Effect of TiO2 on phase evolution and microstructure of MgAl2O4 spinel in different atmospheres

The effect of TiO2 on the formation and microstructure of magnesium aluminate spinel (MgAl2O4) at 1600 °C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl2O4 spinel shifted toward alumina-rich types owing to volatilization of MgO, resulting in an increase in the porosity of fired samples. Addition of graphite to mixtures of MgO and Al2O3 intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl2O4. For TiO2-containing samples on addition of MgAl2O4, magnesium aluminum titanium oxide (MgxAl2(1−x)Ti(1+x)O5, x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO2-containing samples than for samples without TiO2 owing to the formation of a solid solution of TiO2 in MgAl2O4 and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl2O4 spinel, corundum, magnesium orthotitanate spinel (Mg2TiO4) and amorphous phases were identified. Mg2TiO4 spinel formed a complete solid solution with MgAl2O4 spinel but Mg2TiO4 remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO2 containing samples. The results are discussed with emphasis on the application and design of alumina–magnesia–carbon refractory materials, which are used in the steel industry.