398 Torben Boll et al.
closer to the stoichiometric value. The laser pulse energy was typically 200 pJ for the LEAP 3000 and 20 pJ for the LEAP 4000. The laser of the LEAP 4000 has a smaller focal point, which allows lower pulse energies. Pulse frequencies between 100 and 250 kHz were used depending on the applied voltage to avoid a mass cutoff below 300 Da. For the APT reconstructions with IVAS 3.6.12 we used SEM images, if possible. Otherwise the reconstruction was based on the reconstruction parameters (evaporation field of 25.6 V/nm, image compression factor of 1.65, k-factor of 3.3) obtained for a reference measurement of pure alumina at 25K with 20 pJ in the LEAP 4000, in which plane distances could be identified by using cross-correlated spatial distribution maps (SDM) (Boll et al., 2012), which the paper refers to as AtomVicinity (Boll et al., 2007), an algorithm almost identical to SDM (Geiser et al., 2007).
RESULTS
Figure 1 is a STEM-HAADF micrograph showing the oxide structure typical for all the investigated samples. It consists of α-alumina grains that form a protective layer on top of the bright imagingmetal. The grains are elongated in their growth direction, i.e., perpendicular to the metal surface. They are typically about 2 µmhighand 1 µmwide.Ontop of this region, a more finely grained region with equiaxed oxide is visible. This part of the scale originates from the oxide formed during initial oxidation and has been transformed during subsequent exposure. It consists of a mixture of alumina and spinel grains. The region also contains dark imaging voids and bright imaging small precipitates of heavy metal (Y or Hf) oxides. Acquisition of APT data from the oxidized samples
proved to be difficult. Although α-alumina itself runs quite well and measurements with several 100 millions of collected atoms were easily attainable, the successes rate of analysis of the specimens containing oxide phase or GBs was below 25% due to the early specimen fracture. Furthermore, it appeared that premature fractures of the specimens containing the outermost part of the oxide were more frequent than for the specimens containing the inner parts. This was most probably due to the presence of voids at this part of the oxide (see Fig. 1). On the other hand, it was more difficult to prepare samples containing oxide GBs in the inner part of the oxide as the oxide grain size is larger there. Initially analyses were performed at 60K, which gave a
reduces the differences in evaporation fields, the local magni- fication effect and possible surface migration of elements like B or C, the majority of experiments was performed at 25K.
APT Mass Spectra
The APT mass spectra collected with the LEAP 4000 are typically easier to interpret, showing smaller peak tails (Fig. 2). Concerning Hf-molecules, the spectrum is also much simpler. However, usually more hydrides were
composition of α-alumina much closer to the stoichiometric value than the 25K runs. However, as the lower temperature
observed in the measurements, which counteracted this positive effect to some extent. The higher number of peaks leads to a situation where more tails overlap. This makes it effectively more complicated to give an exact value for the Cr and Ni concentration by deconvolution of the peaks. We decided to give these values of the oxide matrix composition as an upper limit as it is known that the solubility of alloying elements in α-alumina is extremely low (Kofstad, 1966). Generally, nonmolecular Hf is rarely detected, instead it
usually field evaporates as molecular HfO+/2+/3+ ions and to some extent as more complex oxide molecules like HfO2 or Hf2O. This results in overlaps with Pt for 3 out of 4 of the significant isotopes, especially for the 2+ state at 97, 97.5, and 98 Da. This is problematic only when plotting the Hf signal close to the protective Pt cap deposited during specimen preparation by FIB. Y has only one significant isotope, for Y2+ a peak is found at 44.5 Da that unfortunately is located in the thermal tail of AlO, which induces a high noise level in the corresponding atom maps. The only other significant peak found is YO2+ at 54.5 Da. Nonmolecular B can be easily identified, but most of themore complex ions show overlaps. Apossible BO+ main peak overlaps with Al at 27 Da, BO2
+
would overlap likewise with AlO at 43 Da. There are also overlaps of the second highest BO+ peak at 26 Da with Cr. As there was no peak for mono-atomic B present, we think it is unlikely that these overlaps are relevant. Ti and its oxides have a typical pattern of five isotopes, which would be easy to recognize, if present. The possible presence of an extremely small number of atoms that do not show up as a peak in the mass spectrum, was considered by looking at the distribution of the atoms at the peak positions. No indication of enrich- ment in GBs or phase boundaries was found for B or Ti.
The Outermost Part of the Oxide Scale (YHf-Sample)
For the reasons described above, the outer oxide’s surface was successfully analyzed only in a YHf-sample (Fig. 3) below the protective Au-capping deposited on the sample surface. The local magnification effects, expected from the high evaporation field of Au, were successfully reduced using 25K during analyses. APT revealed many more features compared to STEM
analysis such as small, <5 nm in diameter, Ni-rich grains observed at the very topmost oxide layer, i.e., below the Au-cap (Figs. 4b–4d). The grains were only found on a small fraction of the surface, accounting for about 30%of the area of the APT measurement when viewed fromthe top. They were surrounded by a Ni-rich spinel containing aluminum. Some enrichment of C and maybe Cr could be found in or around some of these particles (Fig. 4d). Using the isosurface of 32 at % Ni as the boundary of the particles their average composi- tion is Ni48.8Al17.0Cr1.4C1.1Hf0.3O31.3. The compositions are also given in Table 1. The area around this region 1 in Figure 4 has a composition of
Ni16.6Al32.3Cr0.8C0.3Hf0.2O49.8.Below region 1, one finds in region 2 a composition of Ni17.2Al30.7 Cr0.3Y0.1Hf0.1O51.6. There are two kinds of GBs between
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