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Environmental Factors Affecting CaO- and CaSO4-Induced Degradation of Second-Generation Nickel-Based Superalloys

Brennan, Patrick (2020) Environmental Factors Affecting CaO- and CaSO4-Induced Degradation of Second-Generation Nickel-Based Superalloys. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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The goal of this research project was to provide fundamental understanding of CaO- and CaSO4-induced degradation observed in aviation gas-turbine engines and to develop a lab-scale test procedure which accurately replicates the degradation observed in field-exposed components. Based on initial assessments, particular attention was paid to characterizing the nature of breakaway internal oxidation caused by CaSO4 deposits and determining how CaSO4-alloy interactions induce subsurface changes in the alloy composition and microstructure which can make the alloy susceptible to internal attack.
Both SEM and TEM analyses were used to characterize the morphology of degradation in field-exposed components. Emphasis was placed on characterizing the composition and phase distribution in the internal oxidation zone (IOZ). Isothermal experiments were conducted to investigate the interactions that take place between CaO or CaSO4 deposits and single crystal superalloys at elevated temperatures. This was achieved by exposing Rene N5 and N500 coupons with CaO or CaSO4 deposits at 900°C or 1150°C in air for various times and characterizing the reaction product. From the results obtained, a novel bi-thermal test procedure was developed which successfully replicated the degradation that occurs in the field-exposed components. It was determined that degradation of the subsurface caused by CaSO4 at 1150°C made the alloys susceptible to internal oxidation when exposed to conditions that better simulate the gas-turbine environment. Following this result, sets of systematic experiments were developed to determine how the environmental variables of atmosphere, thermal profile, and deposit mass influence the oxidation behavior of a subsurface-depleted alloy and how the behavior is linked to the kinetic competition between internal and external oxidation. Finally, it was observed that N5 is more resistant to CaSO4-induced degradation than N500. Because of this, the influence of alloy composition on the degradation resistance of nickel-based alloys was explored by conducting bi-thermal experiments with CaSO4 deposit on a superalloy with intermediate composition to that of N5 and N500 and by conducting oxidation experiments on model Ni-Cr-Al-Re alloys.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Brennan, Patrickbrennanptb@gmail.comptb14
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Thesis AdvisorGleeson, Brianbmg36@pitt.edubmg36
Committee MemberMeier, Geraldghmeier@pitt.edughmeier
Committee MemberXiong, Weiweixiong@pitt.eduweixiong
Committee MemberWaldeck, Daviddave@pitt.edudave
Date: 31 July 2020
Date Type: Publication
Defense Date: 7 February 2020
Approval Date: 31 July 2020
Submission Date: 18 February 2020
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 180
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: Deposit-induced corrosion CaO CaSO4 Gas-turbine engine single-crystal nickel-based superalloys
Date Deposited: 31 Jul 2020 15:51
Last Modified: 31 Jul 2020 15:51


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