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Effect of heat treatment above the miscibility gap on nanostructure formation due to spinodal decomposition in Fe-52.85 at.%Cr
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. (Structures)ORCID iD: 0000-0001-5098-7980
(Centre for HRTEM, Nelson Mandela University)
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. (Structures)
(ISIS Pulsed Neutron and Muon Facility, Rutherford Appleton Laboratory)
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect of heat treatment at temperatures above the miscibility gap (MG) on subsequent nanostructure formation due to spinodal decomposition (SD) has been investigated in an Fe-52.85 at.%Cr alloy. In-situ total neutron scattering measurements were conducted above and inside the MG to shed light on the high temperature nanostructure. Thereafter, different quenched-in nanostructures were imposed by heat treatments at various temperatures above the MG, followed by rapid quenching. The effect of the quenched-in nanostructure on subsequent SD was investigated ex-situ by small-angle neutron scattering, analytical transmission electron microscopy and hardness testing. The critical temperature of the miscibility gap was found at ~570oC and below that temperature, thermodynamically stable α'-phase forms. It was found that transient clustering of Cr occurs above the MG and that the tendency of clustering increases with decreasing temperature. The quenched-in clustering present in rapidly quenched materials treated above the MG has a significant effect on the kinetics of SD upon further aging within the MG. It is clear that the significant quenched-in Cr clustering present in samples heat treated at 600 and 700oC accelerates SD. However, samples heat treated at 1000oC demonstrate more rapid SD kinetics than samples heat treated at 800oC. Cr clustering and other mechanisms affecting the kinetics of SD are discussed in the light of the results obtained.

Keywords [en]
stainless steel; total neutron scattering; small-angle neutron scattering (SANS); spinodal decomposition; clustering
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-217915OAI: oai:DiVA.org:kth-217915DiVA, id: diva2:1158398
Note

QC 20171121

Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2017-11-21Bibliographically approved
In thesis
1. Phase Separation in Stainless Steels Studied by Small-angle Neutron Scattering
Open this publication in new window or tab >>Phase Separation in Stainless Steels Studied by Small-angle Neutron Scattering
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fe-Cr based steels, i.e. stainless steels, possessing a combination of excellent corrosion resistance and good mechanical properties, have indispensable applications ranging from low-end cooking utensils, to sophisticated components for nuclear power plants. However, the bcc/bct phase containing stainless steels which have a miscibility gap (MG) suffer from the so-called “475 oC embrittlement” leading to hardness increase and toughness deterioration. It occurs due to demixing of Fe and Cr leading to the formation of Fe-rich (α) and Cr-rich (α′) regions in bcc/bct phases. The demixing is referred to as phase separation (PS).

The goal of this work was to study PS in ferrite containing stainless steels mainly by small-angle neutron scattering (SANS). Firstly, the application of different experimental techniques for the study of phase separation in Fe-Cr based steels was reviewed and supplemented by new measurements. SANS was shown to be very sensitive to the nanostructure change caused by PS and capable of characterizing the early stages of PS in Fe-Cr alloys. However, atom probe tomography and transmission electron microscopy are complementary to SANS. Therefore, in order to have a more complete view of the microstructure, the combination of these techniques should be pursued. Secondly, the factors affecting the initial microstructure prior to aging treatment and the effect of the resulted initial microstructure on PS were systematically investigated using binary Fe-Cr model alloys. The critical temperature of the MG was determined to be located between 560 and 580 oC in binary Fe-Cr. The results indicate that the solution treatment temperature above the MG and the cooling rate after solution treatment have significant effects on the initial microstructure and thus on PS during subsequent aging. The mechanisms responsible for the changed aging behavior are Cr clustering, quenched-in vacancy and decomposition during cooling. Therefore, computational simulations should take into account these factors and the initial microstructure to make predictions that are more accurate. Thirdly, the study was extended to PS in commercial duplex stainless steels (DSSs) which are of practical importance in various industries, e.g., nuclear power. It is found that alloying elements have an important effect on PS in DSSs. The grade 2507 (25 %Cr, 7 %Ni) experiences stronger PS than grade 2205 (22 %Cr, 5 % Ni) for the same heat treatment. Moreover, the fracture mechanisms as well as the mechanical properties depend on the extent of PS.  Finally, the fundamental aspects regarding the neutron scattering behavior for Fe-Cr alloys were examined. The results show that the nuclear and magnetic scattering of neutrons depend on the evolution of the nanoscale compositional fluctuation in Fe-Cr alloys. The ratio of the magnitude of nuclear scattering versus magnetic scattering varies with the extent of PS.

Abstract [sv]

Stål baserade på Fe-Cr systemet, det vill säga rostfria stål, som har en kombination av utmärkta korrosionsegenskaper och bra mekaniska egenskaper, har många tillämpningar; allt från köksredskap, till sofistikerade komponenter för kärnkraftverk. Rostfria stål som innehåller Bcc / bct-fasen och som således har en blandningslucka, är känsliga för den så kallade "475 °C försprödningen" som leder till en hårdhetsökning men kraftigt försämrad slagseghet. Detta uppstår på grund av en uppdelning av Fe och Cr som leder till bildandet av Fe-rika (a) och Cr-rika (a’) regioner i bcc / bct-fasen. Denna uppdelning brukar kallas fasseparation.

Målet med detta arbete var att studera fasseparationen i ferrit-innehållande rostfria stål främst genom lågvinkel-spridning av neutroner (SANS). Till att börja med studerades och jämfördes olika experimentella tekniker för undersökning av fasseparation i Fe-Cr-baserade stål med nya SANS- mätningar. SANS visade sig vara mycket känslig för förändringar på nano-skala orsakad av fasseparation och tekniken visade sig även kapabel att karakterisera de tidiga stadierna av fasseparation i Fe-Cr-legeringar. För att få en mer fullständig bild av mikrostrukturen efter fasseparation, bör emellertid en kombination av SANS och komplementära tekniker, såsom atomsond och transmissions-elektronmikroskopi, användas. Vidare undersöktes de faktorer som påverkar den ursprungliga mikrostrukturen före åldringsbehandling, och effekten av den initiala mikrostrukturen på fasseparation studerades systematiskt med användning av binära modell-legeringar av Fe-Cr. Den kritiska temperaturen för blandningsluckan i Fe-Cr bestämdes vara belägen mellan 560 och 580 °C. Resultaten indikerar att temperaturen för upplösningsbehandling ovanför blandningsluckan och kylhastigheten har en signifikant inverkan på den initiala mikrostrukturen och därmed på fasseparationen under efterföljande åldring. Mekanismerna som är ansvariga för det förändrade åldringsbeteendet är: Cr-klustring, insläckta vakanser och fasseparation under kylning. Simuleringar av fasseparationen bör därför ta hänsyn till dessa faktorer och den ursprungliga mikrostrukturen för att göra mer exakta förutsägelser av hur mikrostrukturen utvecklar sig med åldringstiden. Fasseparationen i kommersiella duplexa rostfria stål (DSS), som är av stor praktisk betydelse i olika branscher, t ex kärnkraft, studerades också med SANS. Det visade sig att mängden av olika legeringselement har en viktig effekt på graden av fasseparation i DSS. Legeringen 2507 uppvisade en tydligare fasseparation jämfört med legering 2205 för samma värmebehandling. Brottmekanismerna såväl som de mekaniska egenskaperna visade sig bero på omfattningen av fasseparationen. Slutligen undersöktes de grundläggande aspekterna hos neutronspridnings-beteendet för binära Fe-Cr-legeringar. Resultaten visade att kärn- och magnetisk spridning av neutroner beror på utvecklingen av sammansättningsfluktuationerna på en nanoskala i Fe-Cr-legeringar. Förhållandet mellan magnetisk- och kärnspridning varierar med omfattningen av fasseparationen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 57
Keywords
Fe-Cr alloys, stainless steels, spinodal decomposition, phase separation, small-angle neutron scattering, mechanical properties
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-217616 (URN)978-91-7729-594-5 (ISBN)
Public defence
2017-12-08, B2, Brinellvägen 23, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20171117

Available from: 2017-11-21 Created: 2017-11-15 Last updated: 2017-11-21Bibliographically approved

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