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.