High alumina-forming alloys

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%. 

Since the present day TBCs are used in conjunction with alumina-forming alloys, this imposes an additional constraint on the selection of prospective coatings. They must be thermodynamically stable with alumina at high temperature. Although a great effort has been made in searching out low thermal conductivity materials for high temperature applications, there are three difficulties in selection of candidate materials for TBCs. First, an effective model is needed to understand and... 

Industrial attention focuses mainly upon iron-base (Fe-Cr—Al) alumina-forming alloys. For gas-turbine applications, AI2O3 scale formation is preferred, but the high-strength turbine-blade alloys do not possess adequate oxidation resistance and hence they must... 

J.R. Nicholls, Life Extension of alumina forming alloys - background, objectives and achievements of the BRITE/EURAM Programme LEAFA, in Lifetime Modelling of High Temperature Corrosion Processes, EFC Monograph No. 34, The Institute of Materials, London, 3-15 (2001). 

B. A. Pint, P. F. Tortorelli and I. G. Wright, Effect of cycle frequency on high temperature oxidation behavior of alumina-forming alloys. Oxidation of Metals, 2002, 58, 73. 

Table 70.2 Transport mechanisms established from isotopic marker experiments during high temperature oxidation of alumina-forming alloys...

Pint B A, Tortorelli P F and Wright I G (2002), Effect of Cycle Frequency on High Temperature Oxidation Behavior of Alumina-Forming Alloys, Oxid Met, 58, 73-101. 

Pint B A, More K L and Wright I G (2003), The Use of Two Reactive Elements to Optimize Oxidation Performance of Alumina-Forming Alloys Mater High Temp, 20,... 

Pint B A, Walker L R and Wright I G (2005), Characterization of the Breakaway Al Content in Alumina-Forming Alloys, Mater High Temp, 21, 175-185. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

One of the most serious effects of water vapour on high-temperature oxidation is the increased spalling tendency of AI2O3 and Cr20s scales. This effect is illustrated for the cyclic oxidation of the alumina-forming superalloy CMSX-4 in Figure 7.4. The water vapour is believed to lower the fracture toughness of the alloy-oxide interface. 

From Fig. 7-1 it can be seen that after silica forming materials, those materials with the next most protective oxide scale would be alumina formers. Since alumina scales are less sensitive to impurities, AIN and AI4C3 appear, at first glance, to be ideal high temperature oxidation resistant materials. However, the recession rates in this figure are based on those observed for alumina forming metal alloys. Oxidation of both of these ceramic materials results in the formation of gaseous products which can alter the protective qualities of the alumina scale. 

From 650°C, chromia-forming nickel-based alloys exhibit better corrosion resistance than austenitic steels even with similar chromium content [48]. Indeed, the formation of chromium-rich scale appears to be easier [44,48] and they resist better against carburization [47,48]. The use of alumina-forming nickel-base alloys may be an alternative solution at the highest temperatures, typically >750°C [49]. The consequences of carburization on the mechanical properties at high and low temperature depend on the extent of the damaged zone [47]. 

Wilber J P, Bennett M J and Nicholls J R (2000), Life-time Extension of Alumina Forming FeCrAl-RE Alloys Influence of AUoy Thickness, Mater High Temp, 17, 125-132. 

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