Aluminium nickel aluminides

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... 

NijAl and NiAl can be produced through a termite reaction between nickel and aluminium at a relatively low temperature. Nickel aluminides were produced between the zirconia and the metallic component to achieve the vacuum-tight joining. The morphology, composition, and density of the metal-zirconia joint have been optimized to minimize the GTE mismatch at the metal-ceranuc interface, and they are worth considering in detail as they could be used for sensor applications at temperatures up to 1000°C. 

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.)... 

Nickel aluminides containing more than 25 at.% A1 undergo severe attack by rapid internal oxidation of aluminium. y -NijAl shows no internal oxidation but internal sulfides are formed. 

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]. 

As expected, no carburisation attack at all was detected on iron-aluminium-chromium alloys after 1000 hours exposure in CH4/H2 environments at 850°C, 1000°C and 1100°C. Since the formation of chromia and iron requires relatively high oxygen partial pressures, alumina is the only stable phase at the low partial pressure of the used gas. If once formed, alumina is impervious to carbon, provided the scale remains intact [20], Excellent resistance to carburisation was also found for other alumina forming alloys like nickel aluminides [21] and Ni-Al-Cr alloys [22], The results of the present work show that 10 wt% aluminium are sufficient to prevent carburisation. It is expected, that the minimum aluminium concentration is even lower than 10 wt%. 

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. 

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