α⇋β phase transformations in silicon nitride and sialons

α⇄β structural transformations in silicon nitride are reconstructive and so only occur via a liquid phase. Because α and β sialons differ in composition, the transformation is not only reconstructive but is also chemically controlled. Research at Newcastle and Stockholm shows that the transformation is reversible and can be effected by heat-treatment without further addition of oxides or nitrides. Subsequent work has attempted to assess the roles of many variables. It is now proposed that a key factor, the α⇄β transformation temperature, T_αβ, varies widely with the sialon system and also with the composition within a system. Intrinsic parameters, such as the concentration, valency and size of the interstitial valency-compensating cation, determine the extent of Si-N replacement by A1-N and hence the lattice strain in α and the free energy change of α→β. Extrinsic parameters such as the amount, composition and viscosity of liquid phases, the occurrence of β seed crystals, and the formation of grain-boundary crystalline phases, may modify the rates and extent of the transformation. In both silicon nitride and sialons there are still unexplained features of the α⇄β transformations.