The paper analyzes the effect of nanoadditives of zirconium dioxide, partially stabilized by Y2O3, on the Al2O3–SiO2 matrix during hot pressing by the electroconsolidation method. The microstructure obtained at different compositions and sintering modes is studied. It was established that the introduction of nanopowder ZrO2–3 mol % Y2O3 increases the crack resistance of sintered samples not only due to the transformation of the tetragonal phase to the monoclinic phase, but also due to the formation of a solid solution with aluminum oxide at the interphase boundaries. Starting from a temperature of 1400 °C, complete densification occurs with the formation of nonporous composites while preserving the nanostructure. At this temperature, the formation of mullite with cristobalite phases also begins. The high value of thermocycles makes it possible to use this composite material as a cryogenic one. The fabrication of mullite-corundum composites with additions of ZrO2–3 mol % Y2O3 nanopowders via the method of electrosolidification has enabled the attainment of high fracture toughness, K1C = 14.5 MPa⋅m1/2, and hardness HV10 = 14 GPa. These results evidence excellent mechanical properties, thereby expanding the potential applications of this material.