Formation of AlN-Polytypoid Phases during α-SiAlON Decomposition

Phase transformations from α– to ß–SiAlONs (i.e., from α′ to ß′) have been recently reported in a number of rare‐earth SiAlON systems during postsintering heat treatment. In the present work, this transformation process in a Sm (α+ß)‐SiAlON material is studied by using XRD, TEM, and EDS X‐ray mapping techniques. It is observed that in addition to the formation of ß′ and M′ phases, the α′‐to‐ß′ transformation is accompanied by a significant increase in the amount of an AlN‐polytypoid phase. The results suggest that some α′ phases are thermodynamically unstable at temperatures lower than the material sintering temperature and will decompose when conditions allow. For the composition studied in this work, the α‐SiAlON decomposition can be described in general as α′→ß′+ M′+ AlN polytypoid.