First-principles study, fabrication and characterization of (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramic

The formation possibility of a new (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramic (ZHC-1) was first analyzed by the first-principles calculations and thermodynamical analysis and then it was successfully fabricated by hot pressing sintering technique. The first-principles calculation results showed that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of ZHC-1 was in the range of 0.693R-1.040R. The thermodynamical analysis results showed that ZHC-1 was thermodynamically stable above 959 K owing to its negative mixing Gibbs free energy. The experimental results showed that the as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal structure, some interesting nanoplate-like structures and high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it showed a relatively low thermal conductivity of 15.3 + - 0.3 W/(m.K) at room temperature, which was due to the presence of solid solution effects, nanoplates and porosity. Meanwhile, it exhibited the relatively high nanohardness of 30.3 + - 0.7 GPa and elastic modulus of 460.4 + - 19.2 GPa and the higher fracture toughness of 4.7 + - 0.5 MPa.m1/2, which were attributed to the solid solution strengthening mechanism and nanoplate pullout and microcrack deflection toughening mechanism.Comment: 49 pages,6 figures, 4 table

Structural and Chemical Orders in Ni64.5Zr35.5 Metallic Glass by Molecular Dynamics Simulation

The atomic structure of Ni64.5Zr35.5 metallic glass has been investigated by molecular dynamics (MD) simulations. The calculated structure factors from the MD glassy sample at room temperature agree well with the X-ray diffraction (XRD) and neutron diffraction (ND) experimental data. Using the pairwise cluster alignment and clique analysis methods, we show that there are three types dominant short-range order (SRO) motifs around Ni atoms in the glass sample of Ni64.5Zr35.5, i.e., Mixed-Icosahedron(ICO)-Cube, Twined-Cube and icosahedron-like clusters. Furthermore, chemical order and medium-range order (MRO) analysis show that the Mixed-ICO-Cube and Twined-Cube clusters exhibit the characteristics of the crystalline B2 phase. Our simulation results suggest that the weak glass-forming ability (GFA) of Ni64.5Zr35.5 can be attributed to the competition between the glass forming ICO SRO and the crystalline Mixed-ICO-Cube and Twined-Cube motifs

Atomistic modelling of all dislocations and twins in HCP and BCC Ti

Ti exhibits complex plastic deformation controlled by active dislocation and twinning systems. Understandings on dislocation cores and twin interfaces are currently not complete or quantitative, despite extensive experimental and simulation studies. Here, we determine all the core and twin interface properties in both HCP and BCC Ti using a Deep Potential (DP) and DFT. We determine the core structures, critical resolved shear stresses and mobilities of , , dislocations in HCP and /2 dislocations in BCC Ti. The slip consists of slow core migration on pyramidal-I planes and fast migration on prism-planes, and is kinetically limited by cross-slips among them. This behaviour is consistent with "locking-unlocking" phenomena in TEM and is likely an intrinsic property. Large-scale DFT calculations provide a peek at the screw core and glide behaviour, which is further quantified using DP-Ti. The screw is unstable on pyramidal-II planes. The mixed is nearly sessile on pyramidal-I planes, consistent with observations of long dislocations in this orientation. The edge and mixed are unstable against a pyramidal-to-basal (PB) transition and become sessile at high temperatures, corroborate the difficulties in -axis compression of Ti. Finally, in BCC Ti, the /2 screw has a degenerate core with average glide on {112} planes; the /2 edge and mixed dislocations have non-dissociated cores on {110} planes. This work paints a self-consistent, complete picture on all dislocations in Ti, rationalises previous experimental observations and points to future HRTEM examinations of unusual dislocations such as the mixed and PB transformed cores

Formation criterion for binary metal diboride solid solutions established through combinatorial methods

Establishing the formation criterion is urgent for accelerating the discovery and design of solid-solution materials with desirable properties. The previously reported formation criterion mainly focused on solid-solution alloys, while the formation criterion was rarely established in solid-solution ceramics. To solve this problem, herein, we take a class of solid-solution ceramics, namely binary metal diboride ((MxN1-x)B-2) solid solutions, as a prototype. Through combinatorial methods including high-throughput molten salt syntheses and high-throughput first-principles calculations combined with the machine learning approach, the correlation between influential factors, including atomic size difference (delta), mixing enthalpy at 0 K and 0 Pa (Delta Hmix0K), doping condition (phi), and valence electron concentration (VEC), and the formation ability of (MxN1-x)B-2 solid solutions was first studied systematically, and then their formation criterion was well established. The results showed that the influential degree of the aforementioned four factors on the formation ability of (MxN1-x)B-2 solid solutions could be described as follows: delta \u3e Delta Hmix0K\u3e phi \u3e VEC. In addition, a newly proposed parameter, beta, could well reflect the formation ability of (MxN1-x)B-2 solid solutions: when beta \u3e 0, the single-phase (MxN1-x)B-2 solid solutions could be successfully synthesized in our work and vice versa. This study may provide a theoretical guidance in the discovery and design of various solid-solution ceramics, such as the metal borides, carbides, nitrides, etc, with desirable properties

Structures and magnetic properties of iron silicide from adaptive genetic algorithm and first-principles calculations

We performed a systematic search for low-energy structures of binary iron silicide over a wide range of compositions using the crystal structure prediction method based on adaptive genetic algorithm. 36 structures with formation energies within 50 meV/atom (11 of them are within 20 meV) above the convex hull formed by experimentally known stable structures are predicted. Magnetic properties of these low-energy structures are investigated. Some of these structures can be promising candidates for rare-earth-free permanent magnet

Finite-temperature screw dislocation core structures and dynamics in α-titanium

Abstract A multiscale approach based on molecular dynamics (MD) and kinetic Monte Carlo (kMC) methods is developed to simulate the dynamics of an 〈a〉 screw dislocation in α-Ti. The free energy barriers for the core dissociation transitions and Peierls barriers for dislocation glide as a function of temperature are extracted from the MD simulations (based on machine learning interatomic potentials and optimization); these form the input to kMC simulations. Dislocation random walk trajectories from kMC agree well with those predicted by MD. On some planes, dislocations move via a locking-unlocking mechanism. Surprisingly, some dislocations glide in directions that are not parallel with the core dissociation direction. The MD/kMC multiscale method proposed is applicable to dislocation motion in simple and complex materials (not only screw dislocations in Ti) as a function of temperature and stress state

First‐principles study, fabrication, and characterization of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramic

The formation possibility of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramic (HHC‐1) was first analyzed by the first‐principles calculations, and then, it was successfully fabricated by hot‐pressing sintering technique at 2073 K under a pressure of 30 MPa. The first‐principles calculation results showed that the mixing enthalpy and mixing entropy of HHC‐1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as‐prepared HHC‐1 not only had an interesting single rock‐salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.</p

Formation criterion for binary metal diboride solid solutions established through combinatorial methods

Establishing the formation criterion is urgent for accelerating the discovery and design of solid-solution materials with desirable properties. The previously reported formation criterion mainly focused on solid-solution alloys, while the formation criterion was rarely established in solid-solution ceramics. To solve this problem, herein, we take a class of solid-solution ceramics, namely binary metal diboride ((MxN1-x)B-2) solid solutions, as a prototype. Through combinatorial methods including high-throughput molten salt syntheses and high-throughput first-principles calculations combined with the machine learning approach, the correlation between influential factors, including atomic size difference (delta), mixing enthalpy at 0 K and 0 Pa (Delta Hmix0K), doping condition (phi), and valence electron concentration (VEC), and the formation ability of (MxN1-x)B-2 solid solutions was first studied systematically, and then their formation criterion was well established. The results showed that the influential degree of the aforementioned four factors on the formation ability of (MxN1-x)B-2 solid solutions could be described as follows: delta > Delta Hmix0K> phi > VEC. In addition, a newly proposed parameter, beta, could well reflect the formation ability of (MxN1-x)B-2 solid solutions: when beta > 0, the single-phase (MxN1-x)B-2 solid solutions could be successfully synthesized in our work and vice versa. This study may provide a theoretical guidance in the discovery and design of various solid-solution ceramics, such as the metal borides, carbides, nitrides, etc, with desirable properties.</p