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Effects of platinum, iridium, and hafnium to nickel-aluminium alloys under cyclic oxidation conditions.

A thermally grown oxide (TGO) such as the alumina scale formed on a bondcoat enhances the oxidation and corrosion resistance of thermal barrier coating (TBC)- bondcoat-superalloy substrate systems. As the external alumina scale lies between the thermal barrier coating and bondcoat, its first spallation and subsequent TBC delamination become critical. Once the external alumina scale spalls, it will spall together with the TBC, leaving the system with no temperature barrier protection. Operational factors such as thermal cycling conditions, water vapour in the oxidation atmosphere, and alloying elements comprising the bondcoat system affect alumina scale adherence. Another problem that arises for the majority of bondcoat systems, β (Ni,Pt)Al and MCrAlY (M=Metal), are rich in aluminium. This causes aluminium to diffuse into the substrate, enriching it with aluminium during service, transforming phases in the substrate alloying system. The purpose of this study was to develop bondcoat materials that promote formation of a strongly adherent TGO, but have an aluminium content near the substrate composition. Cyclic oxidation experiments were performed with Ni-Al, Ni-Pt-Al, and Ni-Pt-Al-Ir alloys in dry air and air-12%H2O. Thermal cycles of 1 hr at 1200OC and 10 minutes at 80OC were carried out in flowing gases at a total pressure of 1 atm. Experiments in N2- 12%H2O were performed only on Ni-Al binary alloys. Binary Ni-Al cast alloys were tested for fundamental study purposes, while Ni-Pt-Al and Ni-Pt-Al-Ir cast alloys were intended to be models for aluminide coatings, with attention focused on γ+γ' -Ni-(20 to 23)Al. Comparisons were made with β-Ni-50Al, as it forms an external alumina scale and was found to have the smallest weight loss rate during testing of binary alloys. Assessments of Pt and Pt-Ir additions, with and without hafnium, to the γ+γ' binary alloy were made. Compared to binary alloys, platinum was found to reduce the total weight loss caused by scale spallation. Experiments in air-12%H2O led to more rapid weight loss than in dry air. This was due to enhanced spallation. However, the degradation rate was slower than in platinum-free alloys exposed to the same atmosphere. Partial replacement of platinum with iridium was found to improve alloy scale adherence during exposure in both dry and wet air. Addition of 1wt% hafnium was found to reduce oxide thickness and increase the oxide adherence simultaneously. The hafnium addition was essential in order to reduce spallation rate in wet air. Water vapour in the presence of oxygen generally increased the spallation rate. It weakened the oxide metal interface, causing subsequent spallation to be increased, but only if the gas had access to the alloy-scale interface. Water vapour did not affect the spallation rate of the strongly adherent oxide grown on Ni-22Al-30Pt+1wt%Hf and Ni- 20Al-15Pt-10Ir+1wt%Hf.
Date January 2007
CreatorsKartono, Rahmat, Materials Science & Engineering, Faculty of Science, UNSW
Source SetsAustraliasian Digital Theses Program
Detected LanguageEnglish

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