Oxidation nickel aluminide-based

The selective oxidation of an alloy component, e.g., A1 or Si, requires the alumina or silica to be more stable than the oxides of the other components in the alloy. Figure 2.5 indicates this condition would be met for compounds such as nickel aluminides and molybdenum silicides. However, in the case of Nb- or Ti-base compounds the oxides of the base metal are nearly as stable as those of A1 or Si. This can result in conditions for which selective oxidation is impossible. This situation exists for titanium aluminides containing less than 50 at% A1 as illustrated in Figure 5.27. In this case a two-phase scale of intermixed AI2O3 and I1O2 is generally observed. It should be emphasized that the determination of which oxide is more stable must take into account the prevailing metal activities. 

Pack cementation is the most widely used process for making diffusion aluminide coatings. Diffusion coatings are primarily aluminide coatings composed of aluminum and the base metal. A nickel-based superalloy forms a nickel-aluminide, which is a chemical compound with the formula NiAl. A cobalt-based superalloy forms a cobalt-aluminide, which is a chemical compoimd with the formula CoAl. It is common to incorporate platinum into the coating to improve the corrosion and oxidation resistance. This is called a platinum-aluminide coating. Diffusion chrome coatings are also available. 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

Figure 10.16 Optical micrograph showing the cross-section of a Pt-modified aluminide coating on a nickel-base single-crystal superalloy after oxidation at 1200 °C for 20 h. The original grain structure of the /3-phase is evident and y has begun to nucleate at /6 grain boundaries as a consequence of A1 depletion.

Y ions into an aluminide ((3-NiAl) on a nickel-base alloy and confirm that while initially the implanted reactive element effectively imparts increased scale adhesion, both in air and oxygen at 1000 -1200 C, the beneficial influence is not long lasting. They attributed this loss to the influence of the substrate Ni-base superalloy, since lasting benefits of reduced rates of oxidation and improved scale adherence were maintained when Y was implanted into bulk 3-NiAl (Jedlinski and Mrowec 1987). 

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