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The Effects of Composition and Microstructure on the Reaction Behavior of MCrAlY Alloys Under a Variety of Aggressive Environmental Conditions

Task, Michael N. (2010) The Effects of Composition and Microstructure on the Reaction Behavior of MCrAlY Alloys Under a Variety of Aggressive Environmental Conditions. Master's Thesis, University of Pittsburgh. (Unpublished)

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Cast alloys with compositions similar to those of MCrAlY overlay coatings, which are commonly deposited on Ni-base superalloys in the hot section of gas turbine engines, were exposed to a variety of aggressive conditions. The materials studied include (in wt%): Ni-31Cr-11.25Al-0.65Y, Co-23Cr-13Al-0.65Y, Ni-20Co-18Cr-12.5Al-0.6Y-0.4Hf-0.25Si, Ni-10Co-18Cr-12.5Al-0.6Y-0.4Hf-0.25Si, and Ni-10Co-27.1Cr-12.5Al-0.6Y-0.4Hf-0.25Si . The microstructures of the Ni-base alloys contained FCC γ-Ni (or γ′-Ni3Al, depending on temperature and composition), primitive cubic β-NiAl , and a Ni-rich yttride. BCC α-Cr was also present in some cases. The CoCrAlY contained HCP α-Co, β-CoAl, and a Co-rich yttride. The experimental conditions were: Type I and Type II hot corrosion, cyclic oxidation, and intermittent hot corrosion. The effects of alloy composition and microstructure on the resistance to these modes of degradation were determined using scanning electron microscopy and kinetic data. All alloys showed good resistance to Type I hot corrosion. Conversely, Type II hot corrosion was catastrophic in most cases; however, the coarse-grained NiCrAlY alloy demonstrated reasonably good resistance to Type II attack. When the microstructure was refined via injection casting, however, this alloy showed substantial degradation under Type II conditions. This was attributed partially to differences in the morphology and distribution of the α-Cr phase. The Type II hot corrosion resistance of a NiCoCrAlY alloy was improved by lowering the cobalt content and increasing the chromium content such that a coarse distribution of α-Cr was present in the microstructure. All alloys formed protective Al2O3 scales during cyclic oxidation at 1100°C, with the exception of the NiCrAlY, which showed scale spallation after short exposures. The oxidation resistance of this alloy improved with decreasing temperature. This behavior was explained in part by a phase transformation which results in a volume contraction upon cooling and thus acompressive stress in the oxide. This transformation, as well as other aspects of the phase equilibria in this NiCrAlY system, was examined experimentally, and the results were compared with thermodynamic predictions. No difference in oxidation behavior was observed when cyclic oxidation of the MCrAlY alloys was preceded by a Type I hot corrosion exposure. Conversely, the cyclic oxidation resistance of uncoated René N5 was substantially reduced by such an initial exposure.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Task, Michael N.mnt2@pitt.eduMNT2
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairMeier, G. Hghmeier@pitt.eduGHMEIER
Committee CoChairGleeson, Bbmg36@pitt.eduBMG36
Committee CoChairPettit, F. Spettitfs@pitt.eduPETTITFS
Committee MemberNettleship, Inettles@pitt.eduNETTLES
Committee MemberYang, Jjudyyang@pitt.eduJUDYYANG
Date: 26 January 2010
Date Type: Completion
Defense Date: 19 November 2009
Approval Date: 26 January 2010
Submission Date: 30 November 2009
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: MSMSE - Master of Science in Materials Science and Engineering
Thesis Type: Master's Thesis
Refereed: Yes
Uncontrolled Keywords: gas turbine; hot corrosion; MCrAlY; oxidation
Other ID:, etd-11302009-100514
Date Deposited: 10 Nov 2011 20:06
Last Modified: 15 Nov 2016 13:52


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