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Barkar, T., Höglund, L., Odqvist, J. & Ågren, J. (2018). Effect of concentration dependent gradient energy coefficient on spinodal decomposition in the Fe-Cr system. Computational materials science, 143, 446-453
Open this publication in new window or tab >>Effect of concentration dependent gradient energy coefficient on spinodal decomposition in the Fe-Cr system
2018 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 143, p. 446-453Article in journal (Refereed) Published
Abstract [en]

The Cahn–Hilliard equation is solved with thermodynamic and kinetic input, using the Thermo-Calc and DICTRA software packages rather than simpler models e.g. regular solution. A concentration dependent expression for the gradient energy coefficient is introduced and its effect on simulated decomposition is discussed. Simulations were carried out in 2D and 3D using the FiPy software package modified for non-linear problems.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-219887 (URN)10.1016/j.commatsci.2017.11.043 (DOI)000424900000053 ()2-s2.0-85036461179 (Scopus ID)
Funder
VINNOVA
Note

QC 20171215

Available from: 2017-12-15 Created: 2017-12-15 Last updated: 2018-03-05Bibliographically approved
Xu, X., Westraadt, J. E., Odqvist, J., Youngs, T. G., King, S. M. & Hedström, P. (2018). Effect of heat treatment above the miscibility gap on nanostructure formation due to spinodal decomposition in Fe-52.85 at.%Cr. Acta Materialia, 145, 347-358
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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2018 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 145, p. 347-358Article in journal (Refereed) Published
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 ∼580 °C for the Fe-52.85 at.%Cr alloy and below that temperature, phase separation occurs, where the ferrite decomposes into Fe-rich α-phase and Cr-rich α′-phase. 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 700 °C accelerates SD. However, samples heat treated at 1000 °C demonstrate more rapid SD kinetics than samples heat treated at 800 °C. Cr clustering and other mechanisms affecting the kinetics of SD are discussed in the light of the results obtained. 

Place, publisher, year, edition, pages
Acta Materialia Inc, 2018
Keyword
Clustering, Small-angle neutron scattering (SANS), Spinodal decomposition, Stainless steel, Total neutron scattering, Chromium alloys, Heat treatment, High resolution transmission electron microscopy, Kinetics, Nanostructures, Neutron scattering, Phase separation, Solubility, Temperature, Transmission electron microscopy, Analytical transmission electron microscopy, Critical temperatures, Effect of heat treatments, High temperature, Miscibility gap, Nanostructure formation
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-223139 (URN)10.1016/j.actamat.2017.12.008 (DOI)000424726200035 ()2-s2.0-85039736015 (Scopus ID)
Funder
VINNOVA
Note

 QC 20180326

Available from: 2018-03-26 Created: 2018-03-26 Last updated: 2018-03-26Bibliographically approved
Hou, Z., Babu, R. P., Hedström, P. & Odqvist, J. (2018). Microstructure evolution during tempering of martensitic Fe-C-Cr alloys at 700 A degrees C. Journal of Materials Science, 53(9), 6939-6950
Open this publication in new window or tab >>Microstructure evolution during tempering of martensitic Fe-C-Cr alloys at 700 A degrees C
2018 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 53, no 9, p. 6939-6950Article in journal (Refereed) Published
Abstract [en]

The microstructure evolution of two martensitic alloys Fe-0.15C-(1.0 and 4.0) Cr (wt%) was investigated, using X-ray diffraction, electron backscatter diffraction, electron channeling contrast imaging and transmission electron microscopy, after interrupted tempering at 700 A degrees C. It was found that quenching of 1-mm-thick samples in brine was sufficient to keep most of the carbon in solid solution in the martensite constituent. The high dislocation density of the martensite decreased rapidly during the initial tempering but continued tempering beyond a few minutes did not further reduce the dislocation density significantly. The initial martensitic microstructure with both coarse and fine laths coarsened slowly during tempering for both alloys. However, a clear difference between the two alloys was distinguished by studying units separated by high-angle boundaries (HABs). In the low-Cr alloy, M3C precipitates formed and coarsened rapidly, thus they caused little hindrance for migration of HABs, i.e., coarsening of the HAB units. On the other hand, in the high-Cr alloy, M7C3 precipitates formed and coarsened slowly, thus they were more effective in pinning the HABs than M3C in the low-Cr alloy, i.e., coarsening of HAB units was minute in the high-Cr alloy.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-223769 (URN)10.1007/s10853-018-2036-7 (DOI)000424874900054 ()2-s2.0-85040953153 (Scopus ID)
Funder
VINNOVA
Note

QC 20180307

Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2018-04-18Bibliographically approved
Dahlström, A., Danoix, F., Hedström, P., Odqvist, J. & Zapolsky, H. (2017). An Experimental Assessment of the alpha plus alpha ' Miscibility Gap in Fe-Cr. In: TMS 2017 146TH ANNUAL MEETING & EXHIBITION SUPPLEMENTAL PROCEEDINGS: . Paper presented at 146th TMS Annual Meeting and Exhibition, FEB 26-MAR 02, 2017, San Diego, CA (pp. 711-718). Springer
Open this publication in new window or tab >>An Experimental Assessment of the alpha plus alpha ' Miscibility Gap in Fe-Cr
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2017 (English)In: TMS 2017 146TH ANNUAL MEETING & EXHIBITION SUPPLEMENTAL PROCEEDINGS, Springer, 2017, p. 711-718Conference paper, Published paper (Refereed)
Abstract [en]

Phase separation in the Fe-Cr system has been studied experimentally. The upper limit of the miscibility gap and phase separation kinetics during the early stages of decomposition was studied using Vickers micro-hardness tests and atom probe tomography. The results indicate that the upper limit of the miscibility gap in current thermodynamic descriptions is overestimated, but it is necessary to further analyze the transition across the upper limit of the miscibility gap before drawing conclusions. This work demonstrates the efficient combination of micro-hardness measurements and atom probe tomography to investigate the miscibility gap in the Fe-Cr system.

Place, publisher, year, edition, pages
Springer, 2017
Series
Minerals Metals & Materials Series, ISSN 2367-1181
Keyword
Phase separation, Fe-Cr alloys, Miscibility gap, Atom probe tomography, Phase diagram
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-208737 (URN)10.1007/978-3-319-51493-2_68 (DOI)000401654100067 ()978-3-319-51493-2 (ISBN)978-3-319-51492-5 (ISBN)
Conference
146th TMS Annual Meeting and Exhibition, FEB 26-MAR 02, 2017, San Diego, CA
Note

QC 2017-06-13

Available from: 2017-06-13 Created: 2017-06-13 Last updated: 2017-11-10Bibliographically approved
Ma, T., Borrajo-Pelaez, R., Hedström, P., Blomqvist, A., Borgh, I., Norgren, S. & Odqvist, J. (2017). Liquid Phase Sintering of (Ti,Zr)C with WC-Co. Materials, 10(1), 57
Open this publication in new window or tab >>Liquid Phase Sintering of (Ti,Zr)C with WC-Co
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2017 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 10, no 1, p. 57-Article in journal, Editorial material (Refereed) Published
Abstract [en]

(Ti,Zr)C powder was sintered with WC-Co following an industrial process, including an isotherm at 1410 °C. A series of interrupted sintering trials was performed with the aim of studying the sintering behavior and the microstructural evolution during both solid-state and liquid-state sintering. Reference samples, using the same elemental compositions but with the starting components TiC and ZrC instead of (Ti,Zr)C, were also sintered. The microstructure was investigated using scanning electron microscopy and energy dispersive X-ray spectroscopy. It is found that the (Ti,Zr)C phase decomposes into Ti-rich and Zr-rich nano-scale lamellae before the liquid-state of the sintering initiates. The final microstructure consists of the binder and WC as well as two different γ phases, rich in either Ti (γ1) or Zr (γ2). The γ2 phase grains have a core-shell structure with a (Ti,Zr)C core following the full sintering cycle. The major differences observed in (Ti,Zr)C with respect to the reference samples after the full sintering cycle were the referred core-shell structure and the carbide grain sizes; additionally, the microstructural evolution during sintering differs. The grain size of carbides (WC, γ1, and γ2) is about 10% smaller in WC-(Ti,Zr)C-Co than WC-TiC-ZrC-Co. The shrinkage behavior and hardness of both composites are reported and discussed.

Keyword
cemented carbides, ternary cubic carbide; liquid-phase sintering, scanning electron microscopy, energy dispersive X-ray spectroscopy, dilatometer, differential scanning calorimetry
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-207823 (URN)10.3390/ma10010057 (DOI)000394838800057 ()2-s2.0-85011691103 (Scopus ID)
Note

QC 20170529

Available from: 2017-05-26 Created: 2017-05-26 Last updated: 2017-11-29Bibliographically approved
Malik, A., Odqvist, J., Höglund, L., Hertzman, S. & Ågren, J. (2017). Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel. Metallurgical and Materials Transactions. A, 48A(10), 4914-4928
Open this publication in new window or tab >>Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel
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2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48A, no 10, p. 4914-4928Article in journal (Refereed) Published
Abstract [en]

Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren-Boettinger-McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.

Place, publisher, year, edition, pages
SPRINGER, 2017
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-214868 (URN)10.1007/s11661-017-4214-7 (DOI)000408884300043 ()
Note

QC 20171024

Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2018-03-07Bibliographically approved
Ma, T., Borrajo-Pelaez, R., Hedström, P., Borgh, I., Blomqvist, A., Norgren, S. & Odqvist, J. (2016). Microstructure evolution during phase separation in Ti-Zr-C. International Journal of Refractory Metals and Hard Materials, 61, 238-248
Open this publication in new window or tab >>Microstructure evolution during phase separation in Ti-Zr-C
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2016 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 61, p. 238-248Article in journal (Refereed) Published
Abstract [en]

(Ti,Zr)C powder was synthesized by carbothermal reduction and subsequently aged at 1150–2000 °C. The phase composition and microstructure was investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscatter diffraction. It was found that the as-synthesized (Ti,Zr)C particles have a concentration gradient with a higher concentration of Ti at the surface of the particles. Furthermore, during aging the (Ti,Zr)C decomposes into Ti-rich and Zr-rich lamellae. During aging at 1400 and 1800 °C for 10 h, most Zr-rich and Ti-rich domains precipitate at grain boundaries, inheriting the crystal orientation of the parent grain behind the growth front. When the precipitate grows into another (Ti,Zr)C grain, that grain adopts the same crystal orientation as the parent grain. The crystallographic misorientation between adjacent lamellae is 0–5°. Based on these microstructural observations it is hypothesized that the mechanism of decomposition is discontinuous precipitation.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Carbides, Discontinuous precipitation, Electron backscatter diffraction, Energy-dispersive X-ray spectroscopy, Phase separation, Backscattering, Carbothermal reduction, Electron diffraction, Energy dispersive spectroscopy, Grain boundaries, Grain growth, Microstructure, Precipitation (chemical), Scanning electron microscopy, X ray diffraction, X ray spectroscopy, Concentration gradients, Electron back scatter diffraction, Energy dispersive X ray spectroscopy, Mechanism of decomposition, Micro-structural observations, Micro-structure evolutions, Mis-orientation, Crystal orientation
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-195185 (URN)10.1016/j.ijrmhm.2016.09.019 (DOI)000388048300032 ()2-s2.0-84989814061 (Scopus ID)
Funder
VINNOVA, 2014-03392
Note

QC 20161208

Available from: 2016-12-08 Created: 2016-11-02 Last updated: 2017-06-28Bibliographically approved
Hou, Z., Hedström, P., Chen, Q., Xu, Y., Wu, D. & Odqvist, J. (2016). Quantitative modeling and experimental verification of carbide precipitation in a martensitic Fe-0.16 wt%C-4.0 wt%Cr alloy. Calphad, 53, 39-48
Open this publication in new window or tab >>Quantitative modeling and experimental verification of carbide precipitation in a martensitic Fe-0.16 wt%C-4.0 wt%Cr alloy
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2016 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 53, p. 39-48Article in journal (Refereed) Published
Abstract [en]

Precipitation of carbides during tempering of a martensitic Fe-0.16 wt% C-4.0 wt% Cr alloy has been investigated by experimental analysis and quantitative modeling. It is found that both M7C3 and M23C6 form, at low- and high-angle grain boundaries in the martensite, as well as, at dislocations inside individual laths of martensite, during tempering at 700 °C. The applied Kampmann-Wagner numerical (KWN) modeling, utilizing CALPHAD thermodynamic and kinetic databases together with an assumption of local equilibrium and a constant tie-line, captures the main features of the precipitation, with a transient formation of metastable M23C6, and with M7C3 as the stable carbide. The predicted volume fraction and size are in reasonable agreement with extraction experiments for M7C3. However, for the metastable minority carbide M23C6, the modeling underestimates the size and overestimates the volume fraction within the transient time. With sound thermodynamic databases and physical parameter input, the adopted simplified modeling scheme is a valuable tool for materials design and optimization. Furthermore, by treating conditions at the phase interface more rigorously it is possible to account for different mechanisms of precipitation, such as e.g., non-partitioning local equilibrium, which could be important in systems where interstitial elements diffuse much faster than the substitutional ones.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Electron microscopy, Microstructure, Modeling, Precipitation, Steels, Tempering, Carbides, Chromium alloys, Grain boundaries, Martensite, Models, Phase interfaces, Steel, Volume fraction, Carbide precipitation, Different mechanisms, Experimental analysis, Experimental verification, High angle grain boundaries, Interstitial elements, Quantitative modeling, Thermodynamic database, Precipitation (chemical)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-186937 (URN)10.1016/j.calphad.2016.03.001 (DOI)000377315100005 ()2-s2.0-84960155147 (Scopus ID)
Note

Funding Details: U1260204, NSFC, National Natural Science Foundation of China

QC 20160518

Available from: 2016-05-18 Created: 2016-05-16 Last updated: 2017-11-30Bibliographically approved
Xu, X., Odqvist, J., Colliander, M. H., Thuvander, M., Steuwer, A., Westraadt, J. E., . . . Hedström, P. (2016). Structural Characterization of Phase Separation in Fe-Cr: A Current Comparison of Experimental Methods. Metallurgical and Materials Transactions. A, 47A(12), 5942-5952
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
Westraadt, J. E., Olivier, E. J., Neethling, J. H., Hedström, P., Odqvist, J., Xu, X. & Steuwer, A. (2015). A high-resolution analytical scanning transmission electron microscopy study of the early stages of spinodal decomposition in binary Fe-Cr. Materials Characterization, 109, 216-221
Open this publication in new window or tab >>A high-resolution analytical scanning transmission electron microscopy study of the early stages of spinodal decomposition in binary Fe-Cr
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2015 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 109, p. 216-221Article in journal (Refereed) Published
Abstract [en]

Spinodal decomposition (SD) is an important phenomenon in materials science and engineering. For example, it is considered to be responsible for the 475 degrees C embrittlement of stainless steels comprising the bcc (ferrite) or bct (martensite) phases. Structural characterization of the evolving minute nano-scale concentration fluctuations during SD in the Fe-Cr system is, however, a notable challenge, and has mainly been considered accessible via atom probe tomography (APT) and small-angle neutron scattering. The standard tool for nanostructure characterization, viz, transmission electron microscopy (TEM), has only been successfully applied to late stages of SD when embrit-dement is already severe. However, we here demonstrate that the structural evolution in the early stages of SD in binary Fe-Cr, and alloys based on the binary, are accessible via analytical scanning TEM. An Fe-36 wt% Cr alloy aged at 500 degrees C for 1, 10 and 100 h is investigated using an aberration-corrected microscope and it is found that highly coherent and interconnected Cr-rich regions develop. The wavelength of decomposition is rather insensitive to the sample thickness and it is quantified to 2, 3 and 6 nm after ageing for 1, 10 and 100 h, which is in reasonable agreement with prior APT analysis. The concentration amplitude is more sensitive to the sample thickness and acquisition parameters but the TEM analysis is in good agreement with APT analysis for the longest ageing time. These findings open up for combinatorial TEM studies where both local crystallography and chemistry is required.

Keyword
Spinodal decomposition, Phase separation, Stainless steel, STEM-EELS, Aberration-correction
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-179609 (URN)10.1016/j.matchar.2015.10.001 (DOI)000365365400028 ()2-s2.0-84944321634 (Scopus ID)
Funder
VINNOVA
Note

QC 20160112

Available from: 2016-01-12 Created: 2015-12-17 Last updated: 2017-11-30Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-3598-2465

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