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  • 1.
    Hu, Chun
    et al.
    TU Wien, Austria.
    Lin, Shuyao
    Linköping University, Department of Physics, Chemistry and Biology, Biology. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Podsednik, M.
    TU Wien, Austria; KAI Kompetenzzentrum Automobil & Industrieelektron, Austria.
    Mraz, Stanislav
    Rhein Westfal TH Aachen, Germany.
    Wojcik, T.
    TU Wien, Austria.
    Limbeck, A.
    TU Wien, Austria.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Mayrhofer, Paul H.
    TU Wien, Austria.
    Influence of co-sputtering AlB<sub>2</sub> to TaB<sub>2</sub> on stoichiometry of non-reactively sputtered boride thin films2024In: Materials Research Letters, E-ISSN 2166-3831, Vol. 12, no 8, p. 561-570Article in journal (Refereed)
    Abstract [en]

    Transition metal diboride thin films are promising functional materials for their outstanding mechanical properties and thermal stability. By combining experiment and simulations, we discuss angular distribution of the sputtered species, their scattering in the gas phase, re-sputtering and potential evaporation from the grown films for the complex evolution of film compositions, as well as energetic preference for vacancy formation and competing phases as factors for governing the phase constitution. By co-sputtering from two compound targets, we developed phase-pure crystalline (Ta,Al)B2 solid solution thin films and correlate the stoichiometry changes with the evolution of their microstructure, hardness, and elastic modulus. {GRAPHICAL ABSTRACT}

  • 2.
    Lin, Shuyao
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. Tech Univ Wien, Austria.
    Casillas-Trujillo, Luis
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mayrhofer, Paul H.
    Tech Univ Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Tech Univ Wien, Austria.
    Machine-learning potentials for nanoscale simulations of tensile deformation and fracture in ceramics2024In: npj Computational Materials, E-ISSN 2057-3960, Vol. 10, no 1, article id 67Article in journal (Refereed)
    Abstract [en]

    Machine-learning interatomic potentials (MLIPs) offer a powerful avenue for simulations beyond length and timescales of ab initio methods. Their development for investigation of mechanical properties and fracture, however, is far from trivial since extended defects-governing plasticity and crack nucleation in most materials-are too large to be included in the training set. Using TiB2 as a model ceramic material, we propose a training strategy for MLIPs suitable to simulate mechanical response of monocrystals until failure. Our MLIP accurately reproduces ab initio stresses and fracture mechanisms during room-temperature uniaxial tensile deformation of TiB2 at the atomic scale ( approximate to 103 atoms). More realistic tensile tests (low strain rate, Poisson's contraction) at the nanoscale ( approximate to 104-106 atoms) require MLIP up-fitting, i.e., learning from additional ab initio configurations. Consequently, we elucidate trends in theoretical strength, toughness, and crack initiation patterns under different loading directions. As our MLIP is specifically trained to modelling tensile deformation, we discuss its limitations for description of different loading conditions and lattice structures with various Ti/B stoichiometries. Finally, we show that our MLIP training procedure is applicable to diverse ceramic systems. This is demonstrated by developing MLIPs which are subsequently validated by simulations of uniaxial strain and fracture in TaB2, WB2, ReB2, TiN, and Ti2AlB2.

  • 3.
    Schmid, Barbara
    et al.
    TU Wien, Austria.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Ntemou, Eleni
    Uppsala Univ, Sweden.
    Primetzhofer, Daniel
    Uppsala Univ, Sweden.
    Wojcik, Tomasz
    TU Wien, Austria.
    Kolozsvari, Szilard
    Plansee Composite Mat GmbH, Germany.
    Mayrhofer, Paul Heinz
    TU Wien, Austria.
    Mechanical properties of VC/ZrC and VC/HfC superlattices2024In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 270, article id 119852Article in journal (Refereed)
    Abstract [en]

    Face -centered cubic transition metal carbides exhibit high melting points and hardness, making them prominent candidates for protective coating applications. Vanadium carbide (VC) has typical characteristics of transition metal carbides. It serves as model material in this study, in which we showcase the effect of superlattice architecture on mechanical properties and fracture toughness. While beneficial effects of superlattice arrangement have been demonstrated for a series of nitride -based coatings, transition metal carbides remain largely unexplored territory. Following density functional theory based ab initio predictions on lattice and shear modulus mismatch, we develop VC/ZrC and VC/HfC superlattice coatings synthesized via pulsed DC sputter deposition. The bilayer periods ( Lambda ) of our fully fcc-structured polycrystalline coatings range between 2 - 50 nm (indicated by X-ray diffraction, transmission and scanning electron microscopy). The chemical composition is close to 1:1 stoichiometry (from X-ray fluorescence, elastic back -scattering spectrometry and elastic recoil detection analysis). Both superlattice series exhibit a strong dependence of hardness, elastic modulus, and fracture toughness on their bilayer periods, which can only be correlated with the in -plane stress variations for VC/ZrC. The VC/HfC superlattices provide their peak -hardness of 36.0 +/- 1.6 GPa for Lambda = 6 nm and their peak in fracture toughness of 3.5 +/- 0.5 MPa root m for Lambda = 10 nm.

  • 4.
    Koutna, Nikola
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mayrhofer, Paul H.
    TU Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Phase stability and mechanical property trends for MAB phases by high-throughput ab initio calculations2024In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 241, article id 112959Article in journal (Refereed)
    Abstract [en]

    MAB phases (MABs) are atomically-thin laminates of ceramic/metallic-like layers, having made a breakthrough in the development of 2D materials. Though offering a vast chemical and phase space, relatively few MABs have been synthesised. To guide experiments, we perform high-throughput ab initio screening of MABs that combine group 4-7 transition metals (M); Al, Si, Ga, Ge, or In (A); and boron (B) focusing on their phase stability trends and mechanical properties. Considering the 1:1:1, 2:1:1, 2:1:2, 3:1:2, 3:1:3, and 3:1:4 M:A:B ratios and 10 phase prototypes, synthesisability of a single-phase compound for each elemental combination is estimated through formation energy spectra of competing dynamically stable MABs. Based on the volumetric proximity of energetically-close phases, we identify systems in which volume-changing deformations may facilitate transformation toughening. Subsequently, chemistry- and phase-structure-related trends in the elastic stiffness and ductility are predicted using elastic-constants-based descriptors. The analysis of directional Cauchy pressures and Young's moduli allows comparing mechanical response parallel and normal to M-B/A layers. The suggested promising MABs include Nb 3 AlB 4 , Cr 2 SiB 2 , Mn 2 SiB 2 or the already synthesised MoAlB.

  • 5.
    Stasiak, Tomasz
    et al.
    Masaryk Univ, Czech Republic; Natl Ctr Nucl Res, Poland.
    Debnarova, Stanislava
    Masaryk Univ, Czech Republic.
    Lin, Shuyao
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. Tech Univ Wien, Austria.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Tech Univ Wien, Austria.
    Czigany, Zsolt
    Inst Tech Phys & Mat Sci, Hungary.
    Balazsi, Katalin
    Inst Tech Phys & Mat Sci, Hungary.
    Bursikova, Vilma
    Masaryk Univ, Czech Republic.
    Vasina, Petr
    Masaryk Univ, Czech Republic.
    Soucek, Pavel
    Masaryk Univ, Czech Republic.
    Synthesis and characterization of ceramic high entropy carbide thin films from the Cr-Hf-Mo-Ta-W refractory metal system2024In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 485, article id 130839Article in journal (Refereed)
    Abstract [en]

    We use reactive DC magnetron sputtering to showcase synthesis strategies for multicomponent carbides with the NaCl-type fcc structure and illustrate how deposition conditions allow controlling the formation of metallic and ceramic single phases in the Cr-Hf-Mo-Ta-W system. The synthesis is performed in argon flow and different acetylene flows from 0 to 12 sccm, at ambient and elevated temperatures (700 degrees C), respectively, hindering/promoting the adatom diffusion. Structural and microstructural investigations reveal the formation of the bcc metallic phase ( a = 3.188 - 3.209 & Aring;) in films deposited without acetylene flow, also supported by ab initio density function theory (DFT) analysis of lattice parameters as a function of the C content. Experimentally, a bcc-to-fcc phase transition is observed through the formation of an amorphous coating. Contrarily, samples deposited in higher acetylene flow show an fcc multielement carbide phase ( a = 4.33 - 4.49 & Aring;). The crystalline films reveal columnar morphology, while the amorphous ones are very dense. We report promising mechanical properties, with hardness up to 25 +/- 1 GPa. The indentation moduli reach up to 319 +/- 6 GPa and show trends consistent with DFT predictions. Our study paves the path towards the preparation of Cr-Hf-Mo-Ta-W multicomponent carbides by magnetron sputtering, showing promising microstructure as well as mechanical properties.

  • 6.
    Fiantok, Tomas
    et al.
    Comenius Univ, Slovakia; Comenius Univ, Slovakia.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Mikula, Marian
    Comenius Univ, Slovakia; SAS, Slovakia.
    Ceramic transition metal diboride superlattices with improved ductility and fracture toughness screened by ab initio calculations2023In: Scientific Reports, E-ISSN 2045-2322, Vol. 13, no 1, article id 12835Article in journal (Refereed)
    Abstract [en]

    Inherent brittleness, which easily leads to crack formation and propagation during use, is a serious problem for protective ceramic thin-film applications. Superlattice architectures, with alternating nm-thick layers of typically softer/stiffer materials, have been proven powerful method to improve the mechanical performance of, e.g., cubic transition metal nitride ceramics. Using high-throughput first-principles calculations, we propose that superlattice structures hold promise also for enhancing mechanical properties and fracture resistance of transition metal diborides with two competing hexagonal phases, a and ?. We study 264 possible combinations of a/a, a/? or co/co MB2 (where M = Al or group 3-6 transition metal) diboride superlattices. Based on energetic stability considerations, together with restrictions for lattice and shear modulus mismatch (?a &lt; 4%, ?G &gt; 40 GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, C-ij, where the latter provide indication of in-plane vs. out of-plane strength ( C-11, C-33 ) and ductility ( C-13 - C-44, C-12 - C-66 ). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffiths formula for fracture toughness. The a/a-type TiB2 /MB2 (M = Mo, W), HfB2/WB2, VB2/MB2 (M = Cr, Mo), NbB2/MB2 (M = Mo, W), and a/?-type AlB2/MB2 (M = Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth.

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  • 7.
    Chen, Zhuo
    et al.
    Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria.
    Huang, Yong
    Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria.
    Koutná, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Institute of Materials Science and Technology, TU Wien, A-1060, Vienna, Austria.
    Gao, Zecui
    Institute of Materials Science and Technology, TU Wien, Vienna, Austria.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Fellner, Simon
    Institute of Materials Science and Technology, TU Wien, Vienna, Austria.
    Haberfehlner, Georg
    Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz, Austria.
    Jin, Shengli
    Chair of Ceramics, Montanuniversität Leoben, Leoben, Austria.
    Mayrhofer, Paul H.
    Institute of Materials Science and Technology, TU Wien, Vienna, Austria.
    Kothleitner, Gerald
    Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz, Austria; Graz Centre for Electron Microscopy, Graz, Austria.
    Zhang, Zaoli
    Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria; Department of Materials Science, Montanuniversität Leoben, Leoben, Austria.
    Large mechanical properties enhancement in ceramics through vacancy-mediated unit cell disturbance2023In: Nature Communications, E-ISSN 2041-1723, Vol. 14, no 1, article id 8387Article in journal (Refereed)
    Abstract [en]

    Tailoring vacancies is a feasible way to improve the mechanical properties of ceramics. However, high concentrations of vacancies usually compromise the strength (or hardness). We show that a high elasticity and flexural strength could be achieved simultaneously using a nitride superlattice architecture with disordered anion vacancies up to 50%. Enhanced mechanical properties primarily result from a distinctive deformation mechanism in superlattice ceramics, i.e., unit-cell disturbances. Such a disturbance substantially relieves local high-stress concentration, thus enhancing deformability. No dislocation activity involved also rationalizes its high strength. The work renders a unique understanding of the deformation and strengthening/toughening mechanism in nitride ceramics.

  • 8.
    Leiner, Thomas
    et al.
    Univ Leoben, Austria.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Janovec, Jozef
    UPV, Spain; Brno Univ Technol, Czech Republic.
    Zeleny, Martin
    Brno Univ Technol, Czech Republic.
    Mayrhofer, Paul H.
    TU Wien, Austria.
    Holec, David
    Univ Leoben, Austria.
    On energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing2023In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 215, article id 112329Article in journal (Refereed)
    Abstract [en]

    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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  • 9.
    Gao, Zecui
    et al.
    Tech Univ Wien, Austria.
    Buchinger, Julian
    Tech Univ Wien, Austria.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Tech Univ Wien, Austria.
    Wojcik, Tomasz
    Tech Univ Wien, Austria.
    Hahn, Rainer
    Tech Univ Wien, Austria.
    Mayrhofer, Paul Heinz
    Tech Univ Wien, Austria.
    Ab initio supported development of TiN/MoN superlattice thin films with improved hardness and toughness2022In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 231, article id 117871Article in journal (Refereed)
    Abstract [en]

    Motivated by density functional theory (DFT)-derived ductility indicators for face centered cubic (fcc, rocksalt) structured TiN/MoN0.5 superlattices and Ti0.5Mo0.5N0.75 solid solutions, TiN/MoNy superlattice (SL) thin films with bilayer periods lambda of 2.4, 3.9, 6.6, 9.9, and 23.0 nm and corresponding solid solutions were developed by DC reactive magnetron sputtering. These SLs allow for improved hardness H and critical fracture toughness K-IC, with both peaking at the same bilayer period lambda of 9.9 nm (where the MoN0.5 layers crystallize with the ordered beta-Mo2N phase); H = 34.8 +/- 1.6 GPa and K-IC = 4.1 +/- 0.2 MPa root m. The correspondingly prepared fcc-Ti0.5Mo0.5N0.77 solid solution has H = 31.4 +/- 1.5 GPa and K-IC = 3.3 +/- 0.2 MPa root m. Thus, especially the fracture toughness shows a significant superlattice effect. This is suggested by DFT-by the increase of the Cauchy pressure from -19 to + 20 GPa for the 001-direction (while that in the 100-direction remained high, above 83 GPa) upon increasing lambda from 3 to 4 nm.& nbsp;Together, experimental and computational investigations prove the importance of optimized bilayer periods for highest strength and fracture toughness, as well as optimized N-content for the solid solutions.

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  • 10.
    Koutna, Nikola
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Löfler, Lukas
    Dept Mat Sci, Austria; Rhein Westfal TH Aachen, Germany.
    Holec, David
    Dept Mat Sci, Austria.
    Chen, Zhuo
    Erich Schmid Inst Mat Sci, Austria.
    Zhang, Zaoli
    Erich Schmid Inst Mat Sci, Austria.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mayrhofer, Paul H.
    TU Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Atomistic mechanisms underlying plasticity and crack growth in ceramics: a case study of AlN/TiN superlattices2022In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 229, article id 117809Article in journal (Refereed)
    Abstract [en]

    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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  • 11.
    Fiantok, Tomas
    et al.
    Comenius Univ, Slovakia; Comenius Univ, Slovakia.
    Sroba, Viktor
    Comenius Univ, Slovakia.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Izai, Vitalii
    Comenius Univ, Slovakia.
    Roch, Tomas
    Comenius Univ, Slovakia.
    Truchly, Martin
    Comenius Univ, Slovakia.
    Vidis, Marek
    Comenius Univ, Slovakia.
    Satrapinskyy, Leonid
    Comenius Univ, Slovakia.
    Nagy, Stefan
    Inst Mat & Machine Mech SAS, Slovakia.
    Grancic, Branislav
    Comenius Univ, Slovakia.
    Kus, Peter
    Comenius Univ, Slovakia.
    Mikula, Marian
    Comenius Univ, Slovakia; Inst Mat & Machine Mech SAS, Slovakia.
    Structure evolution and mechanical properties of co-sputtered Zr-Al-B-2 thin films2022In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 40, no 3, article id 033414Article in journal (Refereed)
    Abstract [en]

    Zirconium diboride (ZrB2) represents a promising hard coating material for demanding high-temperature applications and could provide an excellent basis for fine-tuning mechanical properties via the concept of alloying. Here, combining density functional theory and experiments, we investigate the effect of aluminum alloying on thermally induced structure evolution and mechanical properties of alpha-structured Zr1-xAlxB2+Delta. Ab initio calculations predict a strong tendency for spinodal phase separation of hexagonal Zr1-xAlxB2 solid solution into isostructural binaries. Experimental results confirm predictions of the insolubility of aluminum in the ZrB2 phase when the structure of magnetron co-sputtered Zr0.72Al0.28B2.64 films with an aluminum content of 8 at. % has a nanocomposite character consisting of hexagonal alpha-ZrB2 nanocolumns surrounded by an amorphous Al-rich tissue phase. The films are structurally stable up to 1100 degrees C but out-diffusion of Al atoms from boundary regions during annealing was observed. Al alloying causes a significant decrease in hardness when the hardness of the reference as-deposited ZrB2.2 and Zr0.72Al0.28B2.64 is 39 and 23 GPa, respectively. Low hardening effect in ternaries was observed after annealing at 1000 degrees C when the hardness increased from 23.5 to 26.5 GPa due to the locally increased concentration of point defects at the boundaries of the nanocolumns and Al-rich tissue phases. Youngs modulus decrease from 445 (ZrB2.2) to 345 GPa (Zr0.72Al0.28B2.64) indicates a change in the mechanical response of the ternary film toward more ductile behavior.

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