FORMATION OF ALN-POLYTYPOID PHASES DURING ALPHA-SIALON DECOMPOSITION

Phase transformations from (alpha- to beta-SiAlONs (i.e., from alpha' to beta') have been recently reported in a number of rare-earth SiAlON systems during postsintering heat treatment, In the present work, thi s transformation process in a Sm (alpha+beta)-SiAlON material is studi ed by using XRD, TEM, and EDS X-ray mapping techniques. It is observed that in addition to the formation of beta' and M' phases, the alpha'- to-beta' transformation is accompanied by a significant increase in th e amount of an AlN-polytypoid phase, The results suggest that some alp ha' phases are thermodynamically unstable at temperatures lower than t he material sintering temperature and will decompose when conditions a llow, For the composition studied in this work, the alpha-SiAlON decom position can be described in general as (alpha' --> beta' + M' + AIN p olytypoid.