Using titanium dioxide (TiO 2 ) as a sintering additive is common practice in the refractory castable industry, as its lower melting point and ability to increase diffusivity in structures induce densification. In alumina (Al 2 O 3 ) and magnesia (MgO) castables, titania is capable of forming complex phases, such as spinels and pseudobrookites. The in-situ formation of these phases and the mismatch in their thermal expansion coefficients generate stresses that result in microcracks, increasing the material flexibility. However, there is a crucial trade-off: the higher densification promoted by TiO 2 additions increases mechanical strength and, consequently, reduces the structure's ability to accommodate stress, whereas microcracks generated by Ti-containing phases negatively impact mechanical strength, but increase the flexibility of the microstructure. This study aimed to optimize the TiO 2 content in the Al 2 O 3 −MgO−TiO 2 (AMT) system to find the ideal balance point that would maximize thermal shock resistance without compromising mechanical integrity. To do this, compositions were defined using computational thermodynamics (Calphad) and subsequently evaluated for their physical, mechanical, mineralogical, thermomechanical and microstructural features. Microcracking and flexibility of the castables were assessed by in situ elastic modulus and thermal shock resistance tests, respectively. The results showed that increasing TiO 2 content intensifies the formation of pseudobrookite, which increases crack formation at the matrix/aggregate interface, besides promoting interaction between pores and cracks. However, the addition of 1 wt% of titania proved to be the ideal content, enhancing the formation of short, controlled microcracks that effectively dissipate stress energy. As a consequence, the composition with 1 wt% of TiO 2 (80M-20T) retained 90.2 % of its initial elastic modulus after 10 thermal shock cycles (ΔT = 1000°C), while the flexural strength remained at 31 MPa, compared to 20 MPa for the sample without TiO 2 .
Reitmann et al. (Sun,) studied this question.