systems have been studied by combining the statistico-thermodynamical theory of ordering and electronic theory of alloys in the pseudopotential approximation. Simulations have shown that, all ternary alloying element atoms (except Pt) tend to occupy Al sublattice sites of Ni3Al intermetallics, whereas Pt atoms prefer to substitute for Ni sublattice sites. However, in contrast to other X elements, sublattice site occupancy characteristics of Re atoms seem to be both composition and temperature dependent. Distribution of Re atoms at both Al and Ni sublattice sites above critical temperatures, Tc, may lead to localized supersaturation of the parent Ni3Al phase and makes possible formation of topologically close-packed (TCP) phases. In order to validate these results, site occupancy tendencies of afore mentioned alloying X elements have also been determined by performing first-principles ab initio calculations at 0 Kelvin and similar results have been obtained. Furthermore, by making use of charge density difference (CDD), electron localization function (ELF) and density of states (DOS) methods, the effects of alloying X elements on the bonding nature of Ni3Al-X phases have been simulated. Superior mechanical properties of nickel-based superalloys strongly depend on microstructural characteristics of these materials (i.e. γ' volume fraction, size & size distribution, orientation and shape). For this purpose, Ni80Al15X5 alloy systems (X = Al, Co, Cr, Hf, Mo, Nb, Ta, Ti and W, respectively) have been produced and characterized by various techniques. In the light of all theoretical and experimental data, it has been concluded that better micro-hardness properties of nickel-based superalloys can be obtained by appropriate alloying element additions and heat treatment procedures which lead to γ' precipitates having higher lattice parameter, higher volume fraction, smaller size and correspondingly optimum δ parameter.  

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Citation Formats
R. Eriş, “Design and development of high temperature nickel-based superalloys,” M.S. - Master of Science, Middle East Technical University, 2017.