Search Results (20)

Local lattice distortion (LLD) arising from atomic size mismatch is an important structural feature of body-centered cubic (BCC) refractory high-entropy alloys (RHEAs). Reported LLDs are often difficult to compare across alloys because studies use different definitions and reference lattices. In this paper, we computed a consistent static DFT baseline for width-based LLD descriptors for three equimolar quaternary BCC RHEAs: NbTaTiV, TiZrNbMo, and the sparsely reported HfZrNbMo. The alloys were modeled as 128-atom special quasi-random structures and fully relaxed using density functional theory (DFT). Two complementary descriptors were evaluated from the relaxed geometries using a consistently defined reference lattice: a displacement-based metric derived from atomic off-site displacements and a shell-resolved bond length broadening metric for the first and second coordination shells. The resulting LLD descriptors have the lowest values for NbTaTiV, intermediate values for TiZrNbMo, and the highest for HfZrNbMo. Element-resolved analysis shows that individual species contribute differently to the overall distortion, information that is not captured by global descriptors alone. The pretrained MACE machine learning interatomic potential is assessed as a pre-relaxation step prior to DFT relaxation, as well as for screening candidate lattice parameters for HfZrNbMo. Full article

This work investigated the effects of varying tungsten (W) and chromium (Cr) contents on the lattice constant, elastic properties, yield strength, and electronic structure of TiZrNb alloys via first principles and the Special Quasi-Random Structure (SQS). A modified Senkov approach, considering the local atomic environment to estimate yield strength was suggested. Analysis indicated that W and Cr decrease the lattice constant of the TiZrNb alloy. W could improve the elastic modulus of material and solid solution strengthening effect, but Cr only enhanced the bulk modulus at high levels. The alloying of W and Cr was not beneficial for enhancing the plasticity. Cr was more significant in damaging mechanical isotropy. The modified Senkov approach improved the estimation accuracy of yield strength. Electronic property analysis indicated that W and Cr could lower the Fermi level to enhance the stability of the phase. Their covalent interactions helped to enhance strength. At present, the accuracy of the theoretical predictions has improved, relative to the experimental values. This work will provide guidance for the design and optimization of TiZrNb-based alloys. Full article

This study employs first-principles calculations combined with the Special Quasirandom Structure (SQS) technique to investigate the impact of three interstitial elements C, N, and O, on the mechanical properties and stacking fault energy (SFE) of NiCoCr medium-entropy alloys. The results indicate that non-metallic O, C, and N tend to occupy octahedral interstitial sites, which can effectively release stress concentration and enhance the strength and deformability of the material. Differential charge density analysis shows that the dissolution of C, N, and O significantly alters the surrounding electronic environment, strengthening the interaction between solute atoms and metal atoms, thereby hindering dislocation glide and increasing the strength and hardness of the material. Elastic property analysis indicates that NiCoCr alloys doped with C, N, and O exhibit good ductility and anisotropic characteristics. Furthermore, the study of stacking fault energy reveals that the doping with C, N, and O can significantly increase the stacking fault energy of NiCoCr alloys, thereby optimizing their mechanical properties. These findings provide theoretical evidence for the design of advanced high-entropy alloys that combine high strength with good ductility. Full article

The stability and mechanical properties of (TiZrHf)_1−x(AB)_x (AB = NbTa, NbMo, MoTa) refractory high-entropy alloys have been investigated by combining the first-principles with special quasi-random structure (SQS) method. It is found that with the increase in solute concentration x, the ΔH_mix of (TiZrHf)_1−x(AB)_x (AB = NbMo, MoTa) linearly decreases, whereas both ΔH_mix and ΔS_mix of (TiZrHf)_1−x(NbTa)_x increase initially and subsequently decrease, with the crossover occurring at x = 0.56. The ΔH_mix of (TiZrHf)_1−x(NbTa)_x and (TiZrHf)_1−x(AB)_x (AB = NbMo, MoTa) alloys are larger and lower than that of TiZrHf, respectively, while the ΔS_mix of all (TiZrHf)_1−x(AB)_x is larger than that of TiZrHf. The formation possibility parameter Ω of all (TiZrHf)_1−x(AB)_x (AB = NbMo, MoTa) first decreases sharply, followed by a gradual decrease. And the local lattice distortion (LLD) parameter δ remains relatively stable around x = 0.56 for all cases, after which it decreases sharply until x = 0.89. The δ value of (TiZrHf)_1−x(AB)_x is higher than that of TiZrHf for x < 0.56 but becomes lower beyond this composition. The valence electron concentration (VEC), a possible indicator for a single-phase solution, of (TiZrHf)_1−x(AB)_x increases nearly linearly, while the formation energy ΔH_f of (TiZrHf)_1−x(AB)_x shows the opposite tendency, except for (TiZrHf)_0.67(NbTa)_0.33. Furthermore, the VEC of all (TiZrHf)_1−x(AB)_x alloys increases, whereas their ΔH_f decreases compared to that of TiZrHf. The ideal strength σ_p of (TiZrHf)_1−x(AB)_x increases linearly, reaching approximately 2.12 GPa. The bulk modulus (B), elastic modulus (E), and shear modulus (G) also exhibit linear increases, and their values in all (TiZrHf)_1−x(AB)_x alloys are higher than those of TiZrHf, with some exceptions. The Cauchy pressure (C₁₂–C₄₄) and Pugh’s ratio G/B of all (TiZrHf)_1−x(AB)_x alloys increase, whereas the Poisson’s ratio ν exhibits the opposite trend. Moreover, the C₁₂–C₄₄ and G/B ratio of TiZrHf are lower and higher, respectively, than those of (TiZrHf)_1−x(AB)_x, and the ν of TiZrHf is lower than that of (TiZrHf)_1−x(AB)_x. This study provides valuable insights for the design of high-performance TiZrHf-based refractory high-entropy alloys. Full article

Uranium-containing high-entropy alloys (HEAs) exhibit great potential as a novel energetic structural material, attributed to their excellent performance in impact energy release, superior mechanical properties, and high density. This study investigates the effects of Ta content on the phase stability, lattice constant, density, elastic constants, polycrystalline moduli, and electronic structure of (UNbMoHf)_54−xTa_x high-entropy alloys (where x = 2, 6, 10, 14, 18), utilizing a combination of density functional theory (DFT) calculations and the special quasi-random structure (SQS) approach. Our findings confirm that these alloys maintain stable body-centered cubic structures, as evidenced by atomic radius difference and valence electron concentration evaluations. Analysis of elastic modulus, Cauchy pressure, and Vickers hardness indicates that Ta incorporation enhances mechanical properties and increases the anisotropy of these alloys. Furthermore, investigations into the electronic structure reveal that adding Ta reduces metallic character while increasing covalent characteristics, enhancing the contribution of Ta’s d-orbitals to the total density of states and intensifying covalent bonding interactions between Ta and other elements such as Nb, Mo, and U. These findings provide theoretical guidance for the design of high-performance UNbMoHfTa HEAs with tailored properties. Full article

High-entropy alloys (HEAs) have garnered significant global interest due to their outstanding properties. This study investigates the structural stability and mechanical properties of FeCoCrNiMo_x (x = 0, 0.4, 0.5, 0.8, 1.3) HEAs using a first-principles approach coupled with the special quasi-random structure (SQS) method. Of the alloys examined, all except FeCoCrNiMo_1.3 were found to be thermodynamically and dynamically stable. Elasticity calculations revealed that molybdenum improves the ductility and anisotropy of the alloys, though with a slight decrease in strength and stiffness, as confirmed by electronic structure analysis. Defect-free FeCoCrNiMo_0.5 HEAs coatings were then prepared using laser cladding and characterized for their microstructure and hardness. The coating exhibited a transition from columnar crystals at the bottom to equiaxed crystals at the surface, forming a honeycomb-like structure. Inside the crystal cells, high-density dislocations and σ-phase were observed. Elasticity calculations of the σ-phase confirmed its high hardness, low ductility, and classification as a brittle, hard phase. Full article

In this paper, we developed models for 21 quinary high-entropy transition metal carbide ceramics (HETMCCs), composed of carbon and the transition metals Ti, Zr, Mo, V, Nb, W, and Ta, employing the Special Quasirandom Structures (SQS) method. We investigated how the transition metal elements influence lattice distortion, mixing enthalpy, Gibbs free energy of mixing, and the electronic structure of the systems through first-principles calculations. The calculations show that 21 systems can form a stable single phase, among which (TiMoVNbTa)C₅, (ZrMoNbWTa)C₅, and (MoVNbWTa)C₅ exhibit superior stability. The formation energy and migration energy of carbon vacancies in systems with strong single-phase stability were calculated to predict their radiation resistance. The formation energy of carbon vacancies is closely related to the types of surrounding transition metal elements, with values ranging between the maximum and minimum formation energies observed in binary transition metal carbides (TMCs). The range of migration energy for carbon vacancies is wider than that observed in TMCs, which can hinder their long-range migration and enhance the radiation resistance of the materials. Full article

Palladium (Pd) and its alloys, renowned for their good corrosion resistance, catalytic efficiency, and hydrogen affinity, find extensive use in various industrial applications. However, the susceptibility of pure Pd to hydrogen embrittlement necessitates alloying strategies such as Pd-Ag systems. This study investigates the impact of the ordering on the phase stability and elastic properties of Pd-Ag alloys through first-principles calculations. We explore a series of ordered phase structures alongside random solid solutions using Special Quasirandom Structures (SQSs), evaluating their thermodynamic stability and elastic properties. Our findings indicate the possible existence of stable ordered L1₂ Pd₃Ag and PdAg₃ and L1₁ PdAg phases, which are thought to exist only in Cu-Pt alloys. An analysis of the elastic constants and anisotropy indices underscores some pronounced directional dependencies in the mechanical responses between the random solid-solution and ordered phases. This suggests that the ordered phases not only are thermodynamically and mechanically more stable than solid-solution phases, but also display a decrease in anisotropy indices. The results provide a deeper understanding of the atomic behavior of Pd-Ag alloys, and shed light on the design of multiphase Pd-Ag alloys to improve their mechanical properties. Full article

Refractory high-entropy alloys (RHEAs) are strong candidates for use in high-temperature engineering applications. As such, the thermodynamic properties as a function of temperature for a variety of RHEA systems need to be studied. In the present work, thermodynamic quantities such as entropy, enthalpy, heat capacity at constant volume, and linear thermal expansion are calculated for three quaternary and three quinary single-phase, BCC RHEAs: AlMoNbV, NbTaTiV, NbTaTiZr, AlNbTaTiV, HfNbTaTiZr, and MoNbTaVW. First-principle calculations based on density functional theory are used for the calculations, and special quasirandom structures (SQSs) are used to represent the random solid solution nature of the RHEAs. A code for the finite temperature thermodynamic properties using the Debye-Grüneisen model is written and employed. For the first time, the finite temperature thermodynamic properties of all 24 atomic configuration permutations of a quaternary RHEA are calculated. At most, 1.7% difference is found between the resulting properties as a function of atomic configuration, indicating that the atomic configuration of the SQS has little effect on the calculated thermodynamic properties. The behavior of thermodynamic properties among the RHEAs studied is discussed based on valence electron concentration and atomic size. Among the quaternary RHEAs studied, namely AlMoNbV, NbTaTiZr, and NbTaTiV, it is found that the presence of Zr contributes to higher entropy. Additionally, at lower temperatures, Zr contributes to higher heat capacity and thermal expansion compared to the alloys without Zr, possibly due to its valence electron concentration. At higher temperatures, Al contributes to higher heat capacity and thermal expansion, possibly due its ductility. Among the quinary systems, the presence of Mo, W, and/or V causes the RHEA to have a lower thermal expansion than the other systems studied. Finally, when comparing the systems with the NbTaTi core, the addition of Al increases thermal expansion, while the removal of Zr lowers the thermal expansion. Full article

Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf_0.2, Zr_0.2, Ti_0.2, Ta_0.2, Mo_0.2)B₂ made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon. Full article

Thermoelectric materials have attracted extensive attention because they can directly convert waste heat into electric energy. As a brand-new method of alloying, high-entropy alloys (HEAs) have attracted much attention in the fields of materials science and engineering. Recent researches have found that HEAs could be potentially good thermoelectric (TE) materials. In this study, special quasi-random structures (SQS) of PbSnTeSe high-entropy alloys consisting of 64 atoms have been generated. The thermoelectric transport properties of the highest-entropy PbSnTeSe-optimized structure were investigated by combining calculations from first-principles density-functional theory and on-the-fly machine learning with the semiclassical Boltzmann transport theory and Green–Kubo theory. The results demonstrate that PbSnTeSe HEA has a very low lattice thermal conductivity. The electrical conductivity, thermal electronic conductivity and Seebeck coefficient have been evaluated for both n-type and p-type doping. N-type PbSnTeSe exhibits better power factor (PF = S²σ) than p-type PbSnTeSe because of larger electrical conductivity for n-type doping. Despite high electrical thermal conductivities, the calculated ZT are satisfactory. The maximum ZT (about 1.1) is found at 500 K for n-type doping. These results confirm that PbSnTeSe HEA is a promising thermoelectric material. Full article

The technological requirement for ever more efficient materials for the energy and electronics sectors has led to the consideration of numerous compositionally and structurally complicated systems. These systems include solid solutions that are difficult to model using electronic structure calculations because of the numerous possibilities in the arrangement of atoms in supercells. The plethora of such possible arrangements leads to extensive and large numbers of potential supercells, and this renders the investigation of defect properties practically intractable. We consider recent advances in oxide interfaces where studies have demonstrated that it is feasible to tune their defect processes effectively. In this review, we aim to contribute to the ongoing discussion in the community on simple, efficient and tractable ways to realise research in solid solutions and oxide interfaces. The review considers the foundations of relevant thermodynamic models to extract point defect parameters and the special quasirandom structures method to model the supercell of solid solutions. Examples of previous work are given to highlight these methodologies. The review concludes with future directions, systems to be considered and a brief assessment of the relevant methodologies. Full article

The high-entropy transition metal borides containing a random distribution of five or more constituent metallic elements offer novel opportunities in designing materials that show crystalline phase stability, high strength, and thermal oxidation resistance under extreme conditions. We present a comprehensive theoretical and experimental investigation of prototypical high-entropy boride (HEB) materials such as (Hf, Mo, Nb, Ta, Ti)B₂ and (Hf, Mo, Nb, Ta, Zr)B₂ under extreme environments of pressures and temperatures. The theoretical tools include modeling elastic properties by special quasi-random structures that predict a bulk modulus of 288 GPa and a shear modulus of 215 GPa at ambient conditions. HEB samples were synthesized under high pressures and high temperatures and studied to 9.5 GPa and 2273 K in a large-volume pressure cell. The thermal equation of state measurement yielded a bulk modulus of 276 GPa, in excellent agreement with theory. The measured compressive yield strength by radial X-ray diffraction technique in a diamond anvil cell was 28 GPa at a pressure of 65 GPa, which is a significant fraction of the shear modulus at high pressures. The high compressive strength and phase stability of this material under high pressures and high temperatures make it an ideal candidate for application as a structural material in nuclear and aerospace fields. Full article

In recent years, the search for more efficient and environmentally friendly materials to be employed in the next generation of thin film solar cell devices has seen a shift towards hybrid halide perovskites and chalcogenide materials crystallising in the kesterite crystal structure. Prime examples for the latter are Cu${}_2$ZnSnS${}_4$, Cu${}_2$ZnSnSe${}_4$, and their solid solution Cu${}_2$ZnSn(S${}_x$Se${}_{1-x}$)${}_4$, where actual devices already demonstrated power conversion efficiencies of about 13 %. However, in their naturally occurring kesterite crystal structure, the so-called Cu-Zn disorder plays an important role and impacts the structural, electronic, and optical properties. To understand the influence of Cu-Zn disorder, we perform first-principles calculations based on density functional theory combined with special quasirandom structures to accurately model the cation disorder. Since the electronic band gaps and derived optical properties are severely underestimated by (semi)local exchange and correlation functionals, supplementary hybrid functional calculations have been performed. Concerning the latter, we additionally employ a recently devised technique to speed up structural relaxations for hybrid functional calculations. Our calculations show that the Cu-Zn disorder leads to a slight increase in the unit cell volume compared to the conventional kesterite structure showing full cation order, and that the band gap gets reduced by about 0.2 eV, which is in very good agreement with earlier experimental and theoretical findings. Our detailed results on structural, electronic, and optical properties will be discussed with respect to available experimental data, and will provide further insights into the atomistic origin of the disorder-induced band gap lowering in these promising kesterite type materials. Full article

The replacement of cobalt in the lattice of lithium-rich layered oxides (LRLO) is mandatory to improve their environmental benignity and reduce costs. In this study, we analyze the impact of the cobalt removal from the trigonal LRLO lattice on the structural, thermodynamic, and electronic properties of this material through density functional theory calculations. To mimic disorder in the transition metal layers, we exploited the special quasi-random structure approach on selected supercells. The cobalt removal was modeled by the simultaneous substitution with Mn/Ni, thus leading to a p-doping in the lattice. Our results show that cobalt removal induces (a) larger cell volumes, originating from expanded distances among stacked planes; (b) a parallel increase of the layer buckling; (c) an increase of the electronic disorder and of the concentration of Jahn–Teller defects; and (d) an increase of the thermodynamic stability of the phase. Overall p-doping appears as a balanced strategy to remove cobalt from LRLO without massively deteriorating the structural integrity and the electronic properties of LRLO. Full article