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Phase Separation in Stainless Steels Studied by Small-angle Neutron Scattering
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. KTH. (Structures)ORCID iD: 0000-0001-5098-7980
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
Keyword [en]
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: urn:nbn:se:kth:diva-217616ISBN: 978-91-7729-594-5 (print)OAI: oai:DiVA.org:kth-217616DiVA, id: diva2:1157191
Public defence
2017-12-08, B2, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20171117

Available from: 2017-11-21 Created: 2017-11-15 Last updated: 2017-11-21Bibliographically approved
List of papers
1. Structural Characterization of Phase Separation in Fe-Cr: A Current Comparison of Experimental Methods
Open this publication in new window or tab >>Structural Characterization of Phase Separation in Fe-Cr: A Current Comparison of Experimental Methods
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2016 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, no 12, p. 5942-5952Article in journal (Refereed) Published
Abstract [en]

Self-assembly due to phase separation within a miscibility gap is important in numerous material systems and applications. A system of particular interest is the binary alloy system Fe-Cr, since it is both a suitable model material and the base system for the stainless steel alloy category, suffering from low-temperature embrittlement due to phase separation. Structural characterization of the minute nano-scale concentration fluctuations during early phase separation has for a long time been considered a major challenge within material characterization. However, recent developments present new opportunities in this field. Here, we present an overview of the current capabilities and limitations of different techniques. A set of Fe-Cr alloys were investigated using small-angle neutron scattering (SANS), atom probe tomography, and analytical transmission electron microscopy. The complementarity of the characterization techniques is clear, and combinatorial studies can provide complete quantitative structure information during phase separation in Fe-Cr alloys. Furthermore, we argue that SANS provides a unique in-situ access to the nanostructure, and that direct comparisons between SANS and phase-field modeling, solving the non-linear Cahn Hilliard equation with proper physical input, should be pursued.

Place, publisher, year, edition, pages
Springer, 2016
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-197763 (URN)10.1007/s11661-016-3800-4 (DOI)000387856000036 ()2-s2.0-84991082928 (Scopus ID)
Note

QC 20160110

Available from: 2017-01-10 Created: 2016-12-08 Last updated: 2017-11-15Bibliographically approved
2. Effect of cooling rate after solution treatment on subsequent phase separation during aging of Fe-Cr alloys: A small-angle neutron scattering study
Open this publication in new window or tab >>Effect of cooling rate after solution treatment on subsequent phase separation during aging of Fe-Cr alloys: A small-angle neutron scattering study
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2017 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 134, p. 221-229Article in journal (Refereed) Published
Abstract [en]

The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small-angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening.

Place, publisher, year, edition, pages
Acta Materialia Inc, 2017
Keyword
Cooling rate, Phase separation, Small-angle neutron scattering, Spinodal decomposition, Stainless steel
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-209872 (URN)10.1016/j.actamat.2017.06.001 (DOI)000406987700020 ()2-s2.0-85020392602 (Scopus ID)
Funder
VINNOVA
Note

QC 20170627

Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2017-11-15Bibliographically approved
3. Effect of heat treatment above the miscibility gap on nanostructure formation due to spinodal decomposition in Fe-52.85 at.%Cr
Open this publication in new window or tab >>Effect of heat treatment above the miscibility gap on nanostructure formation due to spinodal decomposition in Fe-52.85 at.%Cr
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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.

Keyword
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:nbn:se:kth:diva-217915 (URN)
Note

QC 20171121

Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2017-11-21Bibliographically approved
4. Nanostructure and mechanical properties of duplex stainless steels 25Cr-7Ni and 22Cr-5Ni (wt.%) aged at 325 oC
Open this publication in new window or tab >>Nanostructure and mechanical properties of duplex stainless steels 25Cr-7Ni and 22Cr-5Ni (wt.%) aged at 325 oC
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The nanoscale concentration fluctuations due to phase separation (PS) and the corresponding mechanical property changes of duplex stainless steels, standard grade 2205 and super grade 2507, during aging at 325 oC up to 6000 h have been studied. The nanostructure characterization is performed using small-angle neutron scattering (SANS) and the microstructure, including fracture surface and cross-section, is investigated by scanning electron microscopy and electron backscatter diffraction. The results show that the kinetics of phase separation (PS) in grade 2507 is faster than that in grade 2205, leading to greater hardening and deterioration in toughness for grade 2507 compared to 2205. The evolution of nanostructure in the ferrite changes the deformation mode from the original ductile fracture to a quasi-cleavage type where deformation twins form in the hardened ferrite. Delamination, grain fragmentation and plastic deformation of the austenite are suggested to dissipate most of the energy absorbed by the crack during brittle fracture.

Keyword
duplex stainless steel, phase separation, spinodal decomposition, small-angle neutron scattering (SANS), mechanical property, nanostructure
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-217921 (URN)
Note

QC 20171121

Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2017-11-21Bibliographically approved
5. Nuclear and magnetic small-angle neutron scattering in self-organizing nanostructured FexCr1-x alloys
Open this publication in new window or tab >>Nuclear and magnetic small-angle neutron scattering in self-organizing nanostructured FexCr1-x alloys
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Nuclear and magnetic small-angle neutron scattering (SANS) in self-organizing nanostructured FexCr1-x (x=0.8, 0.6 and 0.5) alloys has been studied. A saturation magnetic field is applied to separate the scattering signals and it is shown that the relation between nuclear and magnetic scattering depend on both, the extent of self-organizing due to demixing of Fe and Cr, and the alloy composition. When the demixing is pronounced with large concentration amplitude, the two scattering signals are identical, whereas when the concentration amplitude is small IM(Q) is negligible compared to IN(Q). The relation between the scattering signals is critical when assessing demixing in FexCr1-x alloys by SANS, and it has been mostly ignored in prior works in the literature.

Keyword
metallic alloys, phase separation, small-angle neutron scattering (SANS), magnetism, 475°C embrittlement
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-217924 (URN)
Note

QC 20171121

Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2017-11-21Bibliographically approved

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