The effect of oxygen partial pressure on the phase transformation of Al2O3 scale on various Fe–Al alloys with and without very thin (∼100 nm) Fe coating was investigated. Fe‐coating on Fe–Al alloys can effectively suppress metastable Al2O3 formation, but little effect was observed when the samples were oxidized in a low partial pressure of oxygen. Under the low ${\rm P}_{{\rm O}_{{\rm 2}} } $ atmosphere, metastable to stable α‐Al2O3 scale phase transformation on both Fe‐coated and non‐coated Fe–Al alloys was significantly delayed. The lattice spacing of α‐Al2O3 scale formed in air decreased with increasing alloy Al content. Further decrease in the lattice spacing of α‐Al2O3 scale was observed when the alloy was oxidized in low ${\rm P}_{{\rm O}_{{\rm 2}} } $. The results obtained clearly indicated that the formation of Fe2O3 or Fe3+ in metastable Al2O3 accelerated the metastable to stable α‐Al2O3 scale transformation.

Abstract. The oxidation behavior of Ni-Fe-41.5Al alloy with different Fe/Ni ratio was investigated in air at 1000°C using in-situ high-temperature X-ray diffraction study by means of Synchrotron radiation. The oxidation mass gain of alloys decreased with increasing Fe content, particularly initial oxidation mass gain significantly decreased with increasing alloy Fe content. In-situ X-ray diffraction study indicated that the stable -Al 2 O 3 formed from beginning of oxidation on the alloy with higher Fe ratio, but metastable Al 2 O 3 formed on the alloy with lower Fe ratio, and it remained longer when alloy Fe ratio was decreased. The effect of Fe on promoting -Al 2 O 3 formation is explained due to initial formation of -Fe 2 O 3 , which has isomorphous structure with -Al 2 O 3 . The additional effect of Fe on the growth rate ofAl 2 O 3 was also discussed.

The oxidation behavior of Ni-Fe-41.5at.%Al alloys with different Fe/Ni ratios was investigated in air at 1000°C in order to clarify the effect of Fe on the phase transformation of Al 2 O 3 scale, using in-situ high-temperature X-ray diffraction by means of synchrotron radiation. The oxidation mass gain of alloys after 25 h of oxidation generally decreased with increasing Fe content; however, the initial oxidation mass gain was significantly decreased by increasing alloy Fe content. Insitu X-ray diffraction analysis indicated that higher alloy Fe contents promoted rapid formation of the stable a-Al 2 O 3 , while lower Fe in the alloy maintained the metastable Al 2 O 3 for longer time oxidation. The effect of Fe on promoting a-Al 2 O 3 formation can be explained by the initial formation of a-Fe 2 O 3 , whose structure is isomorphous with a-Al 2 O 3 . The additional effect of Fe on the growth rate of a-Al 2 O 3 is also discussed.