Alumina nickel aluminides

There should be no significant difference between alumina scales formed on nickel aluminide and those formed on iron aluminide. Compared to Ni3Al, however, Fe3Al has some major advantages ... 

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%. 

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. 

Aluminides based on the intermetallic phases Ni3Al and Fe3Al are considered both as structural materials and as coatings for high temperature applications [1-6]. Their excellent corrosion resistance is due to their forming a dense, protective alumina scale. Alumina, especially ot-Al203, shows low rate constants even at temperatures above 1000°C [7]. Unlike chromia, which is formed on conventional stainless steels and nickel base alloys, alumina does not evaporate above 1000°C [8] and it is even stable in oxygen deficient atmospheres. 

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