| 1. |
- Gorbatov, Oleg I., et al.
(författare)
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Role of magnetism in Cu precipitation in alpha-Fe
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:17, s. 174113-
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Tidskriftsartikel (refereegranskat)abstract
- The temperature-dependent solubility of Cu in alpha-Fe and initial stages of Cu precipitation are investigated in first-principles calculations and statistical thermodynamic and kinetic modeling based on ab initio effective interactions. We demonstrate that the weakening of the phase separation tendency with increasing temperature, especially close to the magnetic phase transition, is related to the strong dependence of the "chemical" interactions on the global magnetic state. At the same time, our calculations demonstrate that the vibrational contribution obtained in the quasiharmonic approximation is relatively small for temperatures near the Curie point. The results of Monte Carlo simulations of Cu solubility and clustering are in good agreement with experimental data.
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| 2. |
- Razumovskii, I. M., 1985-, et al.
(författare)
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Effect of the Particle Size of γ’ Phase on the Mechanical Properties of Ni base Superalloy
- 2011
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Ingår i: Euro Superalloys 2010. - : Trans Tech Publications Inc.. - 9783037851135 ; , s. 96-101
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Konferensbidrag (refereegranskat)abstract
- The effect of gamma’ particle size upon the mechanical properties of Ni base superalloy EP741NP obtained by powder metallurgy was investigated. The particle size of gamma’ phase in gamma-gamma’ microstructure was varied by changing the cooling rate V from the temperature of the solid solution treatment at 1200 C (V = 80, 200 and 400 C \ min.). After solid solution treatment billets were subjected to aging in the standard mode. It was established that as V increases from 80 to 200 C \ min., the average particle’s size of gamma’ phase decreases from 0.54 microns to 0.22 microns in the aged state. This improves the characteristics of creep and low cycle fatigue at 650C: time to rupture under load 1000 MPa increased from 132 hours to 416 hours and low cycle fatigue increased from 42,215 to 82,016 cycles.
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| 3. |
- Razumovskii, I. M., et al.
(författare)
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New generation of Ni-based superalloys designed on the basis of first-principles calculations
- 2008
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 497:1-2, s. 18-24
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to the design of Ni-based single crystal superalloys is proposed. It is based on a concept that under given structural conditions, the creep-rupture characteristics of superalloys are mainly determined by interatomic bonding given by the cohesive energy. In order to characterize the individual contribution of each alloying element to the strength properties at high temperature, we introduce a parameter, X, which is the partial molar cohesive energy of an alloy component. This parameter is then obtained in the total energy first-principles calculations for a usual set of alloying elements. We demonstrate that creep-rupture characteristics of alloys indeed correlate with the total gain partial molar cohesive energy due to alloying and find that W, Ta, and Re have the highest values of X, and should therefore play the major role in providing high-temperature strength of superalloys. Based on this finding, we design three new superalloys with a high content of W and show that they have superior creep-rupture properties compared not only with their counterparts with the lower content of W, but also with the best Ru-bearing Ni-based superalloys.
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| 4. |
- Razumovskiy, Vsevolod I., 1985-, et al.
(författare)
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Analysis of the Alloying System in Ni-Base Superalloys Based on Ab Initio Study of Impurity Segregation to Ni Grain Boundary
- 2011
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Ingår i: EURO SUPERALLOYS 2010. - : Trans Tech Publications Inc.. - 9783037851135 ; , s. 192-197
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Konferensbidrag (refereegranskat)abstract
- A new approach to the design of Ni-based polycrystalline superalloys is proposed. It is based on a concept that under given structural conditions, the performance of superalloys is determined by the strength of interatomic bonding both in the bulk and at grain boundaries of material. We characterize the former by the cohesive energy of the bulk alloy, whereas for the latter we employ the work of separation of a representative high angle grain boundary. On the basis of our first principle calculations we suggest Hf and Zr as “minor alloying additions” to Ni-based alloys. Re, on the other hand, appears to be of little importance in polycrystalline alloys.
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| 5. |
- Razumovskiy, Vsevolod I., 1985-, et al.
(författare)
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Effect of Alloying Elements and Impurities on Interface properties in Aluminum Alloys
- 2011
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Ingår i: Physics of the solid state. - 1063-7834 .- 1090-6460. ; 53:11, s. 2189-2193
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Tidskriftsartikel (refereegranskat)abstract
- The segregation energies of B, Si, P, Cr, Ni, Zr, and Mg on the special grain boundary (GB) Σ5 (210)[100] and on the open (210) surface of aluminum have been determined and the GB splitting energy has been calculated by the density functional theory methods. It has been shown that all elements listed above enrich the GB; for B, Si, P, Cr, Ni and Zr, Mg, interstitial and substitutional sites are preferred, respectively. The effect of alloying elements on the GB binding has been estimated using the parameter η equal to the change in the fracture work of the aluminum GB when adding alloying element atoms. From the viewpoint of strengthening the GB binding forces, Zr, Cr, Ni, and Mg are efficient, Si and B are neutral and phosphorus weakens GBs.
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| 6. |
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| 7. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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First-principles-aided design of a new Ni-base superalloy : Influence of transition metal alloying elements on grain boundary and bulk cohesion
- 2015
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 82, s. 369-377
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of gamma matrix. The ideal work of separation, W-sep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, chi, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Sigma 5 (2 1 0)[1 0] symmetric tilt GB as a representative high-angle GB in Ni, we calculate W-sep, chi, and GB segregation energies, E-seg for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on W-sep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.
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| 8. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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New Pt-Based Superalloy System Designed from First Principles
- 2009
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Ingår i: Advanced Intermetallic-Based Alloys For Extreme Environment And Energy Applications. - 9781605111001 ; , s. 233-238
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Konferensbidrag (refereegranskat)abstract
- Pt-Sc alloys with the gamma-gamma' microstructure are proposed as a basis for a new generation of Pt-based superalloys for ultrahigh-temperature applications. This alloy system was identified on the basis of first-principles calculations. Here we discuss the prospects of the Pt-Sc alloy system on the basis of calculated elastic properties, phonon spectra, and defect formation energies.
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| 9. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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The effect of alloying elements on grain boundary and bulk cohesion in aluminum alloys : An ab initio study
- 2011
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 65:10, s. 926-929
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Tidskriftsartikel (refereegranskat)abstract
- The effect of B, Si, P, Cr, Ni, Zr and Mg impurities on cohesive properties of Al and its special grain boundary (GB) Sigma 5 (2 1 0) [1 0 0], as well as their segregation behavior at the GB and (2 1 0) surface are studied from first principles. Our analysis determines Ni to be the best and P the worst alloying elements in regard to the overall resistance to decohesion of Al alloys.
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| 10. |
- Razumovskiy, Vsevolod I., 1985-, et al.
(författare)
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The influence of alloying elements on grain boundary and bulk cohesion in aluminum all : Ab initio study
- 2012
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Ingår i: Thermec 2011 Supplement. - : Trans Tech Publications Inc.. - 9783037853047 ; , s. 417-422
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Konferensbidrag (refereegranskat)abstract
- The effect of B, Si, P, Cr, Ni, Zr and Mg on cohesive properties of Al and the specialgrain boundary (GB) Σ5 (210)[100], as well as their segregation behavior at the GB and the (210)surface are studied by first principles method. The analysis of these parameters allows us to singleout Ni as the best and phosphorus as the worst interatomic bond strengthening alloying elements.
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| 11. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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Ab initio calculations of elastic properties of Pt-Sc alloys
- 2008
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Ingår i: Intermetallics (Barking). - : Elsevier. - 0966-9795 .- 1879-0216. ; 16:8, s. 982-986
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Tidskriftsartikel (refereegranskat)abstract
- Elastic properties of substitutionally disordered Pt-rich Pt-Sc alloys and L1(2)-ordered Pt3Sc compound are derived from the first-principles calculations based on the exact muffin-tin orbitals (EMTO) method. We demonstrate that these alloys should exhibit a ductile behavior, which combined with relatively high melting temperature and strong cohesive properties make them a very promising candidate for high-temperature applications.
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| 12. |
- Razumovskiy, Vsevolod I., 1985-, et al.
(författare)
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Ab-initio calculations of kinetic properties in ZrC and TiC carbides
- 2011
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Konferensbidrag (refereegranskat)abstract
- Self-diffusion of the metal and carbon atoms in TiC and ZrC carbides is studied by first principles methods. Our calculations yield point defects energies, vacancy jump barriers and diffusion pre-factors in TiC and ZrC. The results are in reasonable agreement with the available experimental data and suggest that the self-diffusion mechanism for metal atoms in these carbides may involve nearest-neighbor vacancy pairs (one metal and one carbon vacancy).
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| 13. |
- Razumovskiy, Vsevolod I., 1985-, et al.
(författare)
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Effect of temperature on the elastic anisotropy of pure Fe and Fe0.9Cr0.1 random alloy
- 2011
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 107:20
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Tidskriftsartikel (refereegranskat)abstract
- The elastic properties of pure iron and substitutionally disordered 10 at. % Cr Fe-Cr alloy areinvestigated as a function of temperature using first-principles electronic-structure calculations bythe exact muffin-tin orbitals method. The temperature effects on the elastic properties are includedvia the electronic, magnetic, and lattice expansion contributions. We show that the degree ofmagnetic order in both pure iron and Fe90Cr10 alloy mainly determines the dramatic change of theelastic anisotropy of these materials at elevated temperatures. The effect of lattice expansion isfound to be secondary but also very important for quantitative modeling.
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| 14. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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Effect of thermal lattice expansion on the stacking fault energies of fcc Fe and Fe75Mn25 alloy
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 93:5
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Tidskriftsartikel (refereegranskat)abstract
- Temperature dependent stacking fault energies in fcc Fe and the Fe75Mn25 random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave ( SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the Fe75Mn25 alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.
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| 15. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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First-principles study of elastic properties of Cr- and Fe-rich Fe-Cr alloys
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:2
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Tidskriftsartikel (refereegranskat)abstract
- Elastic properties of substitutionally disordered Cr- and Fe-rich Fe-Cr alloys are derived from first-principles calculations using the exact muffin-tin orbitals method and the coherent potential approximation. A peculiarity in the concentration dependence of elastic constants in Fe-rich alloys is demonstrated and related to a change in the Fermi surface topology. Our calculations predict high values for the elastic constants of Cr-rich Fe-Cr alloys, but at the same time show that these alloys could be rather brittle according to the Pugh criterion (the ratio between shear and bulk moduli is calculated to be greater than 0.5).
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| 16. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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Vacancy-cluster mechanism of metal-atom diffusion in substoichiometric carbides
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:5, s. 054203-
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Tidskriftsartikel (refereegranskat)abstract
- We find, using ab initio atomistic simulations of vacancy-mediated diffusion processes in TiC and ZrC, that a multivacancy self-diffusion mechanism is operative for metal-atom diffusion in substoichiometric carbides. It involves a special type of a stable point defect, a metal vacancy "dressed" in a shell of carbon vacancies. We show that this vacancy cluster is strongly bound and can propagate through the lattice without dissociating.
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| 17. |
- Ruban, Andrei V., et al.
(författare)
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First-principles based thermodynamic model of phase equilibria in bcc Fe-Cr alloys
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:17, s. 174111-
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Tidskriftsartikel (refereegranskat)abstract
- A first-principles based thermodynamic model for magnetic alloys is applied to the calculation of the Fe-Cr phase diagram restricted by the bcc structure. The model includes magnetic, electronic, phonon, and local atomic relaxations contributions to the free-energy derived from ab initio calculations. Atomic short-range-order effects are found to be relatively small and they have been neglected in the calculations, assuming that alloys are in the completely random state. In contrast, we have taken into consideration magnetic short-range-order effects, which are found to be very important in particular above the Curie temperature. The calculated phase diagram is in reasonable agreement with the latest CALPHAD assessment. Our calculations reproduce a feature known as a Nishizawa horn for the Fe-rich high-temperature part of the phase diagram.
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| 18. |
- Ruban, Andrei V., et al.
(författare)
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Spin-wave method for the total energy of paramagnetic state
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:17, s. 174407-
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Tidskriftsartikel (refereegranskat)abstract
- Spin-wave formalism provides a convenient alternative way of modeling the high-temperature paramagnetic state for a certain type of magnets within the framework of Hamiltonian-type electronic-structure methods. For Heisenberg systems, it is formally equivalent to the so-called disordered local moment approach, which is usually used in the methods based on the coherent potential approximation within the Green's function or multiple-scattering techniques. In this paper, we demonstrate that the spin-wave method has certain advantages when it comes to the calculation of forces and relaxations. It also allows one to take magnetic short-range-order effects into consideration. As examples of the application of the spin-wave method, we calculate the energy of the paramagnetic state in fcc Co and bcc Fe, the vacancy formation energy, elastic constants, and phonon spectrum in bcc paramagnetic Fe. We demonstrate that magnetic short-range-order effects play a crucial role in the mechanical stabilization of the bcc Fe at high temperature in the paramagnetic state.
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| 19. |
- Yeddu, Hemantha Kumar, et al.
(författare)
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Multi-length scale modeling of martensitic transformations in stainless steels
- 2012
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 60:19, s. 6508-6517
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Tidskriftsartikel (refereegranskat)abstract
- In the present work a multi-length scale model is developed to study both the athermal and stress-assisted martensitic transformations in a single crystal of 301 type stainless steel. The microstructure evolution is simulated using elastoplastic phase-field simulations in three dimensions. The input data for the simulations is acquired from a combination of computational techniques and experimental works. The driving force for the transformation is calculated by using the CALPHAD technique and the elastic constants of the body-centered cubic phase are calculated by using ab initio method. The other input data is acquired from experimental works. The simulated microstructures resemble a lath-type martensitic microstructure, which is in good agreement with the experimental results obtained for a stainless steel of similar composition. The martensite habit plane predicted by the model is in accordance with experimental results. The Magee effect, i.e. formation of favorable martensite variants depending on the loading conditions, is observed in the simulations. The results also indicate that anisotropic loading conditions give rise to a significant anisotropy in the martensitic microstructure.
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