Abstract
This thesis studies the effects of chromium to aluminium atomic ratio (Cr-to-Al ratio) on the microstructure evolution and oxidation mechanisms of as-cast CoNiCrAl alloys at 1100 °C. CoNiCrAl alloys serve as bond coat materials in thermal barrier coatings systems and are classified as high entropy alloys (HEAs). Due to their outstanding chemical and mechanical performance, they have been widely used as protective coating materials. To focus specifically on the Cr-to-Al ratio, the weight percentages of cobalt and nickel were held constant, as 39.4% Ni and 31.7% Co throughout this study. As-cast CoNiCrAl alloys with five different Cr-to-Al ratios were prepared by the vacuum arc melting and casting process. High temperature oxidation tests were conducted at 1100 °C in air at time intervals of 2 h, 10 h, 25 h, 50 h and 100h. Prior to the oxidation tests, an initial vacuum heat treatment was carried out to the as-cast CoNiCrAl alloys for microstructure homogenisation.The results showed that the Cr-to-Al ratio significantly influenced the formation of competing oxides, Cr2O3 and Al2O3. A critical threshold at Cr-to-Al ratio of approximately 1.8 was proposed. Below the threshold, an exclusive Al₂O₃ layer was formed, whereas, above the threshold, the formation of outer layer Cr2O3 was allowed by outward diffusion of Cr in the low Al content CoNiCrAl alloy through incomplete Al2O3 layers. Notably, this threshold decreased with increasing oxidation time.
| Date of Award | 15 Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hao Chen (Supervisor) & Adam Rushworth (Supervisor) |