| 1. |
- Abrikosov, Igor A., et al.
(författare)
-
Finite Temperature, Magnetic, and Many-Body Effects in Ab Initio Simulations of Alloy Thermodynamics
- 2013
-
Ingår i: TMS2013 Supplemental Proceedings. - Hoboken, NJ, USA : John Wiley & Sons. - 9781118605813 - 9781118663547 ; , s. 617-626
-
Bokkapitel (refereegranskat)abstract
- Ab initio electronic structure theory is known as a useful tool for prediction of materials properties. However, majority of simulations still deal with calculations in the framework of density functional theory with local or semi-local functionals carried out at zero temperature. We present new methodological solution.s, which go beyond this approach and explicitly take finite temperature, magnetic, and many-body effects into account. Considering Ti-based alloys, we discuss !imitations of the quasiharmonic approximation for the treatment of lattice vibrations, and present an accurate and easily extendable method to calculate free ,energies of strongly anharmonic solids. We underline the necessity to going beyond the state-of-the-art techniques for the determination of effective cluster interactions in systems exhibiting mctal-to-insulator transition, and describe a unified cluster expansion approach developed for this class of materials. Finally, we outline a first-principles method, disordered local moments molecular dynamics, for calculations of thermodynamic properties of magnetic alloys, like Cr1-x,.AlxN, in their high-temperature paramagnetic state. Our results unambiguously demonstrate importance of finite temperature effects in theoretical calculations ofthermodynamic properties ofmaterials.
|
|
| 2. |
- Glazyrin, K., et al.
(författare)
-
Importance of Correlation Effects in hcp Iron Revealed by a Pressure-Induced Electronic Topological Transition
- 2013
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:11, s. 117206-
-
Tidskriftsartikel (refereegranskat)abstract
- We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.
|
|
| 3. |
- Abrikosov, Igor, et al.
(författare)
-
Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys
- 2012
-
Ingår i: Solid State Phenomena. - 1012-0394 .- 1662-9779. ; 190, s. 291-294
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12 face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.
|
|
| 4. |
|
|
| 5. |
- Abrikosov, Igor, et al.
(författare)
-
Phase Stability and Elasticity of TiAlN
- 2011
-
Ingår i: Materials. - : MDPI. - 1996-1944 .- 1996-1944. ; 4:9, s. 1599-1618
-
Tidskriftsartikel (refereegranskat)abstract
- We review results of recent combined theoretical and experimental studies of Ti1−xAlxN, an archetypical alloy system material for hard-coating applications. Theoretical simulations of lattice parameters, mixing enthalpies, and elastic properties are presented. Calculated phase diagrams at ambient pressure, as well as at pressure of 10 GPa, show a wide miscibility gap and broad region of compositions and temperatures where the spinodal decomposition takes place. The strong dependence of the elastic properties and sound wave anisotropy on the Al-content offers detailed understanding of the spinodal decomposition and age hardening in Ti1−xAlxN alloy films and multilayers. TiAlN/TiN multilayers can further improve the hardness and thermal stability compared to TiAlN since they offer means to influence the kinetics of the favorable spinodal decomposition and suppress the detrimental transformation to w-AlN. Here, we show that a 100 degree improvement in terms of w-AlN suppression can be achieved, which is of importance when the coating is used as a protective coating on metal cutting inserts.
|
|
| 6. |
- Abrikosov, Igor, et al.
(författare)
-
Theoretical Modeling of Thermodynamic and Mechanical Properties of the Pure Components of Ti and Zr Based Alloys Using the Exact Muffin-Tin Orbitals Method
- 2014
-
Ingår i: Russian Physics Journal. - : Springer Verlag (Germany). - 1064-8887 .- 1573-9228. ; 56:9, s. 1030-1038
-
Tidskriftsartikel (refereegranskat)abstract
- The exact muffin-tin orbitals (EMTO) method belongs to the third and latest generation of first-principles methods of calculating the electronic structure of materials in the so-called approximation of muffin-tin (MT) orbitals within the framework of the density functional theory. A study has been performed of its applicability for modeling the thermodynamic and mechanical properties of the pure components of Ti and Zr based alloys. The total energies of Ti, Zr, Nb, V, Mo, and Al are calculated in three crystal structures - face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). For all of these elements and crystal structures, we have calculated the theoretical values of the lattice constants, elastic constants, and equations of state. The stable crystal structures have been determined. In all cases, calculations by the EMTO method predict the correct structure of the ground state. For stable structures we compared the obtained results with experiment and with calculations using full potential methods. We have demonstrated the reliability of the EMTO method and conclude that its further application for effective modeling of the properties of disordered alloys based on Ti and Zr is possible.
|
|
| 7. |
- Alling, Björn, 1980-, et al.
(författare)
-
A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203-
-
Tidskriftsartikel (refereegranskat)abstract
- We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
|
|
| 8. |
- Alling, Björn, 1980-
(författare)
-
Configurational and Magnetic Interactions in Multicomponent Systems
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides.In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials.Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb.A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions.Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.
|
|
| 9. |
- Alling, Björn, 1980-, et al.
(författare)
-
Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American institute of physics. - 1098-0121 .- 1550-235X. ; 82, s. 184430-
-
Tidskriftsartikel (refereegranskat)abstract
- Two different methods for the modeling of a magnetically disordered CrN stateusing a supercell approach are investigated. They are found to give equivalentresults of the total energy, being also similar to results obtained with an effectivemedium approach. Furthermore, CrN is shown to be better described using aLDA+U framework for the treatment of electron-electron correlations as comparedto GGA or LDA calculations. Modeling the cubic paramagnetic phase with ourmodels for magnetic disorder and considering the strong electron correlations, thetemperature and pressure induced phase transitions in CrN can be explained.
|
|
| 10. |
- Alling, Björn, 1980-, et al.
(författare)
-
Questionable collapse of the bulk modulus in CrN
- 2010
-
Ingår i: Nature Materials. - London, UK : Nature Publishing Group. - 1476-1122 .- 1476-4660. ; 9:4, s. 283-284
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- In this comment we show that the main conclusion in a previous article, claiminga drastic increase in compressibility of CrN at the cubic to orthorhombic phasetransition, is unsupported by first-principles calculations. We show that if thecubic CrN phase is considered as a disordered magnetic material, as supported bydifferent experimental data, rather then non-magnetic, the bulk modulus is almostunaffected by the transition.
|
|
