OXIDATION STUDIES ON Ag-In AND Ni-BASED ALLOYS AT LOW HOMOLOGOUS TEMPERATURE
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Authors
Ghaffari, Yasaman
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eng
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Oxidation- Ni alloys- Ag alloys- Materials characterization
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Abstract
Internal and intergranular oxidation can result in embrittlement, degradation of mechanical properties and possible premature mechanical failure or stress corrosion cracking. Internal oxidation of Ag-In alloys was studied in an air atmosphere at a relatively low homologous temperature (240 °C) (∼ 0.45), where lattice diffusion is negligible. After exposure, metallic Ag nodules were present on the surfaces of all alloys, expelled to relieve stress due to internal oxidation. Crystallographic orientation and grain boundary misorientation were found to influence nodule expulsion tendency, and the propensity for internal or external oxidation. Moving to more industry relevant alloys, the internal and intergranular oxidation of binary Ni-Al and Ni-Cr model alloys, and ternary Ni-Cr-Al alloys was studied at low homologous temperature (∼ 0.45), 480 ˚C, in a hydrogenated steam environment. In Ni-Al alloys, high angle grain boundaries (HAGBs) were very susceptible to intergranular oxidation, while low energy grain boundaries behaved similar to bulk material. In Ni-Cr alloys, low angle grain boundaries (LAGBs) and coincident site lattice boundaries (CSLBs) were susceptible to intergranular oxidation, while HAGBs were protected by an external oxide. The structure of the intergranular oxide was also different between Ni-Cr and Ni-Al alloys. In Ni-Cr-Al alloys, the intergranular oxide formed at LAGBs consisted of an Al-rich oxide at the center, surrounded by a Cr-rich oxide, while HAGBs were protected by a dual layer external oxide layer. The morphology of the intergranular oxide in the Ni-Cr-Al alloys was found to be similar to that observed in Ni-Al binary alloys exposed to similar conditions. However, the diffusivity of Al was enhanced in Ni-Cr-Al alloys compared to Ni-Al alloys, leading to contribution of Al in external and intergranular oxide formation. Thus, the third element effect, usually reported at high homologous temperature in Ni alloys (> 800 °C), was found to be applicable to short-circuit diffusion pathways, specifically grain boundaries, at low homologous temperature (∼ 0.45) (480 °C).
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ProQuest PhD and Master's Theses International Dissemination Agreement
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