Zirconia materials are frequently used in dental applications due to their excellent strength and their tooth-like aspect. Standard yttria stabilized zirconia (Y-TZP) ceramics suffer, however, from moderate toughness and vulnerability to low-temperature degradation. In this study, 1Y6Ce-TZP materials reinforced with different amounts of alumina and/or strontium hexaaluminate were manufactured by slip casting and pressureless sintering at different temperatures to assess their mechanical properties, microstructure, phase composition, and low-temperature degradation stability. Results show that these materials exhibit a high fracture resistance of 10–12 MPa√m, a bending strength between 700–950 MPa, and a Vickers hardness of 1100–1200 HV10. Strontium hexaaluminate (SA6) precipitates were formed in situ by reaction of alumina and strontium zirconate. Although crack deflection at SA6 platelets was clearly visible, a net toughening was not observed. Accelerated ageing tests at 134 °C/3 bar water vapor pressure showed best results for mixed alumina/SA6 reinforcements and a sintering temperature of 1500 °C. Mehl-Avrami-Johnson plots used to describe the ageing kinetics showed clear indications of different ageing mechanisms due to the introduction of the SA6 phase.