Microstructure and Creep Behavior of Fe-27Al-1Nb Alloys with Added Carbon

Abstract

The effect of Nb and C additions on the phase composition, microstructure, and creep resistance of Fe3Al-type alloys is investigated. Two alloys, which contained (at. pct) (i) 27.6 Al, 1.15 Nb and 0.19 C (Fe balance) and (ii) 27.1 Al, 1.11 Nb, and 0.76 C (Fe balance), were studied in a temperature range from 873 K to 1073 K (600 °C to 800 °C). The carbide in both alloys was identified as Nb6C5. The creep data can be rationalized by introducing a threshold stress, below which, the creep rate is negligible. The threshold stress and an effective stress exponent were found simultaneously by a numerical method. Using the obtained values of the threshold stress, the activation energy of creep was determined to be 328 kJ/mol. The effective stress exponent varied from 2.0 to 3.1. A breakdown of power-law behavior was observed at higher stresses. The transition occurred at the normalized creep rate of \( \dot{\varepsilon }/D = 10^{13} \,{\text{m}}^{ - 2} \), which agrees with the rule suggested by Sherby and Burke and the diffusion coefficient D corresponding to the diffusion of Al in Fe-Al.