The driving force for α⇄β transformation in rare earth α sialon ceramics

In-situ α⇄β sialon transformation in rare earth sialon systems, which occurs when dense α- and α/β-sialon samples are heat-treated at temperatures in the range 1300-1600°C was first observed five years ago, and has generated a considerable level of research activity aimed at defining the reason(s) why the transformation occurs. A mass of experimental data is now available, and many factors have been identified which influence the reaction. However, the exact reason for the transformation is still not entirely clear, and in the present paper, various explanations which have been suggested during the last five years are discussed and the current position evaluated.