Creep cavitation

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... 

Figure 12.4 (a) Intergranular fracture of an alumina sample showing creep cavitation due to compressive creep at 1600°C. Note closely spaced cavities along the two-grain facets. (b) Schematic of cavity formation in viscous grain boundary films as a result of applied tensile stress. 

The parameter is a function of fiber rupture time (<) and temperature 7). Here D = log (Go) = 22 for the SiC and AI2O3 fibers in the high-temperature range where creep cavitation... 

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. 

Figure 10.5 TEM micrograph of creep cavitation sites in the 20 vol % SiC whisker-reinforced composite. ...

Creep cavitation because of tensile stresses in the underlying alloy accompanying scale growth. The stresses are a consequence of the volume change between the alloy and the oxide. 

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. 

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