Exposure iron aluminides

Examination of the specimens after exposure confirmed that thin protective scales had developed and were retained on the iron aluminides and on the other alloys, except where large weight gains were recorded, in all three environments. The scales on the iron aluminides at 700°C were essentially A1203, as confirmed by X-ray diffraction... 

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%. 

Several interesting features were apparent when the aluminide specimens were observed in plan after exposure at 700°C (Figure 3). In all cases, there was little evidence for loss of scale by spallation. However, second-phase particles were observed on all the surfaces, particularly of the lower aluminium-containing alloys, FA 56 and FA 57, while the number and size of such particles for a given alloy were less for preoxidized specimens than for those exposed directly in the mixed gas. Detailed examination and EDX analysis of the particles (e. g. labelled, C, A and B respectively in Figures 3(c), 3(d) and 3(f)) indicated them to be rich in iron and sulphur although too small for precise analysis, they were undoubtedly sulphide nodules. These nodules were always small and discrete, with no evidence that extensive sulphidation of the substrate had... 

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