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EFFECTS OF WATER VAPOR ON THE OXIDATION BEHAVIOR OF ALUMINA AND CHROMIA FORMING SUPERALLOYS AT TEMPERATURES BETWEEN 700C AND 1000C

Hance, Kivilcim Onal (2005) EFFECTS OF WATER VAPOR ON THE OXIDATION BEHAVIOR OF ALUMINA AND CHROMIA FORMING SUPERALLOYS AT TEMPERATURES BETWEEN 700C AND 1000C. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Abstract

Several superalloys and Ni-Cr alloys were tested at temperatures between 700C and 1000C in dry air and in air/water-vapor mixtures, whereby the effects of water vapor on the formation of alumina and chromia scales were investigated. The experimental parameters included temperature of testing, composition of the reactive gases, thermal cycling and the composition of the underlying alloy.Water vapor affected the oxidation characteristics of alumina and chromia in different ways. Selective oxidation of Al was not favored in air/H2O mixtures and at low reaction temperatures. The alloy composition was critical in developing and maintaining continuous protective scales. For alumina-forming systems, higher Al and Cr contents were found to be beneficial for improved resistance against attack. Significant additions of Hf to the alloys resulted in accelerated internal oxidation at 1000C. Transient oxidation was more profound in air/H2O mixtures in comparison to dry air. The adherence of scales was adversely affected by water vapor at 1000C. Water vapor did not affect the selective oxidation of Cr. The major impact of H2O on chromia scales was the accelerated formation of volatile Cr-species which makes the underlying alloy more vulnerable to attack by reactive gases. These reactions were not significant in dry air at 900C and below. The transient oxidation was not adversely affected by water vapor on Ni-Cr systems. The scale spallation was more profound in dry air. The study showed that the main degradation mechanism for chromia in wet air was the formation of vapor Cr species. On the contrary, scale spallation was more detrimental in dry air. Additions of Ce improved the adherence of chromia in each environment. Ce furthermore decreased the chromia formation rate in dry air. It was not clear if the element had the same effect in air/H2O. The presence of water vapor affected the chromia morphology. The thin external TiO2 that developed over chromia on IN 738 reduced the vaporization of the oxide. This indicated that the oxidation resistance of chromia formers can be improved by alloying with elements that would diffuse to the oxide/gas interface and develop an external scale.


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Details

Item Type: University of Pittsburgh ETD
Status: Unpublished
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Hance, Kivilcim Onalkiost4@pitt.eduKIOST4
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee CoChairPettit, F S
Committee CoChairMeier, G H
Committee MemberBarnard, J A
Committee MemberSmialek, J L
Committee MemberEror, N G
Date: 21 June 2005
Date Type: Completion
Defense Date: 12 April 2005
Approval Date: 21 June 2005
Submission Date: 6 April 2005
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: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: alumina; superalloy; volatile Cr-species; water vapor; chromia; oxidation
Other ID: http://etd.library.pitt.edu/ETD/available/etd-04062005-144513/, etd-04062005-144513
Date Deposited: 10 Nov 2011 19:34
Last Modified: 15 Nov 2016 13:38
URI: http://d-scholarship.pitt.edu/id/eprint/6808

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