The effects of mixtures of Ni–Ta and Ni–Zr powders combined with sintered solid solution TaB2 –NbC during spark-plasma sintering of compositions at a pressing loading of 60 MPa in the range 1200 – 1600°C on the phase composition, percent content of Ti, Ta, and Zr in the sialon; microstructure; grain sizes of crystalline phases; relative density; linear shrinkage; physical-mechanical properties; linear correlation of the elasticity modulus and fracture toughness of mullite–β–SiAlON–TiC and mullite–β–SiAlON–c-ZrO2 samples are shown. Synthesized powders of β-SiAlON and TiC are characterized by extensive crystallization of β-SiAlON and TiC. c-ZrO2 spark-plasma sintered at 1400°C and solid solution TaB2 –NbC, at 1800°C, show extensive crystallization of c-ZrO2 and (Nb,Ta)C, B phases. The microstructure of solid solution TaB2 –NbC is crystalline, partially nonuniform, and almost completely sintered. Sintered samples with Ta and Zr additives show extensive mullitization; active crystallization of β-SiAlON, (Nb,Ta)C, B, and Ni(Nb,Ta); and varying crystallization of NiTi, NiTa, NiZr, and NiZr2 phases in the range 1200 – 1600°C. The microstructures of ceramic phases of samples with Ta additive are more uniformly and densely sintered. Particles of NiTi and NiTa in samples with Ta additive are uniformly and densely packed, unlike particles of NiTi, NiZr and NiZr, NiZr2 in samples with Zr additive. Sintering of compositions with Ta additive proceeds uniformly and extensively with formation of polydisperse compositions of crystalline phase grains. Samples with Ta additive show active growth and larger values of physical-mechanical properties, higher crack resistance, and better linear correlation of elasticity modulus and fracture toughness in the range 1200 – 1600°C.