de Leon Nope, Grace Vanessa
(2024)
High-Temperature Oxidation and
Fatigue Performance of Alloy 625: Effect of Alloy Manufacturing Process.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Alloy 625 is a nickel-based superalloy which is strengthened mainly by solid-solution hardening. Multiple efforts have been made to fabricate complex parts by additive manufacturing (AM), taking advantage of the freedom of design, sustainability benefits, and the manufacture of hard-to-machine alloys. However, one of the major drawbacks of the AM implementation for components with both high mechanical and chemical requirements is the lack of full understanding of the processing-structure-properties relationships.
Alloy 625 applications at elevated temperatures required good mechanical properties combined with good corrosion and oxidation resistance. The latter relies on the protectiveness provided by the thermal growth chromia scale. Previous studies have assessed mechanical and chemical performance, finding high variability and inconsistent results within the alloy specifications. Furthermore, previous assessments of these properties have been conducted separately without considering the potential impact of oxidation on mechanical performance.
This study aims to elucidate the impact of the microstructural variables stemming from laser additive manufacturing processes on the high-temperature oxidation behavior and fatigue performance of alloy 625. Two laser-assisted AM processes were studied: laser powder bed fusion (LPBF) and direct energy deposition (DED), and wrought and cast alloy were used as a reference. During the early stages of oxidation, it has been observed that the extent of Nb and Mo segregation affects the formation of transient products, which in turn delays the development of a protective chromia scale. In the steady-state oxidation regime, it has been noted that the AM samples show consistently faster oxidation kinetics. Additionally, the AM samples also exhibit more damaged subsurface and chromia scale, characterized by a larger oxide scale decohesion, non-planar interface, internal voids, and severe intergranular oxidation. The excess of voids formation in oxidized AM samples is attributed to the higher interstitial oxygen gain during the alloy manufacturing processes. The combined effect of these defects (i.e., interfacial voids, delta-precipitates, internal oxidation) accelerates the alloy degradation during high cycle fatigue, ultimately resulting in a significant reduction in fatigue life compared to the wrought sample. The most detrimental factor for fatigue performance is severe intergranular oxidation.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
11 January 2024 |
| Date Type: |
Publication |
| Defense Date: |
5 October 2023 |
| Approval Date: |
11 January 2024 |
| Submission Date: |
18 September 2023 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
240 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Mechanical Engineering and Materials Science |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Oxidation at high-temperature, High-cycle fatigue, additive manufacturing |
| Date Deposited: |
11 Jan 2024 19:30 |
| Last Modified: |
11 Jan 2024 19:30 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/45409 |
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