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Hou, Z., Babu, R. P., Hedström, P. & Odqvist, J. (2019). Early stages of cementite precipitation during tempering of 1C-1Cr martensitic steel. Journal of Materials Science, 54(12), 9222-9234
Open this publication in new window or tab >>Early stages of cementite precipitation during tempering of 1C-1Cr martensitic steel
2019 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 54, no 12, p. 9222-9234Article in journal (Refereed) Published
Abstract [en]

The precipitation of cementite (M3C) from as-quenched martensite during tempering at 500 and 700 degrees C was investigated in a Fe-1C-1Cr (wt%) alloy. Tempering for a short duration at 700 degrees C results in a Cr/Fe ratio in the core region of M3C precipitates which is equal to the bulk alloy composition, while a shell on the surface of the precipitates exhibits a higher Cr concentration. With a prolonged tempering up to 5h, the shell concentration gradually increases toward the equilibrium value, but the core region has not yet reached the equilibrium value. After tempering for 5s at 500 degrees C, there is no Cr enrichment found at the M3C-matrix interface, while a transition to partitioning of Cr is found during the first 5min of tempering at 500 degrees C. These experimental results indicate that M3C grows without significant partitioning of substitutional elements at both temperatures initially, i.e., growth is carbon diffusion controlled. This stage is, however, very short, and soon after 5s at 700 degrees C and 5min at 500 degrees C, Cr diffusion becomes important. Calculations using the diffusion simulation software DICTRA and precipitation simulation software TC-PRISMA were performed. The diffusion simulations using the local equilibrium interface condition show excellent agreement with experiments concerning Cr enrichment of the particles, but the size evolution is overestimated. On the other hand, the precipitation simulations underestimate the size evolution. It is suggested that a major improvement in the precipitation model could be achieved by implementing a modified nucleation model that considers nucleation far from the equilibrium composition.

Place, publisher, year, edition, pages
Springer, 2019
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-249841 (URN)10.1007/s10853-019-03530-8 (DOI)000462633100031 ()2-s2.0-85063202343 (Scopus ID)
Note

QC 20190430

Available from: 2019-04-30 Created: 2019-04-30 Last updated: 2019-06-11Bibliographically approved
Zhou, T., Faleskog, J., Babu, P., Odqvist, J., Yu, H. & Hedström, P. (2019). Exploring the relationship between the microstructure and strength of fresh and tempered martensite in a maraging stainless steel Fe-15Cr-5Ni. Materials Science & Engineering: A, 745, 420-428, Article ID DELL AJ, 1985, METALLURGICAL TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, V16, P2131.
Open this publication in new window or tab >>Exploring the relationship between the microstructure and strength of fresh and tempered martensite in a maraging stainless steel Fe-15Cr-5Ni
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2019 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 745, p. 420-428, article id DELL AJ, 1985, METALLURGICAL TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, V16, P2131Article in journal (Refereed) Published
Abstract [en]

Hierarchical microstructure engineering is an efficient design path for ultra-high strength steels. An excellent example of this is maraging stainless steel, which achieves its high-performance by combining the hierarchic martensitic microstructure and nano-sized precipitates. Relating this complex microstructure with mechanical properties, e.g. strength, is not trivial. In the present work, we therefore explore the relationship between the hierarchic microstructure, evolving with the tempering of a Cu-containing maraging stainless steel 15-5 PH, and its strength. Comprehensive microstructure characterization, including the quantification of dislocation density, effective grain size, precipitates and retained austenite fraction is performed after quenching and tempering at 500 degrees C. The microstructure data is subsequently used as input for assessing the evolution of individual strength contributions and thus the increase in strength of tempered martensite contributed by Cu precipitation strengthening is evaluated. It is found that the Cu precipitation and dislocation annihilation are two major factors controlling the evolution of the yield strength of the tempered martensite. The Cu precipitation strengthening is also modelled using our previous Langer-Schwartz-Kampmann-Wagner model based predictions of the Cu precipitation, and modelled precipitation strengthening is compared with the evaluated Cu precipitation strengthening from the experiments. The work exemplifies the promising approach of combining physically based precipitation modelling and precipitation-strengthening modelling for alloy design and optimization. However, more work is needed to develop a generic predictive framework.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
Mechanical properties, Modelling, Maraging stainless steel, Cu precipitation strengthening, Martensite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-244522 (URN)10.1016/j.msea.2018.12.126 (DOI)000457819400045 ()2-s2.0-85059590155 (Scopus ID)
Note

QC 20190403

Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
Rahaman, M., Mu, W., Odqvist, J. & Hedström, P. (2019). Machine Learning to Predict the Martensite Start Temperature in Steels. Metallurgical and Materials Transactions. A, 50A(5), 2081-2091
Open this publication in new window or tab >>Machine Learning to Predict the Martensite Start Temperature in Steels
2019 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 5, p. 2081-2091Article in journal (Refereed) Published
Abstract [en]

The martensite start temperature (M-s) is a critical parameter when designing high-performance steels and their heat treatments. It has, therefore, attracted significant interest over the years. Numerous methodologies, such as thermodynamics-based, linear regression and artificial neural network (ANN) modeling, have been applied. The application of data-driven approaches, such as ANN modeling, or the wider concept of machine learning (ML), have shown limited technical applicability, but considering that these methods have made significant progress lately and that materials data are becoming more accessible, a new attempt at data-driven predictions of the M-s is timely. We here investigate the usage of ML to predict the M-s of steels based on their chemical composition. A database of the M(s)vs alloy composition containing 2277 unique entries is collected. It is ensured that all alloys are fully austenitic at the given austenitization temperature by thermodynamic calculations. The ML modeling is performed using four different ensemble methods and ANN. Train-test split series are used to evaluate the five models, and it is found that all four ensemble methods outperform the ANN on the current dataset. The reason is that the ensemble methods perform better for the rather small dataset used in the present work. Thereafter, a validation dataset of 115 M-s entries is collected from a new reference and the final ML model is benchmarked vs a recent thermodynamics-based model from the literature. The ML model provides excellent predictions on the validation dataset with a root-mean-square error of 18, which is slightly better than the thermodynamics-based model. The results on the validation dataset indicate the technical usefulness of the ML model to predict the M-s in steels for design and optimization of alloys and heat treatments. Furthermore, the agility of the ML model indicates its advantage over thermodynamics-based models for M-s predictions in complex multicomponent steels. (C) The Author(s) 2019

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-251196 (URN)10.1007/s11661-019-05170-8 (DOI)000463991300003 ()2-s2.0-85063028004 (Scopus ID)
Note

QC 20190523

Available from: 2019-05-23 Created: 2019-05-23 Last updated: 2019-05-23Bibliographically approved
Bonvalet, M., Odqvist, J., Ågren, J. & Forsberg, A. (2019). Modelling of prismatic grain growth in cemented carbides. INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS, 78, 310-319
Open this publication in new window or tab >>Modelling of prismatic grain growth in cemented carbides
2019 (English)In: INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS, ISSN 0263-4368, Vol. 78, p. 310-319Article in journal (Refereed) Published
Abstract [en]

A mean-field model dealing with prismatic grain growth during liquid phase sintering of cemented carbides with a Co-rich binder is presented. The evolution of the size of an assembly of non-spherical grains is obtained using a Kampmann-Wagner approach and by introducing a constant shape factor between the characteristic lengths of prisms. This factor is a function of interfacial energies of the two kind of facets, basal and prismatic, considered. The growth model is based on three different mechanisms, that can be rate limiting, taking place in series: 2D nucleation of a new atomic layer, mass transfer across the interface and long-range diffusion. The driving force for coarsening is distributed between the different facets. These equations are solved numerically, and the simulation results reveal that the specific abnormal grain growth phenomena experimentally observed in cemented carbides may be reproduced with this new more realistic description of the grain shape contrary to the spherical approach developed in the past. It is also shown that the initial powder size distribution, and more specifically its shape has a strong influence on the distribution of the driving force between the different rate limiting mechanisms and thus on the occurrence of abnormal grain growth. In that case, the self-similarity of the normalized grain size distribution over time is not achieved.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2019
Keywords
Grain coarsening, Abnormal grain growth, Cemented carbides, Modelling, Liquid phase sintering
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-239962 (URN)10.1016/j.ijrmhm.2018.10.007 (DOI)000451489300038 ()2-s2.0-85055672266 (Scopus ID)
Funder
VINNOVA
Note

QC 20181211

Available from: 2018-12-11 Created: 2018-12-11 Last updated: 2018-12-11Bibliographically approved
Xu, X., Wessman, S., Odqvist, J., King, S. M. & Hedström, P. (2019). Nanostructure, microstructure and mechanical properties of duplex stainless steels 25Cr-7 Ni and 22Cr-5Ni (wt.%) aged at 325 degrees C. Materials Science & Engineering: A, 754(ALGUE A, 1990, JOURNAL OF MATERIALS SCIENCE, V25, P4977), 512-520
Open this publication in new window or tab >>Nanostructure, microstructure and mechanical properties of duplex stainless steels 25Cr-7 Ni and 22Cr-5Ni (wt.%) aged at 325 degrees C
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2019 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 754, no ALGUE A, 1990, JOURNAL OF MATERIALS SCIENCE, V25, P4977, p. 512-520Article in journal (Refereed) Published
Abstract [en]

The nanoscale concentration fluctuation evolution due to phase separation (PS) and the corresponding mechanical property changes in two duplex stainless steels, 22Cr-5Ni (2205) and 25Cr-7N1 (2507), have been studied after aging at 325 degrees C for up to 6000 h. The nanostructure characterization is performed using small-angle neutron scattering (SANS) and the microstructure and fractography analyses, including observations on fracture surfaces and fracture cross-sections, are performed by scanning electron microscopy and electron backscatter diffraction. The results show that the kinetics of PS in grade 2507 is faster than that in grade 2205, leading to greater hardening and deterioration in toughness for grade 2507 as compared to grade 2205. The evolution of the nanostructure in the ferrite phase changes the deformation mode from the original ductile fracture to a quasi-cleavage type fracture where deformation twins form in the hardened ferrite. Delamination, grain fragmentation in ferrite and plastic slip deformation of the austenite are suggested to dissipate most of the energy absorbed by the crack during brittle fracture.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
Duplex stainless steel, Phase separation, Spinodal decomposition, Small-angle neutron scattering (SANS), Mechanical property, Nanostructure
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-252397 (URN)10.1016/j.msea.2019.03.046 (DOI)000466249400052 ()2-s2.0-85063511012 (Scopus ID)
Note

QC 20190716

Available from: 2019-07-16 Created: 2019-07-16 Last updated: 2019-07-16Bibliographically approved
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: 2018-09-25Bibliographically 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
Keywords
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
Ma, T., Hedström, P. & Odqvist, J. (2018). Effect of synthesis temperature and aging on the microstructure and hardness of Ti-Zr-C. International Journal of Refractory Metals and Hard Materials, 73, 99-105
Open this publication in new window or tab >>Effect of synthesis temperature and aging on the microstructure and hardness of Ti-Zr-C
2018 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 73, p. 99-105Article in journal (Refereed) Published
Abstract [en]

The mixed carbide Ti-Zr-C has been synthesized through carbothermal reduction of TiZrO4 at 2200 °C, 2300 °C, and 2400 °C. As-synthesized carbide was subsequently aged at 1400 °C to study phase separation. Microstructural investigations and nano-indentation measurements were performed. It was found that the synthesis temperature was important for the homogeneity and porosity of the as-synthesized powder, which strongly influenced the subsequent phase separation upon aging. The phase separation occurred via discontinuous precipitation, and high-angle boundaries were preferred. Furthermore, the characteristics of discontinuous precipitation in the present Ti-Zr-C system with a miscibility gap were compared to those of classical discontinuous precipitation. Finally, fully decomposed particles were found to be slightly harder than the unaged carbide particles.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Discontinuous precipitation, Electron backscatter diffraction, Grain boundary misorientation, Powder metallurgy, Ternary carbide
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-227560 (URN)10.1016/j.ijrmhm.2018.02.001 (DOI)000430028800014 ()2-s2.0-85041405008 (Scopus ID)
Funder
VINNOVA, 2014-03392Swedish Foundation for Strategic Research , RMA 15-0062
Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-17Bibliographically approved
Rashidi, M., Odqvist, J., Johansson, L., Hald, J., Andrén, H.-O. -. & Liu, F. (2018). Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels. Materialia, 4, 247-254
Open this publication in new window or tab >>Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels
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2018 (English)In: Materialia, ISSN 2589-1529, Vol. 4, p. 247-254Article in journal (Refereed) Published
Abstract [en]

Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy investigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Besides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.

Place, publisher, year, edition, pages
Elsevier B.V., 2018
Keywords
Coarsening rate, Creep, Diffusivity, Electron microscopy, Precipitate
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-247006 (URN)10.1016/j.mtla.2018.09.024 (DOI)2-s2.0-85060881350 (Scopus ID)
Note

QC 20190626

Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-06-26Bibliographically 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: 2018-08-21Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-3598-2465

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