The dimensions of graphitic layer planes directly affect the reactivity of soot towards oxidation and growth. Quantification of graphitic structure could be used to develop and test correlations between the soot nanosmcmre and its reactivity. Based upon transmission electron microscopy images, this paper provides a demonstration of the robustness of a fringe image analysis code for determining the level of graphitic structure within nanoscale carbon, i.e. soot.Results, in the form of histograms of graphtic layer plane lengths, are compared to their determination through Raman analysis.
Oxidation and the effects of high temperature exposures on notched fatigue life were considered for a powder metallurgy processed supersolvus heat-treated ME3 disk superalloy. The isothermal static oxidation response at 704 °C, 760 °C, and 815 °C was consistent with other chromia forming nickel-based superalloys: a TiO 2 -Cr 2 O 3 external oxide formed with a branched Al 2 O 3 internal subscale that extended into a recrystallized γ'-dissolution layer. These surface changes can potentially impact disk durability, making layer growth rates important. Growth of the external scales and γ'dissolution layers followed a cubic rate law, while Al 2 O 3 subscales followed a parabolic rate law. Cr-rich M 23 C 6 carbides at the grain boundaries dissolved to help sustain Cr 2 O 3 growth to depths about 12 times thicker than the scale.The effect of prior exposures was examined through notched low cycle fatigue tests performed to failure in air at 704 °C. Prior exposures led to pronounced debits of up to 99 % in fatigue life, where fatigue life decreased inversely with exposure time. Exposures that produced roughly equivalent 1 µm thick external scales at the various isotherms showed statistically equivalent fatigue lives, establishing that surface damage drives fatigue debit, not exposure temperature. Fractographic evaluation indicated the failure mode for the pre-exposed specimens involved surface crack initiations that shifted with exposure from predominately single intergranular initiations with transgranular propagation to multi-initiations from the cracked external oxide with intergranular propagation. Weakened grain boundaries at the surface resulting from the M 23 C 6 carbide dissolution are partially responsible for the intergranular cracking. Removing the scale and subscale while leaving a layer where M 23 C 6 carbides were dissolved did not lead to a significant fatigue life improvement, however, also removing the M 23 C 6 carbide dissolution layer led to nearly full recovery of life, with a transgranular initiation typical to that observed in unexposed specimens.
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