MICROSTRUCTURAL FEATURES OF THE ALPHA-SIALON TO BETA-SIALON PHASE-TRANSFORMATION

The microstructural characteristics of Sm (alpha + beta) -SiAlON ceram ics after post-sintering heat treatment at 1450 degrees C are investig ated by analytical electron microscopy (AEM). X-ray diffraction (XRD) analyses indicate that a significant amount of the alpha-SiAlON (alpha ') phase has transformed to beta-SiAlON (beta') during the heat treatm ent. It is found that the transformation is nucleated on the existing beta' phase and the transformed beta' grains have unique micro-structu ral features containing a high density of dislocations and ultra-fine spherical inclusions rich in Sm and O. Similar characteristics have al so been observed in the Yb (alpha' + beta') materials. It is suggested that the transformation from alpha' to beta' proceeds via a nucleatio n-growth mechanism and may require only a small amount of liquid phase to promote atomic diffusion.