Diffusion aluminides

Diffusion aluminide and silicide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 106/yr in North America and about 50 x 106 worldwide. 

The presence of Y in the coatings improves the oxidation resistance of AI2O3 scale. Thus, MCrAlY coatings outperform diffusion aluminides significantly in oxidation tasks as shown in Table 12.8. 

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.

Figure 10.22 Plot of fracture strain vs. temperature for diffusion aluminide and Co-Cr-Al-Y coatings showing the wide range of properties achievable in the M-Cr-Al-Y coatings relative to those achievable in the aluminides.

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. 

Pack cementation as well as fluidised bed chemical vapour deposition have been extended to temperatures below 715°C. Homogeneous diffusion aluminide Fe2Al5 coatings of up to 50 pm thick have been obtained by pack cementation. FBCVD has shown a higher success for the Al-Si co-deposition. Further improvement is, though, required for the Si deposition as well as the pack cementation codiffusion at low temperatures. 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

As concluding remarks about these techniques, their increasing interest and the advantages of their combination with other techniques, we may mention, as an example, that within the European research project COST 535, concerning the Thermodynamics of Alloyed Aluminides , a meeting (Diisseldorf, December 2004) was dedicated to The Diffusion Couple Technique , presenting the principles of the method and the results obtained in the examination of several alloy systems. Zhao (2004) has developed an efficient variant of the diffusion couple technique (the diffusion multiple approach ). 

As a conclusion, notice for all the nine metals of these groups, the large diffusion of the CsCl-type-based (binary and complex) compounds and the solid solutions formed with metals of various groups. Among these, several aluminides can be of interest, possibly also as additional components in high temperature applications. 

Turbine blades of jet engines are coated with a protective layer of platinum aluminide to impart high temperature corrosion resistance. Platinum is electroplated onto the blade using P-salt or Q-salt electroplating solutions (28,29). The platinum is then diffusion-treated with aluminum vapor to form platinum aluminide. Standards for the inspection and maintenance of turbine blades have become more stringent. Blades are therefore being recoated several times during their lifetime. 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

Note that in the framework of purely diffusional considerations any diffusing atoms are assumed to be available for any growing compound layer. In other words, the existence of any interface barriers to prevent diffusion of appropriate atoms is not recognised. From this viewpoint, it would be more logical to compare the diffusion coefficients of aluminium, as the more mobile component, in all the titanium aluminides. In such a case, the absence of most aluminide layers becomes quite unexplainable. It is highly unlikely that the diffusion coefficients of aluminium in different titanium aluminides are so different as to exclude the formation, say, of the TiAl2 layer. 

Titanium aluminides are ordered intermetallics and hence have lower diffusivity and high elastic modulus. These compounds are stronger than the conventional titanium alloys and are suitable for high temperature applications. But these compounds have low ductility due to the planarity of slip in these compounds. 

The large difference in stability between NiO and A1203 makes nearly all Al-Ni alloy compositions in equilibrium with A1203, as shown by the wide A1203 stability region in Fig. 10, even though numerous very stable nickel aluminides can form. Once again, a balance between the alloy liquidus surface composition, matrix formation rate, and the diffusion of transition metal away from the reaction front must be maintained. Typically the presence of a reducible compound in the preform, especially if it is the sole constituent, results in a further refinement of the composite microstructure. 

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

In order to prevent the rapid diffusion a thin Ni layer was electroplated on a TiAl specimen before aluminising [44], However, this trial was not successful, because the diffusion of Ti through the nickel aluminide layer formed was rather fast. 

The diffusion coefficient of aluminium in a ferritic iron-aluminium matrix is some orders of magnitude higher than in austenitic Ni3Al. Because of the low diffusion coefficient, diffusion in nickel aluminides is slow and aluminium depletion beneath the alumina scale and formation of nonprotective nickel oxides has been observed in Ni3Al. [4]. 

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