Microsc. Microanal. 23, 396–403, 2017 doi:10.1017/S1431927617000186
© MICROSCOPY SOCIETY OF AMERICA 2017
Interfaces in Oxides Formed on NiAlCr Doped with Y, Hf, Ti, and B
Torben Boll,1,* Kinga A. Unocic,2 Bruce A. Pint,2 and Krystyna Stiller1,*
1Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden 2Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN 37831, USA
Abstract: This study applies atom probe tomography (APT) to analyze the oxide scales formed on model NiAlCr alloys doped with Hf, Y, Ti, and B. Due to its ability to measure small amounts of alloying elements in the oxide matrix and its ability to quantify segregation, the technique offers a possibility for detailed studies of the dopant’s fate during high-temperature oxidation. Three model NiAlCr alloys with different additions of Hf, Y, Ti, and B were prepared and oxidized in O2 at 1,100°C for 100 h. All specimens showed an outer region consisting of different spinel oxides with relatively small grains and the protective Al2O3-oxide layer below. APT analyses focused mainly on this protective oxide layer. In all the investigated samples segregation of both Hf and Y to the oxide grain boundaries was observed and quantified. Neither B nor Ti were observed in the alumina grains or at the analyzed interfaces. The processes of formation of oxide scales and segregation of the alloying elements are discussed. The experimental challenges of the oxide analyses by APT are also addressed.
Key words: oxidation, diffusion, grain boundaries, reactive elements, atom probe tomography INTRODUCTION
Today, the most important life-limiting factor of advanced alloys for many high-temperature applications is their limi- ted oxidation/corrosion resistance and not their mechanical degradation. Essentially all potential metallic materials are unstable in
high-temperature environments and the various oxidation processes are highly exothermic. The only reason it is possible to have materials that can be used at all is that the reaction product, the oxide, may form a protective layer on the component surface (Kofstad, 1966). Thus, the development of methods for surface protection
against corrosion and oxidation of high-temperature materi- als has become an important area in modern material science. The basic principle is to raise, at the surface of an actual component, the amount of that element that forms a protec- tive oxide scale. The oxidation resistance of such an enriched layer (often existing as a coating) is then dependent of its ability to maintain a continuous surface layer of the oxide, which serves as a diffusion barrier and reduces the rate of reaction between oxygen and the underlying metal. In the case of Ni-base superalloys, developed for various high- temperature applications (e.g., turbine blades for aircrafts), the most successful oxidation protection is obtained by the formation of an Al-enriched intermetallic outer layer. During high-temperature exposure in air this layer creates, a protec- tive Al2O3-oxide layer, an alumina scale. Alumina is protec- tive as it is dense, slow-growing, well-bonded to themetal and resistant against cracking and spallation (Young, 2016).
*Corresponding authors.
torben.boll@
kit.edu;
stiller@chalmers.se Received July 3, 2016; accepted January 16, 2017
Diffusion along alumina grain boundaries (GBs) is
recognized to be the rate-controlling transport mechanism that might be altered by the presence of alloying elements, but the exact mechanisms affecting diffusional fluxes still remain not entirely understood. One of the important issues is the fate of the base metal elements, most commonly Ni or Cr, in the oxide as solubility of these elements in alumina is quite low. Another is segregation of reactive elements (RE) Zr, Y, Hf, Ce, etc., from the alloy to the alumina GBs [often observed using analytical transmission electron microscopy (TEM)], that is known to influence growth of this oxide. However, the complex interplay between the oxide microstructure, ionic transport, and oxidation kinetics is still not completely under- stood to large extent due to the lack of atomic scale information (Pint, 1996; Haynes et al., 2002; Naumenko et al., 2016). The latest development of laser-assisted atom probe
tomography (APT) and new specimen preparation methods using focused ion beam (FIB) milling (Larson et al., 2008) opened the possibility for characterization of the thermally grown oxide scales with high sensitivity and nearly atomic spatial resolution. During the last decade, several successful APT investigations of oxides have been reported (Larson et al., 2008; Lozano-Perez et al., 2009; Marquis et al., 2010; Hono et al., 2011; Chen et al., 2012; Stiller et al., 2012; Dong et al., 2013; Viskari et al., 2013; Chen et al., 2014; Stiller et al., 2016). However,APT analyses of thermally grown oxides are far from being routine and there are stillmany challenges to overcome. This article concerns APT investigations of oxidation of
three NiAlCr alloys with different additions of RE that are mimicking the most commonly used coatings for protec- tion against oxidation at high temperatures in Ni-based superalloys (Goward, 1998). The purpose of the work was dual: (1) to demonstrate feasibility ofAPT studies of grain and
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