Superalloys, nickel aluminides

Mew Materials and Processes. New materials and processes include aligned eutectics, oxide and liber-reinforced superalloys, intermelalhc compounds and other ordered phases including titanium aluminidcs. nickel aluminides. and iron aluminidcs. 

The nickel aluminide NijAl - known as the y phase - crystallizes with the cubic LI2 structure (CujAu-type) which results from the fc.c. structure by ordering (see Fig. 1). Deviations from stoichiometry are accommodated primarily by antisite defects (Lin and Sun, 1993). The density of NijAl is 7.50 g/cm (see Liu et al., 1990) and thus is only slightly lower than that of the superalloys (see Table 2) which, however, is still of interest. The elastic constants have been studied experimentally and theoretically by various authors (e.g. Davies and Stoloff, 1965 Dickson et al., 1969 Kayser and Stassis, 1969 Foiles and Daw, 1987 Wallow et al., 1987 Yoo and Fu, 1991, 1993 Yasuda et al., 1991a, 1992). Young s modulus of cast polycrystalline NijAl at room temperature is about the same as that of pure Ni with a weaker temperature dependence (Stoloff, 1989),... 

Rare earths incorporated into aluminide coatings. The enhanced oxidation resistance and scale adhesion imparted to AI2O3 forming alloys has stimulated a quest for novel approaches for incorporating rare-earth elements into nickel-aluminide (p-NiAl) coatings on superalloy turbine-blade materials. 

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. 

The NiAl intermetallic phases are of particular importance because of their use in strengthening Ni-based alloys, especially the superalloys used in high-temperature gas turbine blades (the Ni-Al phase diagram is shovm in Figure 12.12). Nickel aluminide (Ni-Al) is the simplest of these compounds. It is described by the Strukturbericht symbol B2, which tells us it has the CsCl structure, and the Pearson symbol cP2 tells us we have a cubic lattice with two atoms per unit cell. Therefore NiAl has Ni atoms on the comers of a cube and an A1 atom in the center (or vice versa). (Note this is not a bcc structure because the atom in the center is not the same as those on the comers.) The width of this phase in the phase diagram tells us that some solid solution is possible within the phase. 

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. 

Fig. 10. Schematic diagram showing the as-deposited structure of a low activity (left) and a high activity (right) diffusion aluminide coating on a nickel-base superalloy. The high activity coating would receive a heat treatment to convert the Ni2Al3 to NiAl.

Diffusion Zone f".g.ll. Schematic diagram of a platinum aluminide coating on a nickel-base superalloy. 

Table 2. Properties of alloys based on the titanium aluminides TijAl and TiAl compared with conventional titanium alloys and nickel-base superalloys (Morral, 1980, Lipsitt, 1985a Kim, 1989 Kim and Froes, 1990 Froes et al., 1991).

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). 

As mentioned in the preceding text, pack aluminizing is commonly carried out on nickel- and cobalt-base superalloys. Diffusion-coated superalloys develop an aluminide (NiAl or CoAl) outer layer with enhanced corrosion resistance. It is estimated that more than 90% of all coated gas turbine engine hot section blades and vanes made from superalloys are coated by pack cementation and related processes. Detailed information on protective diffusion coatings for superalloys can be found in Ref 24. 

Intermetallic hardening alloys, for example, nickel-base superalloys, austenitic stainless steels, and iron-aluminides... 

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