Melilite Formation in a Samarium‐Stabilized α‐Sialon Ceramic during Postsintering Heat Treatments

The formation of the melilite solid solution phase (M'), Sm 2 Si 3−x Al x O 3+x N 4−x , in an α‐sialon sample of overall composition Sm 0.6 Si 9.28 Al 2.69 O 1.36 N 14.76 , was studied as a function of time in the temperature interval 1375–1525°C. The alpha‐sialon ceramic contained only minor amounts of the 21R sialon polytype and some residual grain‐boundary glass before heat treatment. In situ studies by high‐temperature X‐ray diffraction were combined with postsintering heat treatment followed by quenching. The M'‐phase was found to be formed by two different mechanisms: either crystallization of the residual grain‐boundary liquid or a direct decomposition of the α‐sialon phase. The liquid crystallized during the first 10–15 min of heat treatment, yielding a rapid M'‐phase formation, and further formation of M'‐phase continued at a much slower rate, related to the decomposition of α‐sialon.