Phase Relationships and Stability of α′‐SiAlON

Abstract

Phase diagrams on the α′‐SiAlON plane have been investigated for ytterbium‐, yttrium‐, and neodymium‐containing systems at different temperatures. Phase relationships between α′‐SiAlON, β′‐SiAlON, AlN polytypoids, and liquid/grain‐boundary phases have been determined. The stability region of the α′‐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 β′‐SiAlON varies with the solubility of aluminum and oxygen in α′‐SiAlON, which causes migration of all the phase lines on the α′‐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 α′→β′ reverse transformations that are sometimes observed in this system.