Phase Relations in the Si3N4-Rich Portion of the Si3N4–AlN–Al2O3–Y2O3

The phase relations in the Si₃N₄-rich portion of the Si₃N₄–AlN–Y₂O₃ rystem were investigated using hot-pressed bodies. The one-phase fields of β3 and α, the twophase fields of β+α, β+M (M=melilite), and α+M, and the three-phase fields of β+α+M were observed in the Si₃N₄-rich portion. The α- and β-sialons are not two different compounds but an allotropic transformation phase of the Si–Al–O–N system, and an α solid solution expands and stabilizes with increasing Y₂O₃ content. Therefore, the formulas of the two sialons should be the same.