Effect and Mechanism of Alumina on the Morphology and Mechanical Properties of Calcium Ferrite

Al 2 O 3 greatly influences the formation of complex calcium ferrite, which is the main binder and iron-bearing phase in sinter. Experiments were carried out in air at 1200 °C with different amounts of Al 2 O 3 mixed with CaO·Fe 2 O 3 (calcium ferrite). The samples contained two phases: CaFe 2 O 4 and Ca 3.18 Fe 15.38 Al 1.34 O 28 (CFA). The results showed that the morphology of the complex calcium ferrite was massive ( w (Al 2 O 3 ) = 0 pct), columnar and plate-like ( w (Al 2 O 3 ) = 0.5 and 1 pct), and acicular and plate-like ( w (Al 2 O 3 ) = 2, 3, and 5 pct). A small amount of Al 2 O 3 solid solution in CaFe 2 O 4 increased the preferred orientation of the crystal. The gradual increase in Al 2 O 3 in the sintered samples caused the complex calcium ferrite composition to be closer to that of silico-ferrite of calcium and aluminum (acicular SFCA-I). When Al 2 O 3 dissolved into calcium ferrite, complex calcium ferrite was formed, which increased the hardness of the sample. The fracture toughness of the sample increased with increasing Al 2 O 3 content from 0 to 2 pct, possibly due to a decrease in grain size. When the Al 2 O 3 content exceeded 3 pct, Al 2 O 3 dissolved into calcium ferrite and decreased the fracture toughness of the sample.