Phase relationships and stability of α′-SiAlON

Phase diagrams on the alpha'-SiAlON plane have been investigated for ytterbium-, yttrium-, and neodymium-containing systems at different temperatures. Phase relationships between alpha'-SiAlON, beta'-SiAlON, AlN polytypoids, and liquid/grain-boundary phases have been determined. The stability region of the alpha'-SiAlON phase increases as the temperature increases and the ionic size of the rare-earth cation decreases. Meanwhile, the solubility of aluminum and oxygen in beta'-SiAlON varies with the solubility of aluminum and oxygen in alpha'-SiAlON, which causes migration of all the phase lines on the alpha'-plane toward the nitrogen-rich side at lower temperatures and with larger modifying cations, These systematic variations of the phase relationships fully explain the occurrence of alpha' --> beta' reverse transformations that are sometimes observed in this system.