Longzhou Ma

Date of Graduation

2001

Document Type

Dissertation/Thesis

Abstract

Recently developed γ-γ′-β three phases governing INCONEL* alloy 783 is the result of an extensive superalloy development program aimed at improving environmental-induced intergranular cracking resistance. Formation of intermetallic nickel aluminide β phase in alloy 783 is essentially resistant for the general oxidation. In this study, commercial alloy 783 was subjected to various metallurgical processing including heat treatments with β aging/without β aging, addition of 3% Cr into alloy, and thermo-mechanical processing (TMP). The corresponding microstructures were evaluated by means of the optical microcopy, SEM and TEM. These processed materials were subjected to the fatigue crack growth and sustained loading crack growth tests to investigate the effects of β precipitates on crack growth process, especially on the time-dependent crack growth behavior. It was found that β precipitates in alloy 783 play a very significant role in controlling SAGBO-induced crack growth process. Properly controlling the microstructure and morphology of β precipitates by different metallurgical processing can improve the resistance to environmental-induced damage. Based on the experimental results, a phenomenological model, which could describe crack growth behavior of alloy 783 subjected to the different metallurgical processing, were developed through introducing a universal time-dependent factor, α(t, T). Combining the physical interpretation of SAGBO-induced degradation mechanism and generated data, a 1-D stress-assisted oxygen diffusion model at crack tip along grain boundaries was proposed. This model was utilized to characterize crack growth behavior and crack incubation time in alloy 783 without β aging under sustained load condition. The good agreement between model prediction and experimental results were obtained. *INCOLOY and INCONEL are trademarks of the Special Metals Corporation. Huntington, West Virginia.

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