Aluminide layers

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. 

The results of this kind have been obtained by R. Tarento and G. Blaise when studying the reactions in thin-film nickel-aluminium couples" (see also Ref. 263). Using an ingenious variant of ion mass spectrometry, they were able to examine the nickel aluminide layers as thin, as 5 nm. For comparison, the lattice spacings of the Ni-Al intermetallics lie in the range 0.3-0.7 nm.142 214... 

The formation of silicides in reaction couples, for example, of the MesAl-SiC type, where Me is a transition metal, is more complicated. In this case, in addition to the Me2Si layer, the MeAl layer (or some other aluminide layer) also grows, i.e. the Me3Al-MeAl-(Me2Si+C)-SiC system is formed. The mechanism of its occurrence is probably as follows. The Me3Al phase is decomposed at the Me3Al-MeAl interface by the reaction... 

Permeation reduction factors of up to 10,000, or 10 , have been realized with the best coatings based on aluminized steels. Ferritic-martensitic steels that were aluminized had the Fe2Al5 phase predominant in the layer sequence, while a 316L steel had FeAlj and FeAl2 as the main aluminide phases. The best permeation barrier resulted from an external alumina film of about 1 micron in thickness grown on the aluminide layers. ... 

FIGURE 5.7 Optical micrographs of the reaction products between Ni and Al. (a) Duplex phases of nickel aluminides formed after a treatment of 1000°C for 1 hour in vacuum (b) a cracked nickel aluminide layer formed after being treated at 640°C for 1 hour in vacuum and (c) a cross-section of a nickel and zirconia joint which was joined together through a nickel aluminide layer at a temperature of 680°C in vacuum. N nickel, R nickel aluminide, A aluminium, and C zirconia. (Reprinted from Mei, J. and Xiao, R, Joining metals to zirconia for high temperature applications, Scripta Materialia 40 (1999) 587-594, with permission from Elsevier Science.)... 

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 aluminizing process for alloy 718 was scaled up from laboratory scale to actual scale for treating the strips for manufacture of PFBR steam generator tube bundle support structures. Aluminizing was carried out using the low activity pack process. The process cycle was standardised and uniform aluminide layer of 80 m thickness was obtained over 600 mm long strip. 

Figure 14.9 shows the oxidation behaviour of Ti-45Al-8Nb specimens coated with different aluminide layers and TBCs under cyclic and quasi-isothermal oxidation conditions. For comparison, mass change data of preoxidised specimens with TBC are included. No spallation of the thermal barrier coatings was observed after 1000 cycles at 900 C for specimens coated with the two types of aluminides investigated. Drops in mass change... 

The life of gas turbine blades is improved by platinum and/or rhodium, applied below or above, or co-deposited with, aluminised, thermal-barrier or AfCrAlY-type layers. The performance of modified aluminides was demonstrated in long-term engine trials . ... 

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

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. 

The growth kinetics of compound layers during the oxidation of zir-conium aluminides have been described in detail by M. Paljevic. 

The important aspect of coating any of the titanium aluminides is that the coating prevent interstitial embrittlement and that the coating does not function as an embrittlement layer. Additional studies are necessary in this area. 

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