Abstract
This paper concerns microstructural characterization of alumina scales formed on a 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050, 1100 and 1150 °C for 100 h. Samples for high resolution characterization of Al2O3 scales were prepared using a focused ion beam (FIB) method. High resolution TEM and S/TEM techniques were used for a detailed characterization and a direct comparison of the phase composition and chemistry of the oxide scales formed during high temperature oxidation. The growth of transient θ-Al2O3 and its transformation to α-alumina is addressed for each oxidation temperature along with the differences in the diffusion of reactive elements, such as Hf, Zr and Y, through grain boundaries of the α phase. The θ to α transformation front was proven to move from the metal-scale to the scale-gas interface. The results presented in this paper indicate that after 100 h of oxidation at 1050 and 1100 °C there are still some θ-alumina grains remaining and even in the regions where the transformation to α was finished the surface retained the whisker-like morphology.
Highlights
High temperature resistant materials applied on jet engine blades and vanes rely on their ability to form an adherent and stable oxide scale, mostly Al2O3, during high temperature exposure at around 1000–1200 °C [1, 2].L
The surface microstructures of the oxide scales formed on the investigated nickel superalloy after 100 h at 1050–1100 °C are characterized by a whisker-like morphology, which is commonly attributed to the transition alumina polymorphs
The dynamic segregation theory (DST) [16] and the results presented in this work indicate that Zr as well as Y readily diffuses through the grain boundaries of alumina grown on bare superalloy even at very low concentrations
Summary
High temperature resistant materials applied on jet engine blades and vanes rely on their ability to form an adherent and stable oxide scale (thermally grown oxide), mostly Al2O3, during high temperature exposure at around 1000–1200 °C [1, 2].L. Time and substrate composition various polymorphs of alumina can form varying in the structure and growth rate. The most desirable polymorph is the hexagonal a-Al2O3 which is characterized by low growth rate and long-term stability at high temperatures. Prior to the formation of aalumina various other fast growing transition (metastable) polymorphs can be observed. These include the c and h-alumina exhibiting whisker or needle-like morphology which can transform to a-Al2O3 with oxidation time [3,4,5,6,7]. While the metastable aluminas grow mostly by outward diffusion of Al3? Cations the growth of a-alumina is governed by an inward O2- diffusion, with minor outward Al3? While the metastable aluminas grow mostly by outward diffusion of Al3? cations the growth of a-alumina is governed by an inward O2- diffusion, with minor outward Al3? contribution [8,9,10]
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