Investigation of microstructural properties and oxidation behavior of Ni-Al-X (X = Nb ve Y) based superalloys

Nickel-based superalloys are important materials used in the high temperature applications due to their superior high temperature mechanical properties, high creep resistance and high component corrosion-oxidation resistance. These superior physical and mechanical properties make Ni-based superalloys as an attractive class of high temperature structural materials for specific applications in aerospace industry, thermal power plants, nuclear power plants and chemical processing parts. The extraordinary physical and mechanical properties of Ni-based superalloys are associated with their bimodal microstructure consisting of coherent L12-type ordered γ' (Ni3Al) precipitates distributed in a Ni-based γ (FCC) matrix phase. Both γ matrix phase and γ' precipitates have cubic crystal structure and the small lattice parameter mismatch between them results in the formation of coherent γ/γ' interfaces. However, γ' precipitates in Ni-based superalloys coarsen unavoidably after long time aging treatments at research high temperatures. Coarsening of the γ' precipitates caused by the balance between interfacial energy and elastic energy destroys the coherency between the γ matrix phase and γ' precipitates. It is known impaired that the high temperature mechanical properties and creep behavior of these materials may be worsened because of the change of the morphology and size of the γ' precipitates as a result of coarsening. At high temperatures, peak strength, arising from precipitation hardening, is reduced due to the coarsening. However, limited experimental study is present in the literature concerning the coarsening behavior of γ′ precipitates in nickel-based superalloys. The Ni-based superalloys should have sufficient oxidation resistance in their service conditions besides the superior mechanical properties and high creep resistance. At elevated temperatures and oxygen-containing atmospheres, oxidation behavior of these materials is one of the service-life limiting factors. The continuous oxidation of Ni-based superalloys under thermal cycling conditions can accelerate crack propagation and eventually cause lead to fracture. In this regard; even, dense, compact and protective oxide scales should be formed on the surfaces of Ni-based superalloys when they are exposed to oxidative environments. Therefore, the development of a Ni-based superalloy that provides the best combination of high temperature strength, microstructural stability, oxidation resistance and castability is crucially desired. To the best our knowledge, there are no comprehensive studies including microstructural evolution, coarsening kinetics and oxidation resistance of Ni-based superalloys. This study focuses on two main topics: (i) investigation of the effect of ternary alloying elements (X = Nb and Y, n = at. % 2 and 4) on the microstructural properties and microhardness of Ni85Al15-nXn superalloys depending on the aging temperature and time, (ii) improving the high temperature oxidation behavior of Ni85Al15-nXn superalloys by adding alloying elements and/or appropriate heat treatment methods. For this purpose, Ni85Al15-nXn superalloys were produced and characterized by various techniques. The effect of ternary alloying element additions on the microhardness values, high temperature oxidation behavior and microstructure of L12-type ordered γ' (Ni3Al) precipitates has been studied in detail in both as-cast and heat treated samples. It has been determined that the type and amount of the ternary alloying element together with the appropriate aging conditions have a significant effect on the relevant properties. It has been shown that the microhardness of the superalloys depend on the size and size distribution, volume ratio, shape and orientation and the channel width of the γ matrix phase of the L12-type ordered γ' (Ni3Al) precipitates distributed in a Ni-based γ (FCC) matrix phase. The cyclic oxidation tests and the structural characterization of the oxide layer formed after oxidation has been investigated by surface and cross-section analysis. According to these analyses, it has been determined that NiO was formed in the outer layer of the film and Al2O3 formed in the inner oxidation region close to the oxidation/metal interface.