Transient oxidation, NiAl

A considerable amount of work has been conducted on the oxidation behavior of NiAl because it is a single-phase, primary-AI2O3 former which is often considered a model alloy for oxidation studies. Whereas in many ways NiAl is a model material, a review of results also shows a few unique characteristics, especially with regard to its transient oxidation behavior. NiAl has also been an excellent matrix material for studying the reactive element (RE) effect discussed in Section 6.2.2. 

Total mass gain (specimen + spalled oxide) after a 500 h isothermal exposure at 1000°C in laboratory air for various precious metal and Hf additions to alloys with 22% Al. NiAl + 0.05% Hf sets a (dashed) baseline in terms of no Ni-rich transient oxide, minimal Hf internal oxidation and slow alumina growth. The alloys with 22% Al primarily show higher mass gains due to the formation of Ni-rich oxide. Pd additions were not as effective as Pt or Ir. Internal oxidation of Hf also could cause increased mass gains. The addition of Cr did not have much effect with -10% Pt, but Y, in addition to Hf, was added to this alloy, which may have resulted in additional internal oxidation. Adapted from Pint et al., 2007d. 

The morphology of alumina scales on NiAl depends on the oxidation temperature and oxidation time. At lower temperatures (<1400 K). the metastable oxide phases 7-and 0-Al2O3 are formed initially. These metastable phases transform into a-AFO, during a transient time which decreases with higher temperature (> 1200 K) [24—26J. 

Figure 6-5. Parabolic rate constants for a-Al203 on various NiAl compositions from 1000-1300 °C. The addition of O.OS at.% Hf reduces the rate constant by an order of magnitude compared with undoped NiAl. The rate at 1100 °C is extremely low because of the initial rapid formation of transient, cubic a-Al203 which then transforms to slow-growing a-Al203. Adding more Hf results in a higher weight gain owing to internal oxidation of Hf.

Hanrahan et al. (1996) studied the effect of addition of 1-10 at.% Be on the oxidation of NiAl. Additions of Be did not result in formation of BeO at any of the temperatures tested (800, 1000, 1200 C). Rather, chrysoberyl (BeO-Al203) was the predominant phase formed. For all specimens tested at up to 7 at.% Be reaction kinetics were equal to or slower than those for undoped NiAl (short term <100-h exposures only). This was attributed to the rapid formation of the chrysoberyl phase then nucleation of a-Al203 at the scale-metal interface, rather than the formation of transient AI2O3 phases. 

The reduced oxidation performance of Ni3Al relative to NiAl is because of its lower Al content which is below that necessary for initial formation of an exclusive, external AI2O3 scale (Pettit, 1967). Instead, a transient Ni-rich oxide is formed (this is observed for Al content up to approximately 35 at.% (Tortorelli and DeVan, 1996). This layer is consistently found to contain NiO... 

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