| 1. |
- Fors, Dan, 1982, et al.
(författare)
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First-principles investigation of the stability of MN and CrMN precipitates under coherency strains in alpha-Fe (M = V, Nb, Ta)
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:11
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Tidskriftsartikel (refereegranskat)abstract
- We perform a systematic ab initio study of the interface energetics of thin coherent rocksalt (nacl) structured MN and tetragonal CrMN films in bcc Fe (M = V, Nb, Ta), motivated by the vital role of MN and CrMN precipitates for the long-term creep resistance in 9%-12%Cr steels. The similarities and differences in the work of separations and the elastic costs for the coherency strains are identified, and the possibility for formation of coherent films are discussed. Our findings provide valuable information of the interface energetics, which in continuation can be combined with thermodynamical modeling to obtain a better understanding of the initial nucleation stage of the MN and CrMN precipitates, and their influence on the long-term microstructural evolution in 9%-12%Cr steels.
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| 2. |
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| 3. |
- Fors, Dan, 1982, et al.
(författare)
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Theoretical investigation of moderate misfit and interface energetics in the Fe/VN system
- 2010
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Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 50:2, s. 550-559
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Tidskriftsartikel (refereegranskat)abstract
- In this study an ab initio based approach to determine the effect of moderate misfit on energies and structures for interfaces is presented and applied to the Fe(0 0 1)/VN(0 0 1) system. The interface energetics of the coherent and semicoherent structures of thin VN films in Fe is investigated in order to determine how the misfit is taken up. The coherent interface is directly treated with ab initio calculations, whereas the semicoherent interface energy is accessed by using a Peierls-Nabarro framework, in which ab initio data for chemical interactions across the interface is combined with a continuum description to account for the elastic distortions. The continuum treatment is here extended to thoroughly account for the anisotropy in the materials. Our approach shows that the elastic contribution to the total interface energy dominates for both the coherent and semicoherent structure and must therefore be accurately accounted for in the interface description. Further, the Peierls-Nabarro framework for the semicoherent interface is evaluated by comparing a full scale two-dimensional solution to one-dimensional approximations. We show that the one-dimensional treatment is sufficient in the present case for accurate results, and consequently interactions at dislocation intersections at the interface do not have to be considered. © 2010 Elsevier B.V. All rights reserved.
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| 4. |
- Fors, Dan, 1982
(författare)
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Theoretical studies of boron and transition-metal carbonitride additions in steels
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Martensitic 9-12%Cr steel alloys are today widely used as critical components in fossil-fired steam power plants with steam temperatures up to 600°C, where sound oxidation resistance and excellent creep strength are required. Their strength and long-term creep resistance are closely connected to the presence and stability of Cr-rich M23C6 (M = Cr, Fe) carbides and densely distributed MX precipitates (M = transition metal, X = C or N), which effectively act as obstacles for dislocation migration. In this thesis, the impact from additions of boron and transition-metal carbonitrides to 9-12%Cr steels are studied from a theoretical perspective by employing first-principlesdensity functional theory (DFT) calculations.In the first part of this thesis we have demonstrated that boron enters predominately as substitutional solute atoms in α-Fe. However, at elevated temperatures the interstitial occupancy is not negligible and the diffusion of boron in α-Fe is under equilibrium conditions found to be governed by the interstitial mechanism. This mixed occupancy explains previous inconsistencies in experimental measurements. Further, the findings for the boron diffusion were combined with a coarsening model to address an observed beneficial impact of boron on the coarsening rate ofM23C6 precipitates in 9-12%Cr steels. The outcome of the model showed that the fast interstitial diffusion in pure α-Fe was too high to explain the enhanced stability of the precipitates, whereas a substitutional mechanism, favored by vacancy generation during dissolution of M23C6, was concluded to be a possible rate-limiting effect for the coarsening process.In the second part of this thesis, we have performed a systematic investigation of the electronic and atomic structure of coherently strained and semicoherent interfaces between α-Fe and nacl-structured MX films. The coherently strained interfaces were treated directly with DFT calculations, whereas the semicoherent interfaces were address by using a Peierls-Nabarro framework. The results showed that the chemical and elastic contributions can be of equal importance for the interface energetics, and hence, it is essential to accurately include both aspects in the interface description. Furthermore, by utilizing the obtained results for Fe/MX interfaces we continued and investigated the interface energetics of coherently strainednacl-structured MN and tetragonal CrMN films in α-Fe, with the aim to improve the understanding of observed nucleation difficulties of CrMN precipitates in 9-12%Cr steel alloys. Analyzes of the chemical interactions across the interface and the elastic energy costs for the coherency strains lent support to that interface energies could provide a fundamental understanding of the early nucleation stage.Taken altogether, the results in this thesis demonstrate that first-principles DFT calculations can provide valuable information about the impact of boron and transition metal carbonitride additions in steels.
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| 5. |
- Fors, Dan, 1982, et al.
(författare)
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Theoretical study of interface structure and energetics in semicoherent Fe(001)/MX(001) systems (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Ta; X=C or N)
- 2010
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 82:19, s. 195410-
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Tidskriftsartikel (refereegranskat)abstract
- We perform a systematic ab initio study of the electronic and atomic structure of semicoherent interfaces between bcc Fe and NaCl MX (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Ta; X=C or N). The interface energetics is accessed by using a Peierls-Nabarro framework, in which ab initio data for the chemical interactions across the interface are combined with a continuum description to account for the elastic distortions. The key factors to the trends in the interface energy are identified and discussed with respect to the size of the misfit and the electronic structure of the MX phase. Our approach shows that the inclusion of lattice misfit can have a significant contribution to the interface energy (up to 1.5 J/m2) and must therefore be thoroughly accounted for in the interface description. The results will have important bearings on our ability to understand and describe precipitate stability in steels.
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| 6. |
- Fors, Dan, 1982
(författare)
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Toward first-principles understanding of carbonitride precipitation in steels
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The most important and widely used group of metallic alloys is steels, whichare alloys of the element iron together with carbon and usually other elementssuch as B, N, V, Nb and Cr. The strength in these alloys is closely connectedto the distribution of point defects, precipitates and grain boundaries whicheffectively act as obstacles for dislocation migration. In the present thesiswe aim to obtain a fundamental understanding of the precipitation of transitionmetal carbonitrides in steels.In particular, we investigate the effect of boron diffusion on the coarseningrate of M23(C,B)6 precipitates (M = transition metal) using densityfunctional theory (DFT) calculations. The results show that boron predominantlyenters as a substitutional solid solution in the iron matrix at low temperatures.At elevated temperatures the interstitial population can however not be neglectedwhich leads to that boron diffusion under equilibrium conditions will be governedby the interstitial mechanism. Further, the corresponding diffusion rate is shownto be too fast in order to explain the coarsening rate of M23(C,B)6 andit's concluded that other possible mechanisms must be explored.In addition, the energetics for semicoherent interfaces between the iron matrixand nacl structured MX precipitates (X = C, N) are studied using DFT incombination with a Peierls-Nabarro model. The electronic structure at the interfaceis characterized by covalent Fe(3d)-X(2p) and metallic Fe(3d)-M(d) bonds, where thestrength of the metallic interaction is connected to the relative position of theM d-band and Fe d-band centers. In addition, it is shown that the elastic energycontained in the dislocation network, due to the lattice misfit at the interface,gives a significant contribution to the interface energy.
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| 7. |
- Liu, Fang, 1975, et al.
(författare)
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Effect of Boron on Carbide Coarsening at 873 K (600°C) in 9 to 12 pct Chromium Steels
- 2012
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43:11, s. 4053-4062
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Tidskriftsartikel (refereegranskat)abstract
- The addition of small amounts of boron to 9 to 12 pct chromium steels has been found to decrease their creep rate at 823 K to 923 K (550 °C to 650 °C). In this article, the behavior of boron during austenitizing, tempering, and isothermal heat treatment at 873 K (600 °C) is studied using high-resolution microscopy and microanalysis as well as using atomistic modeling. It was found that increasing the boron content from 9 to 40 ppm decreased the coarsening constant of M23C6 by a factor of almost 2. Most of the added boron was incorporated in M23C6. Atomistic modeling showed that boron diffusion in ferrite is dominated by an interstitial mechanism at 873 K (600 °C). However, the generation of vacancies when carbide precipitates dissolve may promote a distribution with substitutional boron atoms. The absence of a fast mechanism for the transition from substitutional to interstitial occupancy will make the slow substitutional boron diffusion in the matrix rate controlling for the coarsening process.
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