Interfaces in Oxides Formed on NiAlCr Doped with Y, Hf, Ti, and B 397
phase boundaries of the oxides; (2) to provide detailed infor- mation about the chemistry of the investigated interfaces in the studied
alloys.Major focuswas directed toward analysis of interfaces in the inner part of the oxide scales, i.e., below the outermost region formed during the initial oxidation. This part of the oxide is most important for the protection of the alloy against degradation. The work is a first step in a larger project, aimed at a better understanding of transport pro- cesses in alumina, here coupling between the amount of seg- regating dopants and the oxidation rate is of great interest.
MATERIALS
Three different alloys with different amount of REs were selected. The compositions, as determined by inductively coupled plasma atomic emission spectroscopy, of the alloys are (in at%)
∙ YHf: Ni62.3Cr15.1Al22.5Y0.03Hf0.04C0.024 ∙ YHfTi: Ni61.9Cr15.2Al22.6Y0.020Hf0.043Ti0.31C0.017 ∙ YHfB: Ni60.6Cr16.8Al22.2Y0.015Hf0.037B0.33C0.028
The oxidation kinetics of these alloys in the dry air is very
similar, but addition of Ti did reduce the depth of internal oxidation (Unocic & Pint, 2013). In the case of the B-doped alloy, better oxide adhesion was achieved (Unocic et al., 2014). Still, the question was whether, and if so how, B and Ti are incorporated into the protective Al2O3-oxide layer. In the following text,wewill refer to the samples asYHf-,
YHfTi-, and YHfB-sample.
EXPERIMENTAL PROCEDURES To achieve the formation of an oxide layer all samples were exposed isothermally in O2 for 100h at 1,100°C. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy were performed using a PhillipsCM200 (Phillips,Amsterdam, Netherlands) in Oak Ridge. To prepare the needle shaped samples required for APT
studies, a FEI VERSA 3D FIB milling station (FEI, Hillsboro, OR, USA) was utilized. APT needles containing two different oxide regions, the outermost part close to the surface and the inner oxide, were prepared with the FIB standard method (Larson et al., 2013). To produce APT specimens containing near-surface oxide, a magnetron sputtered Au-coating was applied. The samples were then coated with a 1µmthick,1µmwide, and15µmlong strip of Pt. After this, the sample was cut free with the ion beam and attached to an omniprobe needle. As the GBs inside the protective alumina are not visible during the preparation, the omniprobe was, rotated by 180°. Because the sample is connected to the omniprobe at an angle of 30°–50°, the procedure results in a lift-out sample produced roughly perpendicular to the surface. Subsequently, the sample was attached by Pt-deposition to a Cameca microtip flattop tip (Cameca, Madison,WI, USA) and a 1µm thick part was cut off with the ion beam. Before the annular milling a Pt cylinder was deposited on the attached piece on
top of the desired interface. By selecting this orientation, the chance of hitting a GB in the elongated samples (Fig. 1) is increased by a factor of 2. However, the final APT specimens using this procedure usually contained more Ga contamination than one prepared using the standard method. Thus, for the preparation of the outer oxide the sample pieces were attached without rotation of the omniprobe. Analysis of the oxides with APT proved to be difficult
both in terms of success rate (in our case <25%) and in termsofchoiceofsuitableanalysisparameters. Thus,the measurement parameters were studied to obtain a suitable compromise. Two different atom probes, the LEAP 3000HR (Cameca, Madison, WI, USA) at Chalmers with a green laser and the LEAP 4000HR (Cameca, Madison, WI, USA) in Karlsruhe with a UV-laser were used for the APT analyses. Generally, the LEAP 4000HR mass spectra were less complex and the peaks had smaller thermal tails (Fig. 2). In addition, the success rate of the measurements using LEAP 4000HR appeared to be higher by a factor of at least 2. To reduce possible surface migration of mobile elements
like B or C, and also to reduce differences in evaporation at phase boundaries, the measurements were performed at 25K even though higher temperatures give alumina compositions
Figure 1. High-angle annular dark field scanning transmission electron microscopy of the YHfTi sample. Large grains of α-alumina form a protective inner oxide layer on top of the metal. These grains are elongated perpendicularly to the metal-oxide interface. Between them and the Pt-cover (from the preparation), the outer oxide layer, with smaller grains of alumina spinels (brighter) and the very bright HfO2, is observed. Dark appearing voids are found between these two regions. GBs, grain boundaries.
Figure 2. Mass spectra of (a) the LEAP 3000 with green and (b) the LEAP 4000 with ultraviolet (UV) laser. Typically, the thermal tails are smaller in the LEAP 4000 measurements.
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