PHASE-TRANSFORMATIONS IN SM (ALPHA-SIALON CERAMICS DURING POSTSINTERING HEAT-TREATMENTS(BETA))

Phase transformations in Sm (alpha + beta)-SiAlON ceramics during post -sintering heat treatments at 1300 and 1450 degrees C were investigate d using XRD, SEM, TEM and EDS techniques. The transformation from alph a to beta SiAlON, (i.e. alpha' to beta') phases was observed at both t emperatures. However, other phase transformations occur simultaneously at grain boundaries of the materials and also involve the beta' phase , namely, Glass --> SmAlO3 + beta' and SmAlO3 + beta' --> M'. Their ef fects on the amount of beta' in the materials must therefore be consid ered in the study of the alpha' to beta' transformation. It is thought that the increase in the beta':(alpha' + beta') ratio during a heat t reatment process depends not only on the rate of the alpha' to beta' t ransformation, but also on the amount and composition of the intergran ular phases.