THE EFFECT OF CRYSTALLIZATION OF THE GRAIN-BOUNDARY PHASE ON AMBIENT-TEMPERATURE FATIGUE SHORT-CRACK GROWTH-BEHAVIOR IN Y-ALPHA'-BETA'-SIALON

The cyclic fatigue short-crack growth behaviour of Y-alpha'-beta'-Sial ons with both crystallized and amorphous grain-boundary phases, were i nvestigated to determine whether crystallization of grain boundaries a ffected crack-growth behaviour under cyclic and monotonic loads. Micro mechanisms for fatigue-crack growth in V-alpha'-beta'-Sialon were exam ined by scanning electron microscopy and high-resolution electron micr oscopy. These results show that the wear debris on the fatigue fractur e surfaces gave evidence of a frictional wear crack-growth mechanism. Comparison of fatigue short-crack growth rates for Sialon of crystalli zed grain-boundary phases with that for the amorphous grain-boundary p hases indicated that crystallization of grain-boundary phases does not appear to affect cyclic fatigue growth behaviour, similar to long-cra ck growth behaviour. The similarity of fatigue short-crack growth beha viour in both the crystallized and amorphous grain boundaries sialons is rationalized in terms of the thin residual amorphous grain-boundary regions.