LI, ZHUOQUN
(2015)
Investigation of the Minor-Element Effects on the Oxidation Behavior of γ-NI + γ’-NI3AL Alloys.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The effects of minor-elements, mainly Hf, Y and Si, on the oxidation behavior of Al2O3-scale forming γ-Ni + γ’-Ni3Al-based alloy and coating compositions were investigated. Firstly, the effects of Si addition on Hf+Y co-doped γ+γ’ alloys were assessed on both modified-René N5 superalloy and model alloys. It was found that the addition of Si (≤ 1 at. %) decreased the overall weight gain of modified-René N5 superalloy and Ni-20Al-5Cr-0.05Hf-0.05Y (at. %) model alloy under both isothermal (up to 1000 hours) and cyclic (up to 1000 1h cycles) oxidation conditions at 1150°C. Secondly, the effect of Si (1 at. %) addition on the meta-stable → α-Al2O3 transformation kinetics during the early stage of oxidation was studied on the Ni-20Al-5Cr-0.05Hf-0.05Y model alloy. It was found that the addition of 1 at. % Si delays the θ → α-Al2O3 transformation at temperatures above 900°C. In addition, an alternative mathematical method was developed to determine the θ → α-Al2O3 transformation based on analyzing the time dependence of instantaneous n-value. This method found success in describing the effect of Si addition in delaying the θ → α-Al2O3 transformation on Ni-20Al-5Cr-0.05Hf-0.05Y model alloy. Thirdly, synergistic effect of Si (1 at. %) and Hf/Y on the early stage oxidation behavior of Ni-20Al-5Cr-0.05Hf-0.05Y model alloy was studied by TEM/STEM/EDS. Significantly different microstructures of Al2O3 scale formed on the Si-free and Si-containing alloys were observed after 10 and 30 minutes of oxidation at 1150°C. The Al2O3 scale formed on the Si-free alloy contained a large amount of Ni-rich particles in the region close to the scale/metal interface. By contrast, for the Si-containing alloy, a layer of quickly established large-grained α-Al2O3 was observed in the region close to the scale/metal interface. Segregation behavior of Hf and Y was also influenced by the addition of Si. After 2 hours of oxidation at 1150°C, excessive non-uniform Hf and Y co-segregation was observed at the grain boundaries of Al2O3 scale formed on the Si-free alloy. By contrast, Hf and Y grain boundary segregations were uniform and well-controlled in the Al2O3 scale formed on the Si-containing alloy.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
28 January 2015 |
| Date Type: |
Publication |
| Defense Date: |
28 March 2014 |
| Approval Date: |
28 January 2015 |
| Submission Date: |
25 October 2014 |
| Access Restriction: |
1 year -- Restrict access to University of Pittsburgh for a period of 1 year. |
| Number of Pages: |
20 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Oxidation, Coating, Yttrium, Hafnium, Silicon, Kinetics, Segregation |
| Date Deposited: |
28 Jan 2015 21:33 |
| Last Modified: |
15 Nov 2016 14:25 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/23418 |
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