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

Abstract The effect of TiO 2 on the formation and microstructure of magnesium aluminate spinel (MgAl 2 O 4 ) at 1600 °C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl 2 O 4 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 Al 2 O 3 intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl 2 O 4 . For TiO 2 -containing samples on addition of MgAl 2 O 4 , magnesium aluminum titanium oxide (Mg x Al 2(1− x ) Ti (1+ x ) O 5 , x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO 2 -containing samples than for samples without TiO 2 owing to the formation of a solid solution of TiO 2 in MgAl 2 O 4 and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl 2 O 4 spinel, corundum, magnesium orthotitanate spinel (Mg 2 TiO 4 ) and amorphous phases were identified. Mg 2 TiO 4 spinel formed a complete solid solution with MgAl 2 O 4 spinel but Mg 2 TiO 4 remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO 2 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.