| 1. |
- Landa, A., et al.
(author)
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Stability in bcc Transition Metals : Madelung and Band-Energy Effects due to Alloying
- 2009
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:23
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Journal article (peer-reviewed)abstract
- The phase stability of group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the body-centered-cubic phase relative to low-symmetry rhombohedral phases. We show that band-structure effects determine phase stability when a particular group VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons destabilize and stabilize bcc, respectively. When alloying with neighbors of higher d-transition series, electrostatic Madelung energy dominates and stabilizes the body-centered-cubic phase. This surprising prediction invalidates current understanding of simple d-electron bonding that dictates high-symmetry cubic and hexagonal phases.
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| 2. |
- Kabliman, Evgeniya, et al.
(author)
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Configurational thermodynamics of the Fe-Cr sigma phase
- 2011
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:18, s. 184206-
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Journal article (peer-reviewed)abstract
- Configurational thermodynamics of the Fe-Cr sigma phase is investigated on the basis of an Ising-type configurational Hamiltonian and a single-site mean-field model for the free energy. The parameters of the statistical models are obtained from efficient first-principles calculations using different computational techniques. We demonstrate that the effective pair and multisite interactions in the sigma phase are relatively small, which allows using a simplified model for distributing Fe and Cr atoms among sublattices. We also show that this system exhibits a nontrivial magnetic behavior at high temperatures, which affects the site occupation by Fe and Cr atoms. The structural variation (volume and c/a) that might be present due to neutron irradiation and thermal expansion can lead to an additional atomic redistribution.
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| 3. |
- Landa, A., et al.
(author)
-
Alloying-driven phase stability in group-VB transition metals under compression
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:14, s. 144114-
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Journal article (peer-reviewed)abstract
- The change in phase stability of group-VB (V, Nb, and Ta) transition metals due to pressure and alloying is explored by means of first-principles electronic-structure calculations. It is shown that under compression stabilization or destabilization of the ground-state body-centered-cubic (bcc) phase of the metal is mainly dictated by the band-structure energy that correlates well with the position of the Kohn anomaly in the transverse-acoustic-phonon mode. The predicted position of the Kohn anomaly in V, Nb, and Ta is found to be in a good agreement with data from the inelastic x-ray or neutron-scattering measurements. In the case of alloying the change in phase stability is defined by the interplay between the band-structure and Madelung energies. We show that band-structure effects determine phase stability when a particular group-VB metal is alloyed with its nearest neighbors within the same d-transition series: the neighbor with less and more d electrons destabilize and stabilize the bcc phase, respectively. When V is alloyed with neighbors of a higher (4d- or 5d-) transition series, both electrostatic Madelung and band-structure energies stabilize the body-centered-cubic phase. The opposite effect (destabilization) happens when Nb or Ta is alloyed with neighbors of the 3d-transition series.
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| 4. |
- Moitzi, Franco, et al.
(author)
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Ab initio framework for deciphering trade-off relationships in multi-component alloys
- 2024
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In: npj Computational Materials. - : Springer Nature. - 2057-3960. ; 10:1
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Journal article (peer-reviewed)abstract
- While first-principles methods have been successfully applied to characterize individual properties of multi-principal element alloys (MPEA), their use in searching for optimal trade-offs between competing properties is hampered by high computational demands. In this work, we present a framework to explore Pareto-optimal compositions by integrating advanced ab initio-based techniques into a Bayesian multi-objective optimization workflow, complemented by a simple analytical model providing straightforward analysis of trends. We benchmark the framework by applying it to solid solution strengthening and ductility of refractory MPEAs, with the parameters of the strengthening and ductility models being efficiently computed using a combination of the coherent-potential approximation method, accounting for finite-temperature effects, and actively-learned moment-tensor potentials parameterized with ab initio data. Properties obtained from ab initio calculations are subsequently used to extend predictions of all relevant material properties to a large class of refractory alloys with the help of the analytical model validated by the data and relying on a few element-specific parameters and universal functions that describe bonding between elements. Our findings offer crucial insights into the traditional strength-vs-ductility dilemma of refractory MPEAs. The proposed framework is versatile and can be extended to other materials and properties of interest, enabling a predictive and tractable high-throughput screening of Pareto-optimal MPEAs over the entire composition space.
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| 5. |
- Moitzi, Franco, et al.
(author)
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Accurate ab initio modeling of solid solution strengthening in high entropy alloys
- 2022
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In: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:10
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Journal article (peer-reviewed)abstract
- High entropy alloys (HEA) represent a class of materials with promising properties, such as high strength and ductility, radiation damage tolerance, etc. At the same time, a combinatorially large variety of compositions and a complex structure render them quite hard to study using conventional methods. In this work, we present a computationally efficient methodology based on ab initio calculations within the coherent potential approximation. To make the methodology predictive, we apply an exchange-correlation correction to the equation of state and take into account thermal effects on the magnetic state and the equilibrium volume. The approach shows good agreement with available experimental data on bulk properties of solid solutions. As a particular case, the workflow is applied to a series of iron-group HEA to investigate their solid solution strengthening within a parameter-free model based on the effective medium representation of an alloy. The results reveal intricate interactions between alloy components, which we analyze by means of a simple model of local bonding. Thanks to its computational efficiency, the methodology can be used as a basis for an adaptive learning workflow for optimal design of HEA.
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| 6. |
- Peil, Oleg E., et al.
(author)
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Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu-Mn systems
- 2007
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In: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 310:2, s. 1561-1563
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Journal article (peer-reviewed)abstract
- Manganese-noble metal alloys exhibit a large variety of magnetic structures as a function of Mn concentration. Ab initio studies of these structures are practically absent due to difficulties in handling simultaneously atomic and magnetic ordering effects in disordered alloys. In this work we use the generalized perturbation method (GPM) A.V.Ruban, S. Shallcross, S.I. Simak H.L. Skriver, Phys. Rev. B 70 (2004) 125115 in the framework of the KKR method and the coherent potential approximation (CPA) in order to determine both effective chemical interactions and magnetic exchange interaction parameters of the classical Heisenberg Hamiltonian in Cu-Mn alloys. The effective chemical interactions have been used in Ising Monte Carlo simulations to determine the equilibrium distribution of atoms on the FCC lattice at the experimental aging temperatures. The obtained atomic short-range order for Cu-Mn alloys is in excellent agreement with the existing experimental data. The underlying atomic distribution has subsequently been used in Heisenberg Monte Carlo simulations with the exchange interaction parameters calculated for the corresponding alloy composition by the GPM method. For alloys with low Mn content, up to 20 at%, we find a transition into a spin-glass state with a specific magnetic short order.
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| 7. |
- Peil, Oleg E., 1981-
(author)
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Theory of Disordered Magnets
- 2009
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Doctoral thesis (other academic/artistic)abstract
- Studying magnetic properties of disordered alloys is important both for the understanding of phase transformations in alloys and from the point of view of fundamental issues of magnetism in solids. Disorder in a magnetic system can result in unconventional magnetic structures, such as spin glass, which have rather peculiar features.In this Thesis, a rather general approach to studying disordered magnetic alloys from first principles is presented. Phase transformations and magnetic behavior of crystalline substitutional alloys are considered. This approach is exemplified by calculations of an archetypical spin-glass material: the CuMn alloy.First, a general theoretical framework for the description of the thermodynamics of disordered magnetic alloys is given. It is shown that under certain conditions, a complex magnetic system can be reduced to an effective system containing no magnetic degrees of freedom. This substantially simplifies the investigation of phase transformations in magnetic alloys. The effective model is described in terms of material-specific interaction parameters.It is shown that interaction parameters can be obtained from the ground-state property of a disordered alloy which are in turn calculated from first principles by means of highly accurate up-to-date numerical techniques based on the Green's function method. The interaction parameters can subsequently be used in thermodynamic Monte-Carlo simulations to produce the atomic and magnetic structures of an alloy. An example of calculations for the Cu-rich CuMn alloy is given. It is demonstrated that the atomic and magnetic structure of the alloy obtained by the presented approach agrees very well with the results of neutron-scattering experiments for this system. Moreover, numerical simulations enable one to predict the ground state structure of the alloy, which is difficult to observe in experiment due to large atomic diffusion barriers at temperatures close to the temperature of the phase transformation.A general description of a spin glass is given, and difficulties of modeling this type of magnetic systems are discussed. To overcome the difficulties, improved Monte-Carlo methods, such as parallel tempering, overrelaxation technique, and finite-size scaling method of analysis, are introduced. The results for the CuMn alloy are presented.
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| 8. |
- Reyes-Huamantinco, Andrei, et al.
(author)
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Stacking-fault energy and anti-Invar effect in Fe-Mn alloy from first principles
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:6, s. 060201-
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Journal article (peer-reviewed)abstract
- Based on state-of-the-art density-functional-theory methods we calculate the stacking-fault energy of the prototypical high-Mn steel Fe-22.5 at% Mn between 300 and 800 K. We estimate magnetic thermal excitations by considering longitudinal spin fluctuations. Our results demonstrate that the interplay between the magnetic excitations and the thermal lattice expansion is the main factor determining the anti-Invar effect, the hcp-fcc transformation temperature, and the stacking-fault energy, all of which are in good agreement with measurements.
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| 9. |
- Seth, Priyanka, et al.
(author)
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Renormalization of effective interactions in a negative charge transfer insulator
- 2017
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In: Physical Review B. - 2469-9950. ; 96:20
-
Journal article (peer-reviewed)abstract
- We compute from first principles the effective interaction parameters appropriate for a low-energy description of the rare-earth nickelate LuNiO3 involving the partially occupied eg states only. The calculation uses the constrained random-phase approximation and reveals that the effective on-site Coulomb repulsion is strongly reduced by screening effects involving the oxygen-p and nickel-t2g states. The long-range component of the effective low-energy interaction is also found to be sizable. As a result, the effective on-site interaction between parallel-spin electrons is reduced down to a small negative value. This validates effective low-energy theories of these materials that were proposed earlier. Electronic structure methods combined with dynamical mean-field theory are used to construct and solve an appropriate low-energy model and explore its phase diagram as a function of the on-site repulsion and Hund's coupling. For the calculated values of these effective interactions, we find that in agreement with experiments, LuNiO3 is a metal without disproportionation of the eg occupancy when considered in its orthorhombic structure, while the monoclinic phase is a disproportionated insulator.
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