Oxidation behavior of FeAl+Hf, Zr, B

The oxidation behavior of Fe-40Al-1Hf, Fe-40Al-1Hf-0.4B, and Fe-40Al-0.1Zr-0.4B (at.%) alloys was characterized after 900°, 1000°, and 1100°C exposures. Isothermal tests revealed parabolic kinetics after a period of transitional θ-alumina scale growth. The parabolic growth rates for the subsequent α-alumina scales were about five times higher than those for NiAl+O.1Zr alloys. The isothermally grown scales showed a propensity toward massive scale spallation due to both extensive rumpling from growth stresses and to an inner layer of HfO2. Cyclic oxidation for 200 1-hr cycles produced little degradation at 900 or 1000°C, but caused significant spaliation at 1100°C in the form of small segments of the outer scale. The major difference in the cyclic oxidation of the three FeAl alloys was increased initial spallation for FeAl+Zr, B. Although these FeAl alloys showed many similarities to NiAl alloys, they were generally less oxidation-resistant. It is believed that this resulted from nonoptimal levels of dopants and larger thermal-expansion mismatch stresses.