Isothermal oxidation behavior of a sheet alloy of Fe–40at.%Al at temperatures between 1073 and 1473 K

Abstract

A sheet Fe–40at.%Al alloy with 200μm thickness was oxidized isothermally in air at temperatures between 1073 and 1473 K for up to 608.4 ks. The oxidation behavior was classified into three temperature ranges. At temperatures below 1223 K a mixture of α- and θ-Al 2O 3 grew fast, followed by slow oxidation with the formation of only α-Al 2O 3. The initial rapid oxidation is due to the formation of θ-Al 2O 3. Between 1273 and 1323 K the oxidation obeyed a parabolic rate law over the whole oxidation time with the formation of α-Al 2O 3 scale, except for the very initial stage of oxidation up to 3.6 ks. At temperatures above 1423 K the parabolic rate constants tended to decrease with oxidation time, while the oxide grain size of the columnar structure increased by 3–4 times. The change in the parabolic rate constants with time at 1473 K was reasonably well explained using the oxygen inward diffusion model along the grain boundary with a hypothetical width of 2 nm. The Fe–40Al showed the lowest oxidation rate and the activation energies for a parabolic Al 2O 3 growth are 422 kJ/mol above 1273 K and 241 kJ/mol below 1223 K. Oxide scale exfoliation was not observed for the Fe–40Al oxidized isothermally in air at the experimental temperatures here.