Effect of Al content on oxidation behaviour of ternary Fe–Al–C alloys

Abstract

Iron aluminides produced by the electroslag refining technique, having the compositions: (1) Fe-16Al-1C, (2) Fe-10Al-1C, and (3) Fe-8Al-1C were used to investigate the effect of Al on the oxidation behaviour of the Fe-1C-Al system at 700 to 1000 C. Prior to oxidation studies, phase and microstructure of alloys were analysed. The carbide phase, Fe 3 AlC 0.69 , was found to be distributed in the Fe 3 Al matrix in alloy 1 and (Fe-Al) matrix in alloys 2 and 3. The low Al content alloys displayed inversion in the oxidation kinetics below 800 C, while, high Al content alloy displayed inversion phenomena at 1000 C. The mechanism involving inversion in oxidation kinetics was found to be different in the two cases. In the former, it was attributed to the preferential oxidation of Al, while in the latter, to the phase transformation within the Al 2 O 3 . Carbides in the alloy having low Al content showed instability during oxidation.

Scanning Auger electron spectroscopy study of the oxide film formed on dendritic and interdendritic regions of C containing Fe3Al intermetallic

“…In the first study we have shown that the addition of carbon (0.14-1%) has only moderate effect in the temperature range 700-900°C on the oxidation behaviour of these alloys, but it is responsible for raising the spallation tendency of the alloys at 1000°C [13]. In the second and third communications, we found that the carbide phase is more prone to oxidation in high-Al content alloys, whereas in low-Al content alloys it is the matrix phase [14,15]. Previous papers [13][14][15] are concerned with long-term oxidation behaviour of these dual phase iron aluminides, with variation of C and Al content from one alloy to another.…”

“…Oxidation behaviour of Fe 3 Al and FeAl phases has been well documented by several investigators [9][10][11][12], but little was known on the oxidation behaviour of carbon containing iron aluminides until the oxidation studies of these alloys initiated in our laboratory. In the recent communications, we have reported the effect of carbon on long-term oxidation behaviour of Fe 3 Al [13] and the effect of Al on the oxidation behaviour of Fe-Al-C alloys [14,15]. In the first study we have shown that the addition of carbon (0.14-1%) has only moderate effect in the temperature range 700-900°C on the oxidation behaviour of these alloys, but it is responsible for raising the spallation tendency of the alloys at 1000°C [13].…”

Long-term sulfidation behaviour of Fe3Al-Fe3AlC0.69 iron aluminides

“…10c). Once the activity of Fe comes down in the matrix due to its oxidation, Al starts getting preferentially oxidized and formation of this Al 2 O 3 slows down the rate of sulfidation, as has been explained during the oxidation process of these alloys [8,9]. Therefore, any change in the sulfidation resistance of these alloys due to the lowering of Al content is imputed to the matrix phase rather than the carbide phase.…”

“…9). Earlier we pointed out that stoichiometry of the carbide phase remains the same at 0.377 nm even on reducing the Al content [8,9]. Therefore, lowering the Al content down to 10% (alloy 4), leads to depletion of Al content from the matrix phase, whereas the carbide phase remains the same, in order to maintain stoichiometry of the carbide.…”

“…As the carbon addition has improved the mechanical properties, it is pertinent to know its high temperature behaviour in various environments in order to use these alloys for those applications. In preceding papers we have discussed the effect of C on the oxidation behaviour of the iron aluminides [7] and effect of Al on the oxidation behaviour of Fe-Al-C alloys, respectively, in the range of 700-900 C [8,9]. In many service environments, sulfur co-exists with oxygen and carbon.…”