The Transition from Transient Oxide to Protective Al2O3 Scale on Fe–Cr–Al Alloys During Heating to 1000 °C

The transition behavior of an Al-rich amorphous oxide layer to an external Al2O3 layer on Fe–(4, 24)Cr–(6, 10)Al (at.%) alloys was investigated during heating to 1000 °C at a heating rate of 50 °C/min, by means of in situ high-temperature X-ray diffraction measurement and TEM observation. In the alloy containing 6Al, internal amorphous Al2O3 was initially developed below the Al-rich amorphous surface layer. The amorphous internal precipitates transformed to be crystalline and grew laterally with time. The internal precipitates subsequently connected with each other to form a continuous α-Al2O3 scale. In the case of 10Al alloy, an Al-rich amorphous layer transitioned to a crystalline α-Al2O3 layer from the interface between transient/amorphous layers during heating. The Al2O3 scale developed on high Al alloys contained Fe and Cr with relatively higher contents, but that formed on low Al alloy contained low Fe and Cr. The effect of Cr on promoting an external Al2O3 scale formation was found to be weaker for alloys with higher Al content compared to the alloys with lower Al content, if Al2O3 scale was directly transitioned from the amorphous layer.