On energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing

Transition metal diborides crystallise in the $\alpha$, $\gamma$, or $\omega$ 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 $\alpha$, $\gamma$, and $\omega$ 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 MnB$_2$ and MoB$_2$ possess the overall lowest sliding barriers among the investigated TMB$_2$s. Furthermore, we discuss trends in strength and ductility indicators, including Young's modulus or Cauchy pressure, derived from elastic constants.Comment: 12 pages, 6 figures, accepted for publication in Vacuum, before proo

Machine-learning potentials for nanoscale simulations of deformation and fracture: example of TiB2_2 ceramic

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 TiB$_2$ as a model ceramic material, we propose a strategy for fitting MLIPs suitable to simulate mechanical response of monocrystals until fracture. Our MLIP accurately reproduces ab initio stresses and failure mechanisms during room-temperature uniaxial tensile deformation of TiB$_2$ at the atomic scale ($\approx{10}^3$ atoms). More realistic tensile tests (low strain rate, Poisson's contraction) at the nanoscale ($\approx{10}^4$--10$^6$ 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. To identify useful environments for further up-fitting, i.e., making the MLIP applicable to a wider spectrum of simulations, we asses transferability to other deformation conditions and phases not explicitly trained on

Mauritius as a destination of international tourism

Předmětem zkoumání diplomové práce je Mauricius jako destinace mezinárodního cestovního ruchu. Hlavním cílem práce je charakteristika Mauriciu jako destinace cestovního ruchu a zhodnocení jeho postavení na trhu mezinárodního cestovního ruchu. Práce je rozdělena na dvě části, teoretickou a praktickou. V teoretické části práce jsou vymezeny základní pojmy související s daným tématem spolu se základní charakteristikou ostrovního státu Mauricius. Praktická část práce se již zabývá výhradně cestovním ruchem Mauriciu. Jsou zde zhodnoceny předpoklady pro rozvoj cestovního ruchu, zanalyzován příjezdový cestovní ruch, představena nabídka destinace v českých cestovních kancelářích a zhodnocena konkurenceschopnost země v mezinárodním srovnání. Zjištěné informace jsou závěrem shrnuty ve SWOT analýze.The subject matter of the diploma thesis is Mauritius as a destination of international tourism. The main aim of this work is to characterize Mauritius as a tourist destination and evaluate its position in the international tourism market. The work is divided into two parts, theoretical and practical. The theoretical part of the thesis defines the basic theoretical terms of tourism and presents the basic characteristics of the island state of Mauritius. The practical part of the work already deals exclusively with tourism in Mauritius. In this part, the characteristics of the Mauritius conditions of tourism development are evaluated, the incoming tourism is analyzed, the offer of destination in czech travel agencies is presented and the competitiveness of destination Mauritius in international comparison is evaluated. The obtained information is finally summarized in the SWOT analysis

Ceramic transition metal diboride superlattices with improved ductility and fracture toughness screened by ab initio calculations

Abstract 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, $$\alpha$$ α and $$\omega$$ ω . We study 264 possible combinations of $$\alpha /\alpha$$ α / α , $$\alpha /\omega$$ α / ω or $$\omega /\omega$$ ω / ω MB $$_2$$ 2 (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 ( $$\Delta a40$$ Δ G > 40  GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, $$C_{ij}$$ C ij , where the latter provide indication of in-plane vs. out-of-plane strength ( $$C_{11}$$ C 11 , $$C_{33}$$ C 33 ) and ductility ( $$C_{13}-C_{44}$$ C 13 - C 44 , $$C_{12}-C_{66}$$ C 12 - C 66 ). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffith’s formula for fracture toughness. The $$\alpha /\alpha$$ α / α -type TiB $$_2$$ 2 /MB $$_2$$ 2 (M $$=$$ = Mo, W), HfB $$_2$$ 2 /WB $$_2$$ 2 , VB $$_2$$ 2 /MB $$_2$$ 2 (M $$=$$ = Cr, Mo), NbB $$_2$$ 2 /MB $$_2$$ 2 (M $$=$$ = Mo, W), and $$\alpha /\omega$$ α / ω -type AlB $$_2$$ 2 /MB $$_2$$ 2 (M $$=$$ = Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth

An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is primarily aimed at a disordered solid solution Fe 81.25 Al 18.75 but we have compared our results also with those obtained for the ordered Fe 3 Al intermetallic compound for which experimental data exist in literature. Both phases are found in Fe-Al-based superalloys. The Fe-18.75at.%Al solid solution was simulated using special quasirandom structures (SQS) in three different disordered states with a different distribution of Al atoms. In particular, we have considered a general disordered case (an A2-like variant), the case without the first nearest neighbor Al-Al pairs (a B2-like distribution of atoms) and also the case without both the first and second nearest neighbor Al-Al pairs (the D0 3 -like variant, in fact, an Fe-rich Fe 3 Al phase). The vacancy formation energies as well as the volumes of (fully relaxed) supercells with vacancies showed a large scatter for the disordered systems. The vacancy formation energies decrease with increasing concentration of Al atoms in the first coordination shell around the vacancy (an anti-correlation) for all disordered cases studied. The computed volumes of vacancies were found significantly lower (by 25−60%) when compared with the equilibrium volume of the missing atoms in their elemental states. Lastly, we have analyzed interactions between the vacancies and the Fe atoms and evaluated vacancy-induced changes in local magnetic moments of Fe atoms

An Ab Initio Study of Magnetism in Disordered Fe-Al Alloys with Thermal Antiphase Boundaries

We have performed a quantum-mechanical study of a B2 phase of Fe 70 Al 30 alloy with and without antiphase boundaries (APBs) with the {001} crystallographic orientation of APB interfaces. We used a supercell approach with the atoms distributed according to the special quasi-random structure (SQS) concept. Our study was motivated by experimental findings by Murakami et al. (Nature Comm. 5 (2014) 4133) who reported significantly higher magnetic flux density from A2-phase interlayers at the thermally-induced APBs in Fe 70 Al 30 and suggested that the ferromagnetism is stabilized by the disorder in the A2 phase. Our computational study of sharp APBs (without any A2-phase interlayer) indicates that they have moderate APB energies (≈0.1 J/m 2 ) and cannot explain the experimentally detected increase in the ferromagnetism because they often induce a ferro-to-ferrimagnetic transition. When studying thermal APBs, we introduce a few atomic layers of A2 phase of Fe 70 Al 30 into the interface of sharp APBs. The averaged computed magnetic moment of Fe atoms in the whole B2/A2 nanocomposite is then increased by 11.5% w.r.t. the B2 phase. The A2 phase itself (treated separately as a bulk) has the total magnetic moment even higher, by 17.5%, and this increase also applies if the A2 phase at APBs is sufficiently thick (the experimental value is 2–3 nm). We link the changes in the magnetism to the facts that (i) the Al atoms in the first nearest neighbor (1NN) shell of Fe atoms nonlinearly reduce their magnetic moments and (ii) there are on average less Al atoms in the 1NN shell of Fe atoms in the A2 phase. These effects synergically combine with the influence of APBs which provide local atomic configurations not existing in an APB-free bulk. The identified mechanism of increasing the magnetic properties by introducing APBs with disordered phases can be used as a designing principle when developing new magnetic materials