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The Effects of Alloying Elements and Exposure Atmospheres on Sigma Formation in Ferritic Alloys

Patel, Rita Nalin (2014) The Effects of Alloying Elements and Exposure Atmospheres on Sigma Formation in Ferritic Alloys. Master's Thesis, University of Pittsburgh. (Unpublished)

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Tests were performed on a variety of model and commercial ferritic steels in simulated cathode and anode environments of an intermediate temperature solid oxide fuel cell (SOFC). The alloys were picked for their similarity to currently used interconnect materials. Results from the test provide inside into the effect of exposure atmospheres and alloying elements on the nucleation and growth of sigma phase in the ferritic alloys. Two possible mechanisms for sigma phase transformation were discussed, but the exact method was not able to be determined based on the results shown.
The cathode environment was simulated by either using dry laboratory air, or in the case of a pipe-sealant leak in the SOFC, dry air with 10% water vapor at 700 °C. Three model alloys were tested in these environments to determine the effect of water vapor and alloying elements on the nucleation and growth of sigma phase. It was confirmed that molybdenum is a sigma phase promoter and allows for the nucleation and growth to occur. The presence of water vapor in the exposure environment caused an increased amount of sigma phase formation, although the exact reason why remains unknown.
The anode environment was simulated by using an Ar-4%H2-10%H2O gas-water vapor mixture at 700 °C. A variety of model alloys and commercial alloys were tested in this environment. To simulate the effect of a nickel mesh touching the interconnect in the SOFC cell, nickel was electroplated onto some specimens prior to exposure. The pre-coat of nickel caused the formation of an austenitic interdiffusion zone in all alloys, and in all but the low-Cr alloy, this also lead to the formation of sigma phase. Two possible mechanisms for sigma phase formation were suggested. The first is that the ferrite phase decomposed directly into austenite and sigma phase. The second is that the ferrite phase first transformed into austenite, which then transformed into sigma phase. Neither method could be confirmed based on the results obtained.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Patel, Rita Nalinrnp9@pitt.eduRNP9
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Thesis AdvisorMeier, Geraldghmeier@pitt.eduGHMEIER
Committee MemberPettit, Frederickpettitfs@pitt.eduPETTITFS
Committee MemberGarcia, C. CIGARCIA
Committee MemberFoulke, Larrylrf4@pitt.eduLRF4
Date: 16 June 2014
Date Type: Publication
Defense Date: 28 February 2014
Approval Date: 16 June 2014
Submission Date: 26 March 2014
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 132
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: MS - Master of Science
Thesis Type: Master's Thesis
Refereed: Yes
Uncontrolled Keywords: Solid Oxide Fuel Cell, SOFC, Ferritic Alloys, Ferrite Phase, Sigma Phase, Nickel, Cathode, Anode, Interconnect
Date Deposited: 16 Jun 2014 18:18
Last Modified: 15 Nov 2016 14:18


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