| 1. |
- Forslund, Axel, et al.
(författare)
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Ab initio simulations of the surface free energy of TiN(001)
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:19
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around 78 meV angstrom(-2) at 0 K to 73 meV angstrom(-2) at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.
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| 2. |
- Abdoshahi, Neda, et al.
(författare)
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On the energetics of the cubic-to-hexagonal transformations in TiAl plus Mo alloys
- 2022
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 240
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Tidskriftsartikel (refereegranskat)abstract
- Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transforma-tion energetics between bcc and hcp ordered and disordered phases in the TiAl+Mo model alloy system. By employing two complementary techniques I. VASP-SQS, and II. EMTO-CPA, we can show that chemical disorder flattens the energy landscape but may introduce a small barrier. Unlike that, the energetics of ordered phases are barrier-less and hence would suggest a spontaneous transformation. Finally, we show that Mo stabilizes the bcc phases, leading to a barrier-less transformation hcp -+ bcc for both ordered and disordered states when Mo content exceeds ti 12 at.%.
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| 3. |
- Abdoshahi, Neda, et al.
(författare)
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Structural stability and mechanical properties of TiAl plus Mo alloys : A comprehensive ab initio study
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221, s. 117427-
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Tidskriftsartikel (refereegranskat)abstract
- Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc /3o phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the /3o and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.
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| 4. |
- Bodlos, R., et al.
(författare)
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Ab initio investigation of the atomic volume, thermal expansion, and formation energy of WTi solid solutions
- 2021
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Gruneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at.% Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Gruneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.
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| 5. |
- Dehghani, Mohammad, et al.
(författare)
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Stability and ordering of bcc and hcp TiAl plus Mo phases : An ab initio study
- 2022
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 205
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Tidskriftsartikel (refereegranskat)abstract
- Atomic ordering in bcc and hcp TiAl+Mo alloys near equiatomic TiAl composition is investigated by different ab initio tools. We show that small addition of Mo, about 5 at. %, is enough to make bcc alloys with more than 50 at. % of Ti stable with respect to the hcp alloys. Moreover, such alloying also leads to stabilizing the B2 ordered structure with respect to its B2(2) modification, which is the bcc-based ground state structure of binary TiAl. The site preference of Mo in the B2 and B19 ordered alloys is investigated by different methods: in the dilute limit, using the transfer energy formalism; in concentrated alloys, from the total energies of disordered and partially ordered alloys in the mean-field coherent potential approximation; and from Monte Carlo simulations. These methods produce consistent results for the B2 phase predicting a strong preference of Mo to Al sublattice. The site preference of Mo in the B19 phases varies from a weak preference for Al sites in the single impurity calculations to a quite strong preference for Ti sites in the mean-field approximation and finally to a strong Al preference in Monte Carlo simulations. Mo alloying dramatically increases the order-disorder transition temperatures in bcc and hcp Al-deficient Ti0.5Al0.5-xMox alloys.
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| 6. |
- Delczeg-Czirjak, Erna K., et al.
(författare)
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The effect of longitudinal spin-fluctuations on high temperature properties of Co3Mn2Ge
- 2023
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 226
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Tidskriftsartikel (refereegranskat)abstract
- It is demonstrated that thermally induced longitudinal spin fluctuations (LSF) play an important role in itinerant Co3Mn2Ge at an elevated temperature. The effect of LSF is taken into account during ab initio calculations via a simple model for the corresponding entropy contribution. We show that the magnetic entropy leads to the appearance of a medium size local moment on Co atoms. As a consequence, this leads to a renormalization of the magnetic exchange interactions with a quite substantial impact upon the calculated Curie temperature. Taking LSF into account, the calculated Curie temperature can be brought to be in good agreement with the experimental value.
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| 7. |
- Forslund, Axel, 1991-
(författare)
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Ab initio and phenomenological modeling of materials related to CVD hard coatings
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the work based on two different tools in computational materials science: a first-principles approach, namely that of density functional theory, and the CALPHAD approach.These two methods were used in this work to calculate properties of materials related to hard coatings, in particular coatings produced with chemical vapor deposition for the purpose of wear protection in cutting tools. Several parts of the work is also, in many aspects, of a general character. In a few cases, the material investigations were performed on simpler demonstration systems, with the intention of further application on more involved material systems.A variety of different methods and specific applications are included in this thesis. The reaction-diffusion in Ni-base superalloys deposited by vapor deposition methods was simulated with a continuum approach with CALPHAD thermodynamic and kinetic data. CALPHAD models were also used to predict the stable phases for TiN deposition on a CoCrFeNi substrate. Surfaces and segregation energies were investigated in a random alloy, pseudobinary (Al,Ti)N system. This system was also the subject of calculations of formation energies of structural vacancies, and the configurational dependence of these properties was investigated.Further, surface free energies including all relevant thermal excitations were calculated for TiN(001) and several W surfaces in a newly developed methodology including machine-learning interatomic potentials. For W, the temperature dependence of the surface anisotropy was obtained, which was shown to be decreasing with temperature, with a surface free energy approaching experimental values at the melting temperature.
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| 8. |
- Forslund, Axel, et al.
(författare)
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Ab initio surface free energies of tungsten with full account of thermal excitations
- 2022
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:4
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Tidskriftsartikel (refereegranskat)abstract
- The surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.
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| 9. |
- Forslund, Axel, et al.
(författare)
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Structural vacancies in (Ti,Al)N : An ab initio study
- 2022
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- The zero K formation energies of metal and nitrogen vacancies in several (Ti,Al)N alloys and at the (001) (Ti,Al)N/AlN interface are obtained in ab initio supercell calculations. The dependence of the formation energies of metal vacancies on their local environment and type are analyzed and explained in terms of effective cluster interactions for unrelaxed calculations. The common trend for all investigated types of vacancies is that their formation energy increases with the number of Al nearest neighbors if local lattice relaxations are not allowed. However, local lattice relaxations produce a dramatic effect especially in the case of metal vacancies leading to a complicated nonlinear dependence on the local environment indicating the existence of strong multisite strain-induced interactions.
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| 10. |
- Forslund, Axel, et al.
(författare)
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Surface energetics of AlxTi1-xN alloys
- 2020
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 183
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Tidskriftsartikel (refereegranskat)abstract
- The (100), (110) and (111) surface energies of random AlxTi1-xN alloys with homogeneous concentration profile are determined in first-principles calculations. The (100) surface has the lowest energy of 1.25 J/m(2) in the case of TiN and 1.32 J/m(2) for cubic AlN and exhibit very little concentration dependence. The (110) and (111) surfaces have much higher energy for all the compositions. The segregation energies of Ti and Al were obtained for the (100) surface of pure TiN and cubic AlN as well as Al0.5Ti0.5N and Al0.9Ti0.1 N random alloys. In the latter case, we have used two different methods: direct averaging of the substitution energies with respect to the local environment of the substitution site and the cluster expansion technique (CLE). We find that the segregation of Ti is favorable in the whole concentration range of random AlxTi1-xN alloys. However, it is weak in almost the whole concentration range except in the Al-rich alloys: Al0.9Ti0.1N -/+ and cubic AlN. The strengthening of the surface segregation of Ti in the latter case is related to the sharp increase in phase separation tendency in AlxTi1-xN alloys at compositions close to pure AlN. The increased tendency for Ti segregation close to pure AlN helps explain the formation of a lamella structure in industrially important AlxTi1-xN coatings. As for nitrogen vacancies, their segregation energies are relatively small in the whole concentration range, but for pure AlN where they exhibit strong preference for the surface. The latter is obviously connected with the fact that cubic AlN has a high degree of ionic bonding and a vacancy creation on the nitrogen sublattice is highly energetically unfavorable, especially in the bulk due to a larger number of Al-N ionic bonds.
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