Creep cavitation damage

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

Fig. 6.97 High-magniflcation view of creep damage in an internal section of a tensile sample fractured of ARCO at 82 MPa and 1250 C a near a microcrack and b typical general cavitation damage. Bars = 10 pm [93], With kind permission of John Wiley and Sons...

The bulk of the results obtained in this study were obtained by analytical transmission electron microscopy. Observations were made on both grades of material in the as-received, untested condition and after tensile testing at 1250°C. Test samples selected for examination covered the range of observed creep behavior, and included samples that failed after times ranging from -10 to -200 hours depending on applied stress and also samples from interrupted tests that survived for up to 2000 hours under lower applied stresses. In addition, a comparison was made of non-reinforced samples tested with and without a 500 hour pre-anneal at the test temperature. In all cases, the gauge sections of crept samples were cut parallel to the stress axis to obtain both near-surface and raid-plane sections. Prior to final TEM specimen preparation, these sections were examined optically for evidence of distributed creep cavitation or crack damage. 

Observations on the cavitation damage produced during tensile creep indicate that at stresses sufficient for strain rates exceeding 1 X 10 s full fact cavitation occurs and leads to the development of creep cracks which grow stability or unstably, depending the applied stress. 

NDE inspection at a planned shutdown revealed significant nozzle cracking. A replica-based investigation and engineering analysis identified the nature, quantity and significance of the creep cracking and cavitation damage. Creep ... 

Such predictions may provide inputs for fatigue-relaxation damage modeling, which should he based on the synergy between oxidation and oxide layer fracture in tempered martensite-ferritic steels but creep cavitation in austenitic stainless steels. 

Fig. 8.13 Microstructural damage during creep of Al203/SiC composite at 1400°C in air. Cavitation occurs within glass phase accumulated at a GBI junction. Glassy ligaments bridge the separated interface.27...

Cavitation is the formation of cavities. This phenomenon has been found to take place in ceramics containing a glass phase. The final creep fracture in this case results from the accumulation of cavities. The factors controlling this kind of creep are the microstructure, volume of glass phase, temperature, and applied stress. These factors give rise to bulk and localized damage. 

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