| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
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| 5. |
- Holzapfel, Damian M., et al.
(författare)
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Enhanced thermal stability of (Ti,Al)N coatings by oxygen incorporation
- 2021
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 218
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Tidskriftsartikel (refereegranskat)abstract
- Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800 degrees C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000 degrees C to 1300 degrees C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 6. |
- Koutna, Nikola, et al.
(författare)
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Atomistic mechanisms underlying plasticity and crack growth in ceramics : a case study of AlN/TiN superlattices
- 2022
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Ingår i: Acta Materialia. - : Pergamon-Elsevier Science Ltd. - 1359-6454 .- 1873-2453. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- Interfaces between components of a material govern its mechanical strength and fracture resistance. While a great number of interfaces is present in nanolayered materials, such as superlattices, their fundamental role during mechanical loading lacks understanding. Here we combine ab initio and classical molecular dynamics simulations, nanoindentation, and transmission electron microscopy to reveal atomistic mechanisms underlying plasticity and crack growth in B1 AlN(001)/TiN(001) superlattices under loading. The system is a model for modern refractory ceramics used as protective coatings. The simulations demonstrate an anisotropic response to uniaxial tensile deformation in principal crystallographic directions due to different strain-activated plastic deformation mechanisms. Superlattices strained orthogonal to (001) interfaces show modest plasticity and cleave parallel to AlN/TiN layers. Contrarily, B1-to-B3 or B1-to-B4(B-k) phase transformations in AlN facilitate a remarkable toughness enhancement upon in plane [110] and [100] tensile elongation, respectively. We verify the predictions experimentally and conclude that strain-induced crack growth-via loss of interface coherency, dislocation-pinning at interfaces, or layer interpenetration followed by formation of slip bands-can be hindered by controlling the thicknesses of the superlattice nanolayered components.
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| 7. |
- Leiner, Thomas, et al.
(författare)
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On energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing
- 2023
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Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 215
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal diborides crystallize in the α, γ, or ω type structure, in which pure transition metal layers alternate with pure boron layers stacked along the hexagonal [0001] axis. Here we view the prototypes as different stackings of the transition metal planes and suppose they can transform from one into another by a displacive transformation. Employing first-principles calculations, we simulate sliding of individual planes in the group IV-VII transition metal diborides along a transformation pathway connecting the α, γ, or ω structure. Chemistry-related trends are predicted in terms of energetic and structural changes along a transformation pathway, together with the mechanical and dynamical stability of the different stackings. Our results suggest that MnB2 and MoB2 possess the overall lowest sliding barriers among the investigated TMB2s. Furthermore, we discuss trends in strength and ductility indicators, including Youngs modulus or Cauchy pressure, derived from elastic constants.
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| 8. |
- Ondracka, Pavel, et al.
(författare)
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Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in Ti1-dOxN1-x thin films
- 2022
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 230
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti1-delta OxN1-x). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by ~& nbsp;0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti1-delta OxN1-x thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti1-delta OxN1-x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts. (C)& nbsp;2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.& nbsp;
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| 9. |
- Rachbauer, Richard, et al.
(författare)
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Electronic origin of structure and mechanical properties in Y and Nb alloyed Ti-Al-N thin films
- 2011
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Ingår i: International Journal of Materials Research - Zeitschrift für Metallkunde. - : Hanser. - 1862-5282 .- 2195-8556. ; 102:6, s. 735-742
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Tidskriftsartikel (refereegranskat)abstract
- Ti(1-x)A(x)N thin films are industrially well established protective coatings, whose beneficial mechanical properties are mainly based on the formation of a metastable microstructure and local composition during film synthesis. Alloying of a transition metal (TM) to Ti(1-x)Al(x)N is a promising approach to reach yet higher oxidation and corrosion resistance in high-temperature environments, while maintaining a high intrinsic hardness and elasticity, being essential for a good wear performance. In order to study the effect of alloying with Y and Nb on the structure and mechanical properties of the industrially preferred cubic (c) Ti(1-x)Al(x)N system, quaternary Ti(1-x-z)Al(x)Y(z)N and Ti(1-x)Al(x)Nb(z)N films were deposited by means of plasma-assisted reactive magnetron sputtering and investigated using X-ray diffraction, transmission electron microscopy and nanoindentation. It is shown that Y addition to c-Ti(0.42)Al(0.58)N changes its structure towards single phase wurtzite (w) Ti(0.36)Al(0.55)Y(0.09)N, with deteriorated mechanical properties. In contrast, by the addition of Nb the structure remains cubic up to the studied composition of c-Ti(0.35)Al(0.57)Nb(0.08)N and the film hardness increases from 30.1 to 39.5 GPa. Ab initio studies show that the effect of Y and Nb alloying on structure and mechanical properties of quaternary Ti(1-x-z)Al(x)TM(z)N is not only correlated with strain increase due to lattice mismatch, but rather the changed electronic configuration.
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| 10. |
- Wang, Fei, et al.
(författare)
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Systematic ab initio investigation of the elastic modulus in quaternary transition metal nitride alloys and their coherent multilayers
- 2017
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Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 127, s. 124-132
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Tidskriftsartikel (refereegranskat)abstract
- We give a comprehensive overview of the elastic properties of cubic quaternary transition metal nitride alloys and coherent nitride multilayers for design of wear resistant hard coatings. The elastic stiffness constants of the alloys are calculated using the special quasirandom structure method. For multilayers with sharp interfaces we prove the applicability of a linear-elasticity approximation and show that it can be used with success instead of performing direct computationally demanding ab initio calculations. We explore the trends and the potential of multicomponent alloying in engineering the strength and ductility of both, quaternary alloys and their multilayers. We investigate X(i-x-y)TixAlyN alloys where Xis Zr, Hf, V, Nb or Ta, and present an analysis based on increasing x. We show that with increasing Ti content ductility can increase in each alloy. Elastic isotropy is observed only in (Zr,Hf,V)((i-x-y))TixAlyN alloys in the middle of the compositional triangle, otherwise a high Youngs modulus is observed along [001]. We predict that coherent TiN/X(1-x-y)TixAlyN and ZrN/X(i-x-3)TixAlyN alloy multilayers with the [111] interfacial direction show increasing ductility with increasing x, while the multilayers with the [001] orientation become more brittle. We show that the Youngs moduli variation in the parent bulk quaternary nitride alloy provide a reliable descriptor to screen the Youngs modulus of coherent multilayers in high-throughput calculations. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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