Al-Fe-O (Aluminum-Iron-Oxygen)

The early results on this ternary system were reviewed by [1989Rag], with emphasis on the phase equilibria at metal saturation. The review presented isothermal sections at 1500, 1380, 1280, 900 and 700 °C and a hypothetical liquidus projection and reaction scheme. Recently, [2004Han] and [2009Rha] investigated the pseudo-binary section in air along the Fe₂O₃-Al₂O₃ join.

Binary Systems

In the Al-O system, there is one intermediate compound corundum (αAl₂O₃; D5₁-type rhombohedral), which melts congruently at 2054 °C. The Fe-O phase diagram [1991Wri] exhibits the following compounds. Wustite (FeO) is a metal-deficient monoxide with the B1, NaCl-type cubic structure. Magnetite (Fe₃O₄) has the H1₁, Al₂MgO₄-type cubic structure. Hematite (αFe₂O₃) is D5₁-type rhombohedral. Other less-common polymorphic forms of the above compounds are known [1991Wri].

The Fe₂O₃-Al₂O₃ Pseudo-Binary Section in Air

With starting powders of Fe₂O₃ and Al₂O₃ of 99.5+% purity, [2004Han] pelletized powder mixtures and equilibrated them in air at temperatures between 1550 and 1100 °C. The microstructures were examined with optical and scanning electron microscopy. The phase compositions were measured by electron probe microanalysis (EPMA). With starting powders of 99.98% Fe₂O₃ and 99.99% Al₂O₃, [2009Rha] equilibrated a limited number of pellets between 1400 and 1200 °C and measured the compositions of the co-existing phases by EPMA. Figure 1 shows the pseudo-binary section in air along the Fe₂O₃-Al₂O₃ join, incorporating the results of [1956Mua], [2004Han] and [2009Rha]. The ternary phase Fe₂Al₂O₆ is stable between 1410 and 1318 °C [1956Mua]. The results of [2004Han] and [2009Rha] indicate a larger solubility of Al₂O₃ in hematite than that found by [1956Mua].

Fig. 1

Al-Fe-O pseudo-binary section in air along the Fe₂O₃-Al₂O₃ join