Micromechanisms of creep-fatigue crack growth in α′-β′-SiA1ON at 1200°C

The cyclic fatigue crack growth characteristics of gas pressure sintered (GPS) Y-α′-β′-Sialon at 1200°C were examined by 'controlled' surface cracking from Vickers indentations in bars loaded in four-point bending. The stress intensity factor for semielliptical surface cracks in bending was calculated in terms of the stress and dimensions of the subcritically growing cracks. For comparison, static fatigue crack growth experiments were also carried out on the material. Similar to the fatigue crack growth behaviour of Si₃N₄ at ambient temperature, the high-temperature static and cyclic fatigue crack growth rates showed three characteristic regions having a plateau with a lowest Paris exponent. In the lower stress intensity factor region, cyclic fatigue crack growth rate was slower than that of static fatigue at the same stress intensity factor. In the higher stress intensity factor region, the crack growth rates, both static and cyclic fatigue, were almost the same. Electron microscopy analysis in the crack-tip damage zone indicated that, in addition to the subcritical crack growth of the main crack assisted by oxidation, creep damage mechanisms such as cavity formation and microcracking may be responsible for static fatigue crack growth in present study. However, it is not significant in the case of cyclic fatigue. Besides, deformation process such as dislocation activity within the matrix grains appeared to play an active role in determining the crack growth kinetics for cyclic loading. It seems that the fatigue stresses will help aid the crystallization process of the amorphous phase and lead to increase the role of dislocation plasticity during high temperature fatigue fracture.