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Spatial correlation between local misorientations and nanoindentation hardness in nickel-base alloy 690
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).ORCID iD: 0000-0002-2641-7838
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). Ringhals AB, Sweden.ORCID iD: 0000-0003-1498-5691
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0003-2170-0076
2016 (English)In: Journal of Materials Science and Engineering: A, ISSN 2161-6213, Vol. 674, 171-177 p.Article in journal (Refereed) Published
Abstract [en]

Misorientation increases with plastic strain in metals, and this observation has been used as an empirical assessment of plastic strain in recent years. The method has been validated for a sample area corresponding to a 100 µm×100 µm square, but on the micrometer scale misorientations no longer seem to correlate with plastic strain. Misorientations are however not dependent on plastic strain but rather on dislocation density, which means it should also be related to hardness. Therefore, we have in this work compared maps of predicted hardness calculated from misorientation determination with maps of actual hardness measured by nanoindentation. It was shown that the predicted and measured hardness maps do indeed correlate spatially in nickel-base Alloy 690, although the measured values have a significantly smaller hardness variation. This is explained by a presumably high and uniform density of statistically stored dislocations, which contribute to hardness but do not affect the misorientation determination from electron backscatter diffraction. Thus local misorientation can be used to qualitatively map the local effective plastic strain distribution, for example to identify regions of increased hardness.

Place, publisher, year, edition, pages
Elsevier, 2016. Vol. 674, 171-177 p.
Keyword [en]
EBSD, Nanoindentation, Local Misorientation, Hardening, Plasticity, Nickel Based Superalloys
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-193478DOI: 10.1016/j.msea.2016.07.123ISI: 000383292800022Scopus ID: 2-s2.0-84982839184OAI: oai:DiVA.org:kth-193478DiVA: diva2:1014967
Note

QC 20161004

Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2017-11-30Bibliographically approved
In thesis
1. On the low primary water stress corrosion cracking susceptibility of weld deformed Alloy 690
Open this publication in new window or tab >>On the low primary water stress corrosion cracking susceptibility of weld deformed Alloy 690
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

It has been shown in recent years that the nickel-base Alloy 690 can become susceptible to stress corrosion cracking (SCC) in the primary water of nuclear power plants with pressurized water reactors if it has been sufficiently deformed at room temperature. Although the material is not intentionally used in a deformed state, it can become deformed by various manufacturing processes. Welding is believed to be the process that is most likely to cause susceptibility, yet it does not seem quite that detrimental in experiments. The overall purpose of this work was to investigate why weld-induced deformation does not seem to cause the same degree of susceptibility as cold deformation.

The work started with a microstructural investigation, presented in Paper A, to assess if any of the changes caused by welding can explain the difference in behavior. While a beneficial change in the microstructure was observed, it was not enough to explain the differences.

The focus was then turned towards addressing knowledge gaps of the method used to assess weld-induced deformation. This method is based on measuring misorientations using electron backscatter diffraction (EBSD). It was shown in Paper B that kernel average misorientation (KAM) is closer related to the degree of hardening than the degree of deformation, and that it can be used to obtain a qualitative map of hardness at the micrometer scale. Improvements to the KAM-based method were presented in Paper C along with estimates near welds from component mockups.

The validity of using a misorientation-based method on warm deformation was tested in Paper D. It was shown that the method gives a rough estimate of the degree of strain hardening, although the data suggests it is a small overestimation. The overestimation would mean that weld deformation may have a lower hardness than the strain estimate implies, which is beneficial for SCC resistance.

Abstract [sv]

Under senare år har det visats att nickelbasen Alloy 690 kan bli känslig för spänningskorrosion (SCC) i primärvatten av kärnkraftverk med tryckvatten-reaktorer om det har blivit tillräckligt deformerat i rumstemperatur. Även om materialet inte medvetet används i ett deformerat tillstånd, så kan det bli deformerat av olika tillverkningsprocesser. Svetsning är den process som anses mest sannolik att orsaka känslighet, dock verkar den inte vara riktigt så skadlig i experiment. Det övergripande målet med det här arbetet var att undersöka varför svetsinducerad deformation inte verkar orsaka samma nivå av känslighet som kalldeformation.

Arbetet började med en mikrostrukturell undersökning, presenterad i Artikel A, för att bedöma om någon av förrändringarna orsakade av svetsning kan förklara skillnaderna i beteende. Även om en gynnsam förrändring i mikrostrukturen observerades så var det inte tillräckligt för att förklara skillnaderna.

Fokusen vändes istället mot att adressera kunskapsbristerna i metoden som användes för att uppskatta den svetsinducerade deformationen. Denna metod är baserad på att mäta misorienteringar med bakåtspridd elektrondiffraktion (EBSD). Det visades i Artikel B att kärn-medel-misorientering (KAM) är närmare besläktat med nivån av hårdnande än nivån av deformation, och att det kan användas för att erhålla en kvalitativ karta över hårdhet på mikrometerskalan. Förbättringar till en KAM-baserad metod presenterades i Artikel C tillsammans med uppskattningar nära svetsar från komponentattrapper.

Giltigheten i att använda en misorienteringsbaserad metod för varmdeformation testades i Artikel D. Det visades att metoden ger en grov uppskattning av nivån av hårdnande, dock antyder data att det är en liten överskattning. Överskattningen skulle betyda att svetsdeformerat material kan ha en lägre hårdhet än töjningsuppskattningen antyder, vilket är gynnsamt för SCC-resistens.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. 42 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 613
National Category
Metallurgy and Metallic Materials
Research subject
Solid Mechanics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-202573 (URN)978-91-7729-294-4 (ISBN)
Public defence
2017-03-29, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170301

Available from: 2017-03-01 Created: 2017-02-28 Last updated: 2017-03-01Bibliographically approved

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