Abstract: Available information concerning the elastic moduli of refractory carbides at temperatures (T) of relevance for practical applications is sparse and/or inconsistent. Ab initio molecular dynamics (AIMD) simulations at T = 300, 600, 900, and 1200 K are carried out to determine the temperature-dependences of the elastic constants of rocksalt-structure (B1) TiC, ZrC, HfC, VC, TaC compounds, as well as high-entropy (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C. The second-order elastic constants are calculated by least-square fitting of the analytical expressions of stress/strain relationships to simulation results obtained from three tensile and three shear deformation modes. Sound-velocity measurements are employed to validate AIMD values of bulk, shear, and elastic moduli of single-phase B1 (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C at ambient conditions. In comparison with the predictions of previous ab initio calculations – where the extrapolation of finite-temperature elastic properties accounted for thermal expansion while neglecting intrinsic vibrational effects – AIMD simulations produce a softening of shear elastic moduli with T in closer agreement with experiments. The results show that TaC is the system which exhibits the highest elastic resistances to tensile and shear deformation up to 1200 K, and indicate the (V,Nb,Ta,Mo,W)C system as candidate for applications that require superior toughness at room as well as elevated temperatures.

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Topics: Elastic modulus (55%)

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10 results found

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Davide Sangiovanni^{1}, William M. Mellor^{2}, Tyler Harrington^{2}, Kevin Kaufmann^{2} +1 more•Institutions (2)

Abstract: Bottom-up design of high-entropy ceramics is a promising approach for realizing materials with unique combination of high hardness and fracture-resistance at elevated temperature. This work offers a simple yet fundamental design criterion – valence electron concentration (VEC) ⪆9.5 e-/formula unit to populate bonding metallic states at the Fermi level – for selecting elemental compositions that may form rocksalt-structure (B1) high-entropy ceramics with enhanced plasticity (reduced brittleness). Single-phase B1 (HfTaTiWZr)C and (MoNbTaVW)C, chosen as representative systems due to their specific VEC values, are here synthesized and tested. Nanoindentation arrays at various loads and depths statistically show that (HfTaTiWZr)C (VEC = 8.6 e-/f.u.) is hard but brittle, whilst (MoNbTaVW)C (VEC = 9.4 e-/f.u.) is hard and considerably more resistant to fracture than (HfTaTiWZr)C. Ab initio molecular dynamics simulations and electronic-structure analysis reveal that the improved fracture-resistance of (MoNbTaVW)C subject to deformation may originate from the intrinsic material’s ability to undergo local lattice transformations beyond tensile yield points, as well as from relatively facile activation of lattice slip. Additional simulations, carried out to follow the evolution in mechanical properties as a function of temperature, suggest that (MoNbTaVW)C may retain good resistance to fracture up to ≈900-1200 K, whereas (HfTaTiWZr)C is predicted to remain brittle at all investigated temperatures.

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Topics: Brittleness (53%), Valence electron (52%), Plasticity (51%) ... read more

4 Citations

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Abstract: High-entropy ceramics with five or more cations have recently attracted significant attention due to their superior properties for various structural and functional applications. Although the multi-component ceramics have been of interest for several decades, the concept of high-entropy ceramics was defined in 2004 by producing the first high-entropy nitride films. Following the introduction of the entropy stabilization concept, significant efforts were started to increase the entropy, minimize the Gibbs free energy and achieve stable single-phase high-entropy ceramics. High-entropy oxides, nitrides, carbides, borides and hydrides are currently the most popular high-entropy ceramics due to their potential for various applications, while the study of other ceramics, such as silicides, sulfides, fluorides, phosphides, phosphates, oxynitrides, carbonitrides and borocarbonitrides, is also growing fast. In this paper, the progress regarding high-entropy ceramics is reviewed from both experimental and theoretical points of view. Different aspects including the history, principles, compositions, crystal structure, theoretical/empirical design (via density functional theory, molecular dynamics simulation, machine learning, CALPHAD and descriptors), production methods and properties are thoroughly reviewed. The paper specifically attempts to answer how these materials with remarkable structures and properties can be used in future applications.

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3 Citations

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07 Dec 2015-

Abstract: Ti_(1−x)Al_xN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the symmetry imposed force constant temperature dependent effective potential method, which include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C_(11) decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.

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Topics: Lattice constant (57%), Anharmonicity (55%), Thermal expansion (54%) ... read more

3 Citations

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Abstract: In this study, a new type of (La0.2Ce0.2Gd0.2Er0.2Tm0.2)2(WO4)3 single-phase high-entropy ceramic (HEC) powder was designed, prepared by solid-phase synthesis, and evenly mixed into an epoxy resin (EP) matrix to test the thermal neutron and γ-ray shielding performance. In terms of γ-ray shielding, (La0.2Ce0.2Gd0.2Er0.2Tm0.2)2(WO4)3 showed a higher lead equivalent value than EP at 65 keV, indicating a clear shielding effect in the feeble absorbing area of lead-based and bismuth-based materials; moreover, (La0.2Ce0.2Gd0.2Er0.2Tm0.2)2(WO4)3 showed good γ-ray shielding performance in both low-energy and medium-energy regions. In terms of thermal neutron shielding, the composite with the highest HEC content (EP/W3) had a nearly 100% shielding efficiency, while that of EP was approximately 50%, indicating that this material has favorable thermal neutron and γ-ray shielding performance. Moreover, the thermal conductivity of the resulting composite material was lower than 0.3 W·m−1·K−1, the tensile strength was higher than 10 MPa, and the density was lower than 1.5 g·cm−3. All these results suggest that (La0.2Ce0.2Gd0.2Er0.2Tm0.2)2(WO4)3 is a candidate radiation shielding material.

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Topics: Shielding effect (63%), Electromagnetic shielding (61%), Neutron temperature (54%)

2 Citations

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William M. Mellor^{1}, Kevin Kaufmann^{1}, Olivia F. Dippo^{1}, Samuel D. Figueroa^{1} +2 more•Institutions (1)

Abstract: In the last decade, single-phase high-entropy materials have attracted considerable research interest owing to their unexpected existence and unique combinations of properties. Recent development of 5-cation high-entropy carbides (HECs) has demonstrated alluring properties compared to the rule of mixtures and binary carbides. Proposed here is the development of ultrahigh-entropy carbides (UHECs) containing 6+ principal elements with greater combinatorial possibilities. The use of 6+ multi-cation compositions allows for the design of ceramics with further tunable properties, while likely possessing higher orders of entropic stabilization. There are 133 possible carbide compositions containing 6, 7, 8, or 9 refractory metal cations in equiatomic ratios. Candidate selection for fabrication and material testing was accelerated using a machine learning model that was originally trained to predict the synthesizability of five cation disordered metal carbides. Two compositions from each category of six through eight cations, one containing Cr and one without, plus the one possible nine cation carbide were fabricated and characterized. The potential for these 6+ cation UHECs as improved materials for oxidative environments is demonstrated by comparing the oxidation performance of a 5- and 7-cation system after 10 min at 1973 K in air. The oxidation behavior is correlated with Ellingham diagrams, and it is demonstrated that the 7-cation carbide has the ability to form a transitional stable 5+ cation HEC layer as elements preferentially form oxides, which results in significantly improved oxidation resistance.

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Topics: Carbide (58%)

1 Citations

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126 results found

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Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif

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Topics: Local-density approximation (60%), Orbital-free density functional theory (57%), Density functional theory (56%) ... read more

117,932 Citations

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Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

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Topics: DIIS (51%)

64,484 Citations

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Abstract: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.

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Topics: Projector augmented wave method (66%), Pseudopotential (58%)

48,474 Citations

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Abstract: VMD is a molecular graphics program designed for the display and analysis of molecular assemblies, in particular biopolymers such as proteins and nucleic acids. VMD can simultaneously display any number of structures using a wide variety of rendering styles and coloring methods. Molecules are displayed as one or more "representations," in which each representation embodies a particular rendering method and coloring scheme for a selected subset of atoms. The atoms displayed in each representation are chosen using an extensive atom selection syntax, which includes Boolean operators and regular expressions. VMD provides a complete graphical user interface for program control, as well as a text interface using the Tcl embeddable parser to allow for complex scripts with variable substitution, control loops, and function calls. Full session logging is supported, which produces a VMD command script for later playback. High-resolution raster images of displayed molecules may be produced by generating input scripts for use by a number of photorealistic image-rendering applications. VMD has also been expressly designed with the ability to animate molecular dynamics (MD) simulation trajectories, imported either from files or from a direct connection to a running MD simulation. VMD is the visualization component of MDScope, a set of tools for interactive problem solving in structural biology, which also includes the parallel MD program NAMD, and the MDCOMM software used to connect the visualization and simulation programs. VMD is written in C++, using an object-oriented design; the program, including source code and extensive documentation, is freely available via anonymous ftp and through the World Wide Web.

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Topics: Rendering (computer graphics) (52%), Molecular graphics (52%), Visualization (51%) ... read more

36,939 Citations

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11 Feb 1988-

Abstract: Introduction Statistical mechanics Molecular dynamics Monte Carlo methods Some tricks of the trade How to analyse the results Advanced simulation techniques Non-equilibrium molecular dynamics Brownian dynamics Quantum simulations Some applications Appendix A: Computers and computer simulation Appendix B: Reduced units Appendix C: Calculation of forces and torques Appendix D: Fourier transforms Appendix E: The gear predictor - corrector Appendix F: Programs on microfiche Appendix G: Random numbers References Index.

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20,549 Citations