Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide
Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm2/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.
KeywordsInternal oxidation Diffusion YSZ GDC
Wagner  described the internal oxidation of a less noble metal in solid solution with a nobler base metal. With increasing content of the less noble metal, the rate of internal oxidation is supposed to diminish and the formation of an oxide scale is favored. If the less noble element is initially present in an intermetallic phase, Gesmundo and Gleeson  as well as Anžel et al.  predict the so-called in situ internal oxidation of this intermetallic phase and subsequent coarsening of internal oxide particles with increasing content of the less noble alloying metal.
Kloss et al.  observed in situ internal oxidation of a binary Ni–Zr alloy at 1000 °C under low oxygen partial pressure. They found that even minor additions of Y accelerate internal oxidation considerably. The theory of Gesmundo et al.  could not explain the behavior of these ternary Ni–Zr–Y alloys. Konrad et al.  later detected that internal oxidation rates of Ni–Zr–Y alloys are not dependent on the total Zr and Y contents but more so on the Zr/Y ratio. The internal oxidation kinetics of Ni–Zr–Y alloys varied with the Zr/Y ratio very much in the same way as the oxygen ion diffusivities of bulk yttria-stabilized zirconia (YSZ) ceramics vary with the zirconia/yttria ratio. In both materials, fastest kinetics are observed at the ration of about 9/1. Furthermore, the absolute oxygen diffusivities in bulk YSZ and internally oxidized Ni–Zr–Y alloys turned out to be very close. Another interesting observation was that the oxide phase behind the internal oxidation front in Ni–Zr–Y alloys forms a continuous and interpenetrating network. Konrad et al.  concluded that oxygen diffusion through the evolving oxide phases themselves is controlling internal oxidation in the Ni–Zr–Y system.
Fuhrmann et al.  too investigated binary Ni–Zr and ternary Ni–Zr–Y systems. Both systems showed a transition from internal to external oxidation at unusually high contents of the less noble elements. Binary Ni–Zr alloys exhibited a transition from internal to external oxidation at around 35 at% Zr, whereas in ternary Ni–Zr–Y alloys this transition occurred at even higher contents of Zr + Y of about 55 at%. The transition in ternary Ni–Zr–Y came along with a major decrease of oxidation rates due to the formation of a dense yttria scale.
The question arises whether the Ni–Zr–Y system is unique showing exceptionally fast internal oxidation or whether other ternary alloys being able to form a high oxygen conductive oxide like YSZ or gadolinia-doped ceria (GDC) [7, 8] behave similar. Therefore, ternary alloys M–Zr–Y and M–Ce–Gd were studied here.
The as-cast compositions of ternary alloys
Noble base metal
Zr or Ce
Minor reactive metal
Y or Gd
Oxide content after internal oxidation
Oxygen partial pressures
T in °C
p(O2) in Pa
4.1 × 10−16
1.2 × 10−13
1.3 × 10−11
Cross sections of the oxidized samples were embedded in phenolic resin with carbon filler, ground with SiC paper (until grit 1200), and polished with diamond paste (6, 3, 1 µm). Microstructures were investigated using light microscopy and scanning electron microscopy (SEM, 1540 EsB Cross-Beam, Zeiss) equipped with an energy-dispersive X-ray spectroscopy (EDX). The depths d of the internal oxidation fronts were measured on SEM micrographs. For every data point, five micrographs from different specimen areas were taken. The error of the mean penetration depth was ±25 µm. X-ray diffraction (XRD) measurements were carried out on bulk specimens with a D8 advance diffractometer (Bruker AXS, Germany) set up in Bragg–Brentano geometry with a copper anode (λCu Kα1 = 0.15418 nm), a Ge monochromator, and a 0.3° divergence slit in the primary beam. An axial Soller slit with 2.5° opening angle was inserted into the secondary beam in front of a 1D detector. Possible phases were identified with the aid of the PDF-4+ 2015 powder diffraction database from The International Centre for Diffraction Data.
Phases present in the as-cast state according to XRD measurements and/or EDX analysis
Peaks in XRD diffractograms of FeZr10Y1 could be associated to (Fe) solid solution and Fe2Zr. However, for some peaks present in the diffractograms no corresponding phase could be found in the PDF-4+ 2015 powder diffraction database. On backscattered electron SEM micrographs (Fig. 1), a third phase is identified. EDX revealed a composition of approximately 85 at% Fe, 9 at% Y, and 6 at% Zr. Ternary Cu–Zr–Y and Ni–Ce–Gd phase diagrams could not be found in literature; however, EDX analysis (Fig. 4) indicates that Y and Gd are dissolved preferentially in the intermetallic phases.
Phases in the alloys after internal oxidation heat treatment at 900 °C
Phases detected by XRD
Internal oxidation rates, kp, of ternary alloys at 700, 800, and 900 °C in µm2/s
All investigated ternary alloys undergo exceptionally fast internal oxidation. Stott et al.  internally oxidized Ni–Al and Ni–Cr alloys in Ni/NiO Rhines packs . They generally found faster internal oxidation with decreasing content of the reactive metal in the binary alloys in accordance with the predictions of Wagner . The fastest internal oxidation was measured in ~1 wt% Cr- or ~wt% Al-containing Ni-based alloys. However, internal oxidation depths at 800 and 900 °C are very much smaller compared to all the here investigated alloys even though the contents of reactive metals in these ternary alloys are much higher (see Figs. 8, 9). Konrad  too found much slower internal oxidation of binary Ni–Zr alloys at 800 °C under low oxygen partial pressure (see Fig. 8). According to Nagorka et al. , internal oxidation of Cu-1 at% Zr and Cu-1 at% Y wires at 800 °C in Rhines packs proceeded faster than in the above-mentioned Ni–Al and Ni–Cr binary alloys, however still much slower than in the ternary alloys of this study (see Fig. 8).
Kloss et al.  first assumed that oxygen diffusion through the internally formed oxide phase itself plays an important role in the internal oxidation of Ni–Zr–Y alloys. They suggested that high ionic conductivity in YSZ ceramics is responsible for fast internal oxidation in noble metal-based alloys with the additions of both Zr and Y. Later, Konrad et al.  could explain such incredible fast internal oxidation in Ni–Zr–Y alloys by means of a model derived under the assumptions that oxygen diffusion through the oxides itself is the rate-controlling mechanism and the oxide phase behind the internal oxidation front is continuous, i.e., provides continuous diffusion paths to the surface.
These assumptions are fulfilled in the here investigated ternary alloys. Microstructure investigations (Fig. 6) prove that all ternary alloys in this study form continuous oxide phases behind the internal oxidation fronts in the temperature range from 700 to 900 °C. In Zr + Y containing alloys, a continuous YSZ is formed by internal oxidation and in the Ce + Gd containing alloy continuous GDC is observed. The highest internal oxidation rates of the NiCe8Gd1 alloy may be explained straightforwardly by the higher oxygen ion conductivity of GDC compared to YSZ, the oxygen ion conductivity in GDC is roughly one order of magnitude higher than that in YSZ [8, 16, 17].
From the ternary alloys forming a continuous YSZ phase, CuZr9Y1 oxidizes fastest internally. The study of Nagorka et al.  confirms our observation. In , the binary Cu-1 at% Zr and Cu-1 at% Y alloys show faster internal oxidation compared to binary Ni–Al, Ni–Cr, and Ni–Zr alloys (see Fig. 8). Even though the literature values are not consistent, oxygen diffusivities turned out to be generally much higher in Cu than in Ni. According to Narula et al. , the diffusivity of oxygen in copper DOCu is ~10−5 cm2/s at 900 °C, while the diffusivity of oxygen in Ni DONi is ~10−9 cm2/s at 900 °C according to Park et al. , i.e., orders of magnitude lower. In the Cu-based alloys, oxygen diffusion through the noble base metal may therefore play a more pronounced role in the internal oxidation kinetics than in Ni-, Co-, and Fe-based ternary alloys.
Ternary alloys containing two reactive metals being able to form a high oxygen conductive oxide are prone to exceptionally fast internal oxidation. Fast internal oxidation is even more expected if the internal oxides form continuous diffusion paths from the surface to the alloy interior. Internal oxidation of NiCe8Gd1 is much faster than that of CoZr10Y1, CuZr9Y1, FeZr10Y1, and NiZr9Y1. Internal oxidation kinetics of such ternary alloys are apparently more dependent on the oxygen conductive oxide that is formed through internal oxidation than on the noble base metal. Noble base metals with high oxygen diffusivities may further accelerate internal oxidation. Fast internal oxidation in these ternary alloys is not limited to low concentrations of the reactive alloying metals.
The financial support from the Deutsche Forschungsgemeinschaft (DFG) within the project GL 181/32-1 is gratefully acknowledged.
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