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The thermodynamic properties of the Zr-Nb alloy are investigated at temperatures below 890 K with ab initio calculations. The solution energies of the bcc Nb-rich and hcp Zr-rich solid solutions obtained within the framework of density functional theory are in good agreement with experimental data, although insufficient for a quantitative description of the miscibility gap, for which non configurational entropy has to be accounted for. Whereas electronic free energies can be neglected, we show, using the harmonic approximation and the density functional perturbation theory, that both solution free energies are strongly modified by the contribution related to atomic vibrations. Considering this vibrational free energy leads to a good description of the phase diagram. [ABSTRACT FROM AUTHOR]

X-ray single crystal data for “ThBC 2 ” defined the crystal structure as orthorhombic (space group Pnnm ; a = 1.30655(3) nm, b = 0.39757(1) nm, c = 0.36507(1) nm; R F2 = 0.0130) with the correct formula Th 2 B 2 C 3 . According to the new findings phase relations for the Th-B-C system at 1400 °C have been revised confirming the hitherto known thorium-boron-carbon compounds: ThBC, ThB 2 C and Th 3 B 2 C 3 . The crystal structure of Th 2 B 2 C 3 as a unique structure type is characterized by a C2-branched infinite chain … B-C1-B-B-C1-B … whereby each boron atom is additionally linked to a C2-atom. Boron atoms are in a typical triangular prismatic metal coordination, C1-atoms center a bi-pyramid, Th 4 B 2 , and C2-atoms are surrounded by 5 pyramidal Th-atoms and one B-atom. Density functional theory calculations were made within the pseudopotential approach of VASP utilizing the general gradient approximation for the exchange correlation functional. Structural parameters were optimized resulting in the orthorhombic lattice parameters of a = 1.3144 nm, b = 0.3989 nm, and c = 0.3656 nm. Relativistic calculations by including spin-orbit coupling were performed for the electronic structure. Atomic volumes and charges were computed by the concept of Bader yielding the ionic charges Δq(Th) = +0.99; Δq(B) = +0.61; Δq(C1) = −1.20; Δq(C2) = −1.10. The analysis of the electronic structure shows features of chains and corresponding structural subunits with σ-like bonding. The compound appears weakly metallic with a Sommerfeld constant of γ = 3.12 mJ/(molK 2 ) per formula unit. [ABSTRACT FROM AUTHOR]

A novel doping strategy based on paired charge compensating dopants is proposed to improve the solar thermochemical H 2 O and CO 2 splitting performance of ceria. Density functional theory based ab initio calculations show that by co-doping trivalent and pentavalent cations into ceria the behavior of the trivalent dopant resembles that of tetravalent dopants. In this study, which investigated combinations of group IIIA and VA elements, it was found that the trivalent dopant identity has the largest effect on the reduction energy, while the pentavalent dopant only slightly modifies it. The trivalent dopant in these materials suppresses the reduction energy due to decreased ionic attraction of the trivalent cation and O 2− as compared to Ce 4+ and O 2− . We predict relative reducing capabilities in the following decreasing order: Hf-CeO 2 > ScX-CeO 2 > Zr-CeO 2 > YX-CeO 2 > LaX-CeO 2 > undoped CeO 2 (where X = V, Nb or Ta). Experimental thermogravimetric analysis confirms the computational predictions. [ABSTRACT FROM AUTHOR]

We present a modeling framework and discuss the energetics and structural features of the surface terminations of Ca, Ba, Sr and La hexaborides using density functional theory analysis in combination with X-ray photoelectron spectroscopy. There is significant uncertainty in the literature about the nature of the surface terminations in metal hexaborides in terms of metal versus boron terminations. We show from electronic structure calculations that segregated regions of metal and boron-terminations produce the lowest energies for di-cations of CaB 6 , SrB 6 and BaB 6 , while trivalent LaB 6 minimizes the surface energy by arranging the metal ions in parallel rows on the surface. XPS measurements show that CaB 6 and SrB 6 have surfaces that are close to stoichiometric for the compound, while BaB 6 has surfaces that are Ba-rich. Energetic barriers are calculated for transitions between each of the surface geometries considered. There is a substantial increase in the activation energy for the lanthanum migrations compared to the divalent cations. We also find that the boron octahedra units in these materials tend to contract or expand from their bulk values depending on the proximity to regions of high metal concentrations. These materials have many attractive features, such as low work functions, high hardness, low thermal expansion coefficients, and high melting points, among many other properties of interest for industrial applications. Promising uses of these materials also include catalytic applications for chemical dissociation reactions of various molecules such as hydrogen, water and carbon monoxide, for example, thus, the interest in determining relevant surface properties. [ABSTRACT FROM AUTHOR]

Transition-metal (TM) carbides are an important class of hard, protective coating materials; however, their brittleness often limits potential applications. We use density functional theory to investigate the possibility of improving ductility by forming pseudobinary cubic M 1 M 2 C alloys, for which M 1 = Ti or V and M 2 = W or Mo. The alloying elements are chosen based on previous results showing improved ductility of the corresponding pseudobinary nitride alloys with respect to their parent compounds. While commonly-used empirical criteria do not indicate enhanced ductility in the carbide alloys, calculated stress/strain curves along known slip systems, supported by electronic structure analyses, indicate ductile behavior for VMoC. As VMoC layers are sheared along the 1 1 ¯ 0 direction on {111} planes, the stress initially increases linearly up to a yield point where the accumulated stress is partially dissipated. With further increase in strain, the stress increases again until fracture occurs. A similar mechanical behavior is observed for the corresponding TM nitride VMoN, known to be a ductile ceramic material [1]. Thus, our results show that VMoC is a TM carbide alloy which may be both hard and ductile, i.e. tough. [ABSTRACT FROM AUTHOR]

Lattice site position of O 18 in the presence of vacancy defects in Fe(100) crystal is measured by ion channeling and the corresponding oxygen-vacancy cluster configuration is predicted with Density Functional Theory (DFT) calculations. Energy dependent dechanneling and Slow Positron Doppler broadening analysis show the defects to be vacancy dislocation loops. In O 18 ion implanted Fe, O 18 is found to be in tetrahedral interstitial position. If self ion is irradiated together with O 18 to increase vacancy defects, O 18 is found to be trapped at 1.2 Å away from the lattice position along <111>. From DFT calculations, similar lattice site of O is predicted with a displacement of 1.1 Å along <111> for the interaction of oxygen with ½ <111> vacancy loop structure. Upon increasing vacancy defects further by increasing the fluence of self ions, O 18 is found to shift near octahedral interstitial site displaced by 0.6 Å along <100>. DFT calculations show a similar displacement of 0.46 Å along <100> for the interaction of oxygen with <100> vacancy loop structure. Our combined experimental and DFT studies provide strong evidence of O trapping at vacancy dislocation loops and O trapping site is sensitive to ½ <111> and <100> type vacancy dislocation loops. [ABSTRACT FROM AUTHOR]

We study, using molecular-dynamics (MD) simulation, the spreading kinetics and equilibrium shape of liquid Pb droplets on an Al (111) substrate. The Al-Pb solid-liquid interface was found previously [Phys. Rev. Lett, 110 , 096102 (2013)] to exhibit solid-liquid interfacial premelting at temperatures below the Al melting point. Because the Al(111) free surface does not exhibit premelting, the spreading of a Pb droplet is accompanied by a simultaneous surface transition from a faceted to a premelted Al substrate. Here, we examine how the coupling of the droplet spreading to the premelting affects the spreading kinetics and compare the results to two standard limiting mechanisms of droplet spreading: hydrodynamic spreading , in which the spreading energy dissipation is dominated by viscous relaxation, and kinetic spreading , where interfacial friction dominates. These two mechanisms predict different power-law dependency for the droplet radius versus time. For temperatures where premelting is present (between 875 K and the melting point of aluminum), kinetic spreading is observed at intermediate times. Because Pb droplets only partially wet the surface, the droplet radius at long times relaxes exponentially to the equilibrium droplet shape. Spreading simulations of a faceted system at 625 K [below the Al(111) roughening temperature] show that this system is consistent with a hydrodynamic spreading mechanism. However, when we examine a system at 922.38 K in which premelting is artificially suppressed by introducing harmonic constraints to give surface vibrations consistent with the Al(111) free surface, we observe a kinetic spreading mechanism - as in the premelted system. When premelting is suppressed, the equilibrium contact angle is observed to be significantly larger than when premelting is present even at the same temperatures. Thus, we conclude that the presence of the premelting layer has a significant effect on the thermodynamics Al(111)/Pb solid-liquid interface, but little effect on the mechanism of spreading. We also observe that the structure of the droplet contact line in the presence of the premelting layer is described by two contact angles (instead of the usual one) due to the presence of the additional premelting layer, and resembles structures seen in reactive wetting systems. [ABSTRACT FROM AUTHOR]

We investigate the total energies and electronic structures of Ti-Fe pair defects in α -Al 2 O 3 by carrying out density-functional-theory calculations for various configurations of spin, charge, and orientation of Ti-Fe pair. From the total energy results, we show that the high-spin state of Ti IV -Fe II with an anti-ferromagnetic alignment between Ti and Fe is the ground state for both edge- and face-sharing orientations. Among several meta-stable states, the high-spin state of Ti III -Fe III with an anti-ferromagnetic alignment is connected to the Ti IV -Fe II ground state. An optical transition from the highest occupied Fe defect level to the lowest unoccupied Ti leads to an excited state of the high-spin Ti III -Fe III configuration together with a donation of electron from Fe II to Ti IV . It explains that the intervalence charge transfer from Ti IV -Fe II to Ti III -Fe III is responsible for the color of blue in sapphire. In addition, the total energies of Ti IV -Fe II ground state are found to be quite close within 0.02 eV for both edge- and face-sharing orientations, but the optical excitation energies for edge- and face-sharing orientations are 1.99 and 1.36 eV, respectively. [ABSTRACT FROM AUTHOR]

The third-order elastic constants (TOECs) and elastic anharmonic behavior in four body-centered cubic refractory high-entropy alloys (HEAs) based on elements of the fourth, fifth, and sixth groups are investigated using density-functional simulations. We find that the values of the TOECs C 111 are the largest in magnitude among the studied six independent TOECs and strongly increase with increasing average valence electron concentration (VEC). Interestingly, the TOEC C 456 undergos a sign change as a function of the VEC. Using the obtained TOECs, we investigate the mode Grüneisen constants γ i as well as the low temperature limit γ ¯ , derive the long-wavelength acoustic nonlinearity parameters β , and reveal the pressure derivatives of effective elastic constants and polycrystalline moduli as a function of the VEC. Our results show that β displays a different directional order along the pure mode [ 100 ] , [ 110 ] , and [ 111 ] directions for the four considered refractory HEAs. Furthermore, we show that the directional order of β is not correlated to the crystal symmetry. With the help of the obtained pressure derivatives of polycrystalline moduli, we predict the low temperature volume expansion coefficient. [ABSTRACT FROM AUTHOR]

A recent investigation has confirmed that solute atoms segregated to the interfaces between aluminum and θ ′-Al 2 Cu can extend the stability of metastable θ ′ to higher temperatures. Herein, we present an extensive first-principles database of the segregation energies of 34 elements to the Al/ θ ′ interface and identify key descriptors that can guide the design of higher temperature Al-Cu alloys. We find that the segregation energies are strongly correlated with the size and volume of solute atoms and their solubilities within θ ′. We provide a physical/chemical basis to the experimentally observed elemental hierarchy in terms of the ability of individual elements to stabilize the Al/ θ ′ interface. Finally, we suggest a new microalloying strategy that offers opportunities to further increase the critical temperature limit of Al-Cu alloys. [ABSTRACT FROM AUTHOR]

The mechanical properties of hexagonal Ti alloys depend substantially on the glide of < a > type screw dislocations. The configurations and stabilities of these line defects are, however, known only in pure Ti and Ti + O solutions, where the locking-unlocking mechanism and a strong pinning effect control their activity. In this study, we investigated the unclear, screw dislocation mediated solution strengthening of substitutional α-Ti alloys. To this end, a first principle computational scheme was used to determine the structures and energies of the considered line defects during planar and cross-slip processes in the vicinity of the solute element. Two phenomena were determined that are crucial in terms of plastic deformation: (i) enhanced polymorphism of the dislocation cores leading to multiple new core configurations, and (ii) relatively large positive and negative interaction energies between the solutes and the line defects. Both these effects are strongly affected by the valence configuration of the alloying elements. Due to their pronounced structure and energy variations, dislocation planar and cross slip processes can occur under different scenarios, through diverse non-planar core geometries. Accordingly, an improvement in material strength is related to the overall energy amplitude of the dislocation states that can occur in the neighbourhood of the alloying element, which agrees well with experimental data. The calculations performed also indicate In as a potential alloy element for improving both the strength and ductility of Ti by stabilizing a special, compact core geometry able to spread on an arbitrary glide plane with a low energy barrier. All of the above effects are discussed in terms of the physical factors (solute size misfit, stacking fault energy and electronic structure) that affect the energy and geometry of dislocation cores. [ABSTRACT FROM AUTHOR]

Microalloying is a routine method to optimize precipitation and mechanical properties in light metals. Here we study how In and Sb (0.025 at.%) additions in an Al-1.7 at.% Cu alloy benefit precipitation during ageing and investigate the underlying mechanism using scanning transmission electron microscopy and density functional theory (DFT) calculations. The combined additions accelerate precipitation kinetics and increase peak hardness through two different ways. In samples aged directly at low temperatures (≤200 °C) after solid solution treatment, cubic close-packed InSb nanocrystals form first. The truncated {002} surfaces of InSb particles induce the preferential nucleation of Guinier-Preston (GP) zones, θ″ and θ′ successively. However, in samples aged at 250 °C, precipitation of θ′ precedes that of InSb particles. Supersaturated vacancies trapped by In and Sb solute atoms/clusters play a critical role in switching the precipitation sequence. By tuning how strong the binding between a vacancy and solute atoms is, we successfully invert the temperature dependence of the precipitation sequence. These findings will contribute to understanding precipitation mechanisms and optimizing precipitate distribution in aluminium precipitation hardenable alloys. [ABSTRACT FROM AUTHOR]

Integrating III-V semiconductors into next-generation silicon-based transistors is a promising alternative being considered as a route to faster and more energy-efficient electronic devices. These III-V materials will be doped, typically with Si as a dopant. However, dopant activation remains an issue, compounded by the fact there is still insufficient knowledge of the ease and preferred mechanistic pathways by which dopants, like Si, become activated within the III-V matrix. Using Density Functional Theory calculations, we have determined many of these critically important properties, namely, the energy barriers associated with the diffusion of both intrinsic point defects and silicon impurities in a prototypical ternary III-V material, here a CuAuI-ordered In 0.5 Ga 0.5 As . Refuting assumptions in the current literature, vacancy-assisted diffusion was found to be an unfavorable mechanism for this ordered ternary alloy, unlike in GaAs. Hence, substitutional Si atoms are essentially immobile on experimental timescales. While vacancies are nearly immobile, interstitials, especially split interstitials, can move easily within the crystal lattice. These defects become significant at high Si concentration leading to the unexpected phenomenon of enhanced diffusion of Si at high concentrations, which explains experimental observations. [ABSTRACT FROM AUTHOR]

We examine the equilibrium morphology of Al 2 Cu ( θ ′) precipitates in Al-Cu alloys using a phase field method with the parameters supplied by first-principles density functional theory (DFT) calculations. The phase field method employed allows for an interfacial energy that is highly anisotropic: there are missing high-energy orientations and corners on the Wulff shape. This high degree of anisotropic interfacial energy yields a plate-shaped equilibrium θ ′ precipitate in two-dimensions and a disk-like shape in three dimensions. Also, we consider the effects of a mismatch in elastic-moduli (elastic inhomogeneity) of Al and θ ′, elastic anisotropy, as well as tetragonal misfit strain anisotropy to gain a fuller picture of the elastic energy contributions to the morphology of θ ′ precipitates. Based on our phase-field modeling, the results show that the aspect ratio of the precipitate morphology with the anisotropy of interfacial and strain energies as given by DFT is significantly smaller than the aspect ratio observed in the experiment after long aging times (∼50 h). Specifically, the computed length (54 nm) is almost ten-times smaller than the length (∼580 nm) observed in the Al-Cu experiment with similar precipitate thickness (∼10 nm). Thus, we conclude that the experimental morphology after long aging times (∼50 h) is strongly influenced by the kinetics of precipitate growth. [ABSTRACT FROM AUTHOR]

Knowledge of the thermodynamic and structural properties of iron carbide and nitride phases is crucial for understanding phase transformations and related microstructure formation in steels. While the existence and crystal structure of the primitive cubic fcc-based γ′-Fe 4 N 1- z phase is experimentally well-established, there is no consensus in contemporary literature about an analogous γ′-Fe 4 C compound. Here, we present DFT calculations for all fcc-like Fe 4 C and Fe 4 N superstructures with up to two formula units per primitive unit cell, providing energy values and the relaxed atomic structures, which were analysed mathematically and by visual inspection of the atomic arrangement. Notably, all considered Fe 4 C and Fe 4 N superstructures are metastable with respect to α-Fe and cementite-Fe 3 C/ε-Fe 3 N. Unsurprisingly, we find the well-known γ′ compound's crystal structure to be most favourable among these metastable Fe 4 N superstructures. However, we find the equivalent superstructure to be quite unfavourable in Fe 4 C. The most favourable among these metastable Fe 4 C structures are stabilised by a partial Bain-like distortion into the direction of a body-centred cubic arrangement of Fe atoms. This makes the particular C-ordering interesting for comparison with the short-range order in Fe-C martensites. However, even the lowest-energy Fe 4 C structure releases about 0.056 eV/atom upon decomposition into α + Fe 3 C, much more than it is the case for Fe 4 N (0.019 eV/atom). That energy difference is difficult to overcome even at T > 0 K, in agreement with the lack of clear experimental evidence for existence of a Fe 4 C phase. [ABSTRACT FROM AUTHOR]

Shear failure of the interfacial regions of CrN/Cu/Si and CrN/Ti/Si ceramic-coating/metal-adhesion-layer/substrate systems was measured quantitatively and observed concurrently through instrumented compression of cylindrical micro-pillars in-situ a scanning electron microscope. Results indicate that shear failure of the interfacial region occurred in two stages: an initial shear deformation of the entire metal interlayer followed by an unstable shear-off close to the metal/ceramic interface. The shear-off is suggested to be concomitant with the metal/ceramic interface going from being “locked”, with no relative displacement between materials on the two sides of the interface, to being “unlocked”, with significant relative displacements. Density functional theory and molecular dynamics studies on a related metal/ceramic interface, Ti/TiN, showed that a weak interaction plane exists in the metal layer near the chemical interface in a coherent Ti/TiN structure. Consequently, the free energy and theoretical shear strength of the semi-coherent Ti/TiN interface is found to depend on the physical location of the misfit dislocation network (MDN). The minimum energy and strength of the interface occur when the MDN is near, but not at the chemical interface. The relevance of the present work to other metal/ceramic interfaces is discussed. [ABSTRACT FROM AUTHOR]

The formation of a ternary μ-phase is documented for the system Co-Ti-W. The relevant compositional stability range is identified by high-throughput energy dispersive X-ray spectroscopy, electrical resistance and X-ray diffraction maps from a thin-film materials library (1 μm thickness). Bulk samples of the identified compositions were fabricated to allow for correlative film and bulk studies. Using analytical scanning and transmission electron microscopy, we demonstrate that in both, thin film and bulk samples, the D8 5 phase (μ-phase) coexists with the C36-phase and the A2-phase at comparable average chemical compositions. Young's moduli and hardness values of the μ-phase and the C36-phase were determined by nanoindentation. The trends of experimentally obtained elastic moduli are consistent with density functional theory (DFT) calculations. DFT analysis also supports the experimental findings, that the μ-phase can solve up to 18 at.% Ti. Based on the experimental and DFT results it is shown that CALPHAD modeling can be modified to account for the new findings. [ABSTRACT FROM AUTHOR]

The physics of Fe-C surface interactions is of fundamental importance to phenomena such as corrosion, catalysis, synthesis of graphene, new steels, etc. To better understand this question, we perform an extensive characterization of the energy landscape for carbon diffusion from bulk to surfaces for bcc iron at low C concentration. C diffusion mechanisms over the three main Fe-surfaces – (100), (110) and (111) – are studied computationally using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm. Migration and adsorption energies on surfaces as well as absorption energies into the subsurfaces are predicted and then compared to density functional theory (DFT) and experiment. The energy landscape along C-diffusion pathways from bulk to surface is constructed allowing a more extensive characterization of the diffusion pathways between surface and subsurface. In particular, effective migration energies from (100), (110) and (111) surfaces, to the bulk octahedral site are found to be around ∼1.6 eV, ∼1.2 eV and ∼1.3 eV respectively suggesting that C insertion into the bulk cannot take place in pure crystalline Fe, irrespective of the exposed surface. [ABSTRACT FROM AUTHOR]

We report on a combined computational and experimental examination of coherent precipitation in a Mg-Nd alloy, a prototypical Mg-rare earth alloy. Three-dimensional phase field simulations are conducted to predict the composition and morphology of β ′ ′ ′ precipitates, a unique family of hierarchically ordered phases that are metastable in a wide Nd concentration range. Predictions are compared to experimental high-angle annular dark-field scanning transmission electron microscopy observations. The phase field model thermodynamic description is parameterized entirely from first-principles calculations. The simulations predict an elevated Nd composition in β ′ ′ ′ that is above the stress-free equilibrium value. The elevated Nd concentration in β ′ ′ ′ is in very good agreement with experimental observations and arises from a large misfit strain energy penalty and the low curvature of the β ′ ′ ′ free energy well. The phase field simulations predict that isolated precipitates are lenticular with a (100) habit plane, and are approximately equiaxial when viewed along the [100] direction. The predicted habit plane and precipitate dimensions are shown to be generally consistent with experimental observations, within the uncertainty introduced by density functional theory calculations of the stress-free transformation (misfit) strain and by precipitate interactions not accounted for in the simulations. Contrary to the predictions for isolated precipitates, some experimentally observed precipitates are elongated along the [001] direction relative to the [010] direction. This elongation is frequently correlated with a particular arrangement of two orientation variants of β ′ ′ ′ . A phase field simulation of two precipitates in this arrangement is shown to exhibit similar [001] elongation. [ABSTRACT FROM AUTHOR]

The vacancy effects on the site preferences of alloying elements Re, Mo, Ta and Cr at γ/γ′ interface are studied using first-principles density functional theory. The alloying strengthening mechanism mediated by vacancy is also evaluated via calculating the Griffith work of fracture for γ/γ′ interface as well as the energy barrier of vacancy migration at the interface. The calculated results show that a Ni vacancy on (001)γ′ plane does not change the most preferable substitution position of the alloying elements at corner-point Ni site on (001)γ except Cr. These alloying elements in γ′ enhance the interfacial bonding strength of γ/γ′ interface even with the presence of a neighboring vacancy, among which Re exhibits the best strengthening effect. The improvement in the γ/γ′ interfacial strength is ascribed to the increased chemical bonding strength between the alloying atom and the nearest-neighbor host atom. Furthermore, a single Ni vacancy has higher migration energy barriers around the alloying solutes especially Re solute substituting on (001)γ at γ/γ′ interface, although the alloying solutes at these preferable sites have no contribution to the interfacial bonding strength. [ABSTRACT FROM AUTHOR]

We present calculations within density functional theory on the thermodynamic stability and mechanical properties of t-ZrO 2 (001)/Mo(001) interfaces. The interfacial strength is evaluated by applying energy-based (work of separation) and stress-based (theoretical strength) criteria for different cleavage planes. The lowest energy for crack propagation is obtained for a cut creating a stoichiometric ZrO 2 (001) surface. Our results reveal that molybdenum grain boundaries contaminated with oxygen are less stable against brittle fracture than pure Mo GBs. Addition of Zr, however, can strengthen Mo grain boundaries that contain oxygen by forming an ultrathin zirconia film between Mo grains. The stress required to cleave an ultrathin zirconia film is equal to that required for a pure Mo GB. [ABSTRACT FROM AUTHOR]

We conduct a multiscale modeling of different iron systems exposed to carbon-rich atmospheres by means of density functional theory and reactive molecular dynamics in order to evaluate the effects of temperature, gas content and surface defects such as emerging grain boundaries and grooves on CO dissociation rate. Comparative density functional theory calculations of carbon adsorption energies on clean surface and the groove area show that grooves have preferential binding sites, explaining why emerging grain boundaries are more severely attacked by metal dusting corrosion. Molecular dynamical simulations using a ReaxFF potential on iron Σ3 and Σ5 emerging grain boundaries on (111) and (210) surfaces, respectively, also demonstrate the enhanced CO dissociation rate within the grooves area. Analysis of CO dissociation and recombination events on these systems demonstrates quantitatively the dual role of hydrogen as a CO dissociation enhancer and reactant with dissociated carbon atoms. By carefully characterizing reaction mechanisms as a function of reactant content, we provide a linear correspondence between CO dissociation variation as function of temperature and the experimental measurement of metal dusting corrosion rate. [ABSTRACT FROM AUTHOR]

The crystal structure and atomic coordinates of β F ′ in aged samples of a Mg–3wt.%Nd alloy are examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the first-principles density functional theory (DFT). It is found that the lattice parameters and the atomic positions in the orthorhombic β F ′ unit cell deviate from the atomic model that has been generally accepted. The newly proposed β F ′ structure is similar to that of β 1 which also forms in the alloy, and a simple atomic replacement and structural relaxation can transform the structure of β F ′ to that of β 1 . A specific variant of the β F ′ phase is often observed to be attached to the end of a β 1 precipitate after ageing for longer time. The stress field analysis and interaction energy calculations indicate that this attachment can reduce the stress field around the end facet of the β 1 precipitate and may influence β 1 lengthening. DFT computation results suggest that the β F ′ phase is more stable than β′, but less stable than β 1 , in the Mg–Nd system, and this finding is extendable to other Mg–RE (where RE represents rare earth elements) systems such as Mg–La, Mg–Ce, Mg–Pr, Mg–Pm, Mg–Sm and Mg–Eu alloys. [ABSTRACT FROM AUTHOR]

Superplastic deformation of polycrystalline materials is usually accommodated by diffusion-assisted grain boundary (GB) sliding at high temperatures. Lowering the temperature requirement for commercial superplastic forming enables green and cost-effective manufacturing. Recently, room-temperature (RT) superplasticity was realized in ultrafine-grained Al–Zn based alloys, but the underlying mechanism remains unclear. Here, we conducted in-situ tensile straining and post-mortem electron microscopy characterization, and atomistic density functional theory simulation to understand the RT superplasticity of an Al–15 Zn (at%) alloy. Results showed that the superplasticity is achieved by GB sliding and grain rotation, assisted by the continuous diffusion of Zn. In-situ observations showed Zn atoms diffusing from within grains to GBs, resulting in a Zn nanolayer at the GBs that acts as a solid lubricant to decrease the energy barrier of GB sliding. This research advances our understanding of diffusion-assisted deformation mechanism that is a prerequisite for the rational design of new materials with RT superplasticity. [Display omitted] [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) composed of multiple principal elements have the potential to offer extraordinary radiation resistance. Using NiCoFeCrMn HEA as a model, here we introduce carbon and nitrogen interstitial alloying solutes to impart additional chemical heterogeneities in the form of local chemical order (LCO) and associated compositional variations. Density functional theory simulations reveal the chemical short-range order (CSRO) surrounding carbon and nitrogen atoms. Atomic-resolution chemical mapping of the elemental distribution confirms marked sub-nanometer-scale compositional variations well beyond statistical fluctuations. Irradiation experiments at elevated temperatures demonstrate a remarkable reduction in void swelling by at least one order of magnitude compared to the base HEA without carbon and nitrogen alloying. The underlying mechanism is that the interstitial-solute-induced chemical heterogeneities roughen the lattice as well as the energy landscape, impeding the movements of, and constraining the path lanes for, the normally fast-moving self-interstitials and their clusters. The irradiation-produced interstitials and vacancies therefore recombine more readily, delaying void formation. Our findings indicate that HEAs can be alloyed with other LCO / fluctuation-provoking interstitial or even substitutional elements, and thus open a promising avenue towards highly radiation-tolerant alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Deformation twinning plays a crucial role in the strong anisotropic strength, poor deformability, and texture and microstructure evolutions of hexagonal metals. Modifying twinning behaviour thus may tailor mechanical behaviour of hexagonal metals. Since propagation and growth of deformation twins inevitably interact with other plastic deformation carriers, such as dislocations, stacking faults and phase transformation bands if they can be activated simultaneously, we hypothesize that lowering basal stacking faults energy (SFE) may promote the activity of partial dislocations, thus generating basal stacking faults (BSFs) and even face centred cubic (fcc) phase. Based on first-principles density functional theory calculations, Al solutes can reduce basal SFE and the cohesive energy difference between hexagonal close packed (hcp) and fcc phases of Ti alloys. Microscopy characterizations of deformed Ti-10at.%Al alloy demonstrated that lowering basal SFE actually promotes the formation of profuse BSFs and fcc nanobands in hcp-phase Ti-10at.%Al alloy during mechanical deformation. Consequently, these defects constrain the propagation and growth of {10 1 ¯ 2} deformation twins. More importantly, BSFs and fcc nanobands are activated from twin boundaries during twinning, pinning deformation twins. As a result, deformation twins in Ti-10at.%Al alloy have an average thickness of ∼130 nm, much thinner than twins in pure Ti and other Ti alloys with the similar grain size and under similar strains. In addition, BSFs and fcc nanobands are activated during twin-twin interactions, releasing local stress/strain concentration. Using topological model and atomistic simulations, we further explored interaction mechanisms between deformation twinning and BSFs or fcc nanobands that formed prior to twinning, and the mechanisms of the emission of BSFs and fcc nanobands from twin boundaries during twinning. This work provides insights into understanding the influence of lowering basal SFE on twinning behaviours in hexagonal metals. [Display omitted] [ABSTRACT FROM AUTHOR]

The atomic-level structure of platinum/γ-alumina interfaces is characterized in a model system of dense γ-alumina embedded with faceted Pt NPs produced by implantation of platinum ions into sapphire followed by thermal annealing in air at 800 °C. Aberration-corrected scanning transmission electron microscopy (STEM) was used to collect atomic-resolution images, which are compared to STEM image simulations of two experimentally-based bulk models of γ-alumina by Smrčok et al. and Zhou and Snyder. A density functional theory (DFT) based model of (111) interfaces with different chemical terminations (O, Al 1 , Al 2) of the γ-alumina developed by Oware Sarfo et al. is also compared to experimental STEM data from Pt/γ-alumina interfaces. The Smrčok γ-alumina model provides a better fit than the Zhou structure to the bulk of the γ-alumina. The oxygen-terminated Oware Sarfo model best fits the experimental data and is a very good model close to the interface. However, the fit of the interface model to the experimental data is poorer beyond the third atomic layer in the γ-alumina. This is attributed to compromises required in the design of the model to limit the cell size and computational time for DFT calculations. Understanding the accuracy and limits of the structural models of γ-alumina and Pt/γ-alumina interfaces is important to further the understanding of the structure/property relationships in this system. [Display omitted] [ABSTRACT FROM AUTHOR]

Novel transparent semiconducting oxides have emerged as key materials for a variety of optoelectronic applications due to their ultra-wide band gap. Owing to the critical role of native defects and impurities in the optoelectronical properties, investigation in detail of the electronic structure of these oxides has become a challenge, being luminescence technique a powerful way to gain insight into the electronic levels. Here, the photoluminescence temperature dependence of Li doped Zn 2 GeO 4 microrods is analysed and discussed in the framework of the electronic levels structure related to native defects and Li impurities. Unlike the undoped material, the blue emission (2.8 eV) of Li-doped germanate exhibits a negative thermal quenching as its photoluminescence intensity increases with increasing temperature. The mechanisms underlying this anomalous behaviour are discussed with the aid of X-ray photoemission spectroscopy and first principles calculations. This study has helped us to understand the role of Li in the electronic structure and optical reponse of Zn 2 GeO 4. Our results highlight the versatility of Zn 2 GeO 4 luminescence through Li doping, which could be exploited in optoelectronic devices with bespoke properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Dendrite growth and the deterioration of anode/electrolyte interface impede the development of all-solid-state Li metal batteries. Tremendous designs of anode have been proposed to tackle these challenges. Herein, considering the issues of solid/solid contact and Li depositing nature, we realize a domain-limited laminar Li deposition by designing a three-dimensional (3D) skeleton supporting hybrid anode with embedded lithiophilic seeds. Specifically, LiC 6 particles connect into a skeleton to limit the volume of anode for its sufficient contact with Li 6 PS 5 Cl electrolyte. Assisted by extensively dispersed Li 22 Sn 5 inside the hybrid anode, the electrochemical reaction area is expanded to the whole bulk, giving full play to the 3D design of anode. Besides, density functional theory (DFT) calculations confirm that Li 22 Sn 5 functions as lithiophilic seeds to regulate the laminar Li deposition. Ultimately, the hybrid anode exhibits outstanding compatibility towards Li 6 PS 5 Cl electrolyte as confirmed by the long lifespan of symmetric cell for 400 h at 0.5 mA cm‒2. And a capacity retention of 93.6% is achieved in the full cell with Li 2 ZrF 6 coated LiNi 0.9 Co 0.05 Mn 0.05 O 2 cathodes after 50 cycles at 0.1 C. This work paves the way for designing composite metal anode with dynamically stable interface and suppressed Li dendrites for all-solid-state batteries. [Display omitted] [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical investigation is presented which examines the compressibility of two SrUO 4-x polymorphs, α and β, under hydrostatic conditions and explains the contrasting chemical and mechanical behaviours of these in terms of differences in oxygen defect formation chemistry. Via Rietveld refinements against in situ neutron powder diffraction data, the uranyl bonds in α-SrUO 4 and β-SrUO 4 are shown to expand by approximately 20% when subjected to hydrostatic pressures of up to 6 GPa. This is consistent with a reduction of the uranium formal oxidation state from +6 towards +4. Supported by ab initio calculations using density functional theory, the origin of the reduction is ascribed to the development of oxygen vacancies leading to the formation of SrUO 4-x phases. Remarkably, very different apparent bulk moduli of 591(100) and 60(3) GPa were found for α-SrUO 4-x and β-SrUO 4-x respectively, which are attributed to the difference in the preferred lattice sites for oxygen defect formation. In α-SrUO 4-x oxygen vacancies preferentially form at the in-plane equatorial sites of the UO 8 polyhedra rather than the uranyl sites leading to enhanced electrostatic interlayer repulsion with increasing pressure. In contrast, the oxygen vacancies in β-SrUO 4-x form at the uranyl sites of the UO 6 polyhedra, which negates the electrostatic repulsion and therefore it is more easily compressed. [Display omitted] In situ neutron diffraction measurements have demonstrated that the uranyl bonds in α-SrUO 4 and β-SrUO 4 expand to around 20% when exposed to hydrostatic pressures of up to 6 GPa. Supported by ab initio calculations, the bond expansion is attributed to oxygen vacancy defect formation which results in the reduction of the U valence state from +6 towards +4. Depending on the location of the oxygen defects, very different apparent bulk moduli are produced for the two polymorphs with a diamond-like compressibility for α-SrUO 4. [ABSTRACT FROM AUTHOR]

Solute-dislocation interaction energies are calculated using Density Functional Theory and the 2-D lattice Greens function approach for Al, Ti, Cr, Fe, Zr, Nb, Mo, Ru, Hf, Ta, W, Re solutes in BCC Nb, Ta, Mo, and W. The predictions are then used in an extension of the Rao-Suzuki model to analyze the experimentally observed solid solution strengthening and softening in Nb-X, Fe-X, Ta-X, Mo-X and W-X crystals, X= Mo, W, Ta, Si or Re. DFT predictions for the solute dislocation interaction energies for Fe-Si, taken from the literature are also analyzed. The original Rao-Suzuki model considers kink migration controlled mobility of screw dislocations in body-centered cubic (BCC) structures and is valid for high solute concentrations (>5 at%) as well as above a critical temperature. Here, we extend the Rao-Suzuki model to include kink-pair nucleation statistics in the presence of solutes which is expected to be the rate limiting process at low temperatures (<0.2T melt). Such an extension should be valid up to low concentration of solutes as well as fairly low temperatures. The key parameters required for the model are solute – ½[111] screw dislocation interaction energies in pure Nb, Ta, Mo or W. These parameters were derived in this paper using first principles calculations. Using the calculated interaction energies, the model satisfactorily reproduces experimentally observed solution strengthening data in Nb-Mo, Nb-W, Nb-Re crystals at temperatures from 77 to 600K, for solute concentrations in the range 8.5 to 19 at% for Mo, 6 to 15 at.% for W and 5 to 9 at.% for Re. Similar agreement is found for strengthening/softening data for Ta-W, Ta-Re, Ta-Nb, Fe-Si, Mo-Re and W-Re crystals in the temperature range 77 – 600K. We also explore sensitivity of these predictions to the variation in the solute-dislocation energies. In Ta-W, a 10% variation (±0.015 eV) produces a modest change in solution hardening, which is within experimental error. The refined Rao-Suzuki model can be used in designing high strength, multi-component BCC alloys for high temperature applications. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Half-antiperovskites (HAPs) are a class of materials consisting of stacked kagome lattices and thus host exotic magnetic and electronic states. We perform high-throughput calculations based on density functional theory (DFT) and atomistic spin dynamics (ASD) simulations to predict stable magnetic HAPs M 3 X 2 Z 2 (M = Cr, Mn, Fe, Co, and Ni; X is one of the elements from Li to Bi except noble gases and 4 f rare-earth metals; Z = S, Se, and Te), with both thermodynamical and mechanical stabilities evaluated. Additionally, the magnetic ground states are obtained by utilizing DFT calculations combined with the ASD simulations. The existing spin frustration in an AFM kagome lattice manifests as competing behavior of the in-plane FM and AFM couplings. For a total number of 930 HAP compositions considered, we have found 23 compounds that are stabilized at non-collinear antiferromagnetic (AFM) state and 11 compounds that possess ferromagnetic (FM) order. [ABSTRACT FROM AUTHOR]

[Display omitted] NdFe 11 Ti and YFe 11 Ti serve as prototypes for rare-earth (RE) lean or REfree magnets with the ThMn 12 -type structure. Although NdFe 11 Ti has been studied for a long time the origin of its complex magnetism at low temperature is so far not well-understood. We present a comprehensive theoretical and experimental study of the magnetic properties of NdFe 11 Ti and RE-free YFe 11 Ti to elucidate the influence of the 4 f electrons. The partially localized 4 f electrons of Nd are the driving force behind the complex behavior of the magnetocrystalline anisotropy which changes from cone to uniaxial above 170 dK. The spontaneous magnetization and the five leading anisotropy constants were determined from high-quality single crystal samples over a wide temperature range using field dependencies of magnetization measured along the principle crystallographic directions. The experimental data are compared with density functional theory combined with a Hartree-Fock correction (+U) and an approximate dynamical mean-field theory. [ABSTRACT FROM AUTHOR]

Understanding the interaction of hydrogen (H) with defects has been recognized as a central aspect in hydrogen-induced damages in metals. In this work, the role of coherent and semi-coherent interfaces between W and ZrC on H trapping, desorption and clustering in nanostructured W alloys was investigated by density functional theory calculations. It is found that, the trapping ability of pure interface is comparable with the intrinsic defects (grain boundary, dislocations) in bulk W, with segregation energies of H ranging from -1.37 to -0.98 eV. While the interfaces with W/C vacancy show stronger trapping capabilities, especially for the semi-coherent interfaces. This leads to low peak temperatures of 390–567 K for H desorption from the pure interfaces and high peak temperatures of 470–836 K from the vacancy-containing interfaces. As the number of H atoms increases, they preferentially segregate to the interfacial W vacancy and divacancy, and then stay at the interface and interface-like zones discretely until the adsorption rate reaches 3/8. The strong traps of vacancy-containing phase interfaces could effectively absorb H from the screw dislocation cores and bulk vacancies in W, hence preventing H-induced damage. This work advances atomic-level understanding of the role of phase interfaces in H trapping and clustering, not only offering direct explanations for experimental H desorption observations, but also pointing to a feasible route towards interface engineering against H-induced damage in structural metals. [Display omitted] [ABSTRACT FROM AUTHOR]

Understanding the formation and evolution of Cu precipitates in Fe-based alloys is crucial as they are key factors responsible for hardening and embrittlement. Dilute FeCu alloys are model materials for structural components in various application areas, including that of reactor pressure vessel steels for light water nuclear reactors. Positron annihilation spectroscopy (PAS) is a powerful tool to study the nucleation stage of both homogeneous and heterogeneous Cu precipitation, which are beyond the reach of most other experimental techniques. In this work, we present a first-principles study of positron annihilation in Fe-Cu systems. The positron annihilation characteristics (positron lifetimes, Doppler broadening spectra) are calculated for both homogeneous vacancy-free Cu clusters and heterogeneous vacancy-Cu complexes using two-component density functional theory. The theoretical results excellently agree with the available reference PAS experimental results. Our calculations show that the types of Cu precipitates can be clearly distinguished by positron annihilation, and the sizes of Cu precipitates can also be reasonably well estimated with our calculations. Moreover, we also successfully analyze the evolution of the experimental signals during isochronal annealing where the small Cu clusters change character. This work improves the understanding of the early-stage Cu precipitation in Fe matrix. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] We investigate the influence of elastic properties of point defects on dislocation climb under stress and irradiation. For this purpose, elastic dipole tensors and diaelastic polarizabilities are evaluated in aluminum for vacancies and self-interstitial atoms in their stable and saddle configurations, using density functional theory calculations. These parameters are introduced in an object kinetic Monte-Carlo code and a continuous diffusion model to estimate the stress dependence of dislocation climb, using a dipole of straight dislocations. We show that both parameters have an influence on absorption of point defects under stress, in agreement with previous analytical models. However, the effect of dipole tensor is found only 5 times larger than polarizability, whereas models predict a factor up to 30. In addition, including polarizability reverses the stress angular dependence when a uniaxial stress is applied orthogonal to the dislocation line, so in general polarizability cannot be ignored for simulations under applied stress. Further comparison with analytical models shows that they give a good description of angular dependence, provided saddle point configuration of point defects is not too anisotropic. For vacancies, which are strongly anisotropic in their saddle configuration, models fail to reproduce quantitatively lattice effects on stress angular dependence observed in simulations. Calculations show that dislocation climb velocity under irradiation is expected to be the highest if the stress is approximately orthogonal to the dislocation line, especially along the Burgers vector, and the lowest if the stress is applied close to the 〈 100 〉 direction with the largest projection on the dislocation line. [ABSTRACT FROM AUTHOR]

A multiscale approach based on the phase-field model is developed to simulate homogeneous and heterogeneous formation of θ′ precipitates during high temperature ageing in Al-Cu alloys. The model parameters that determine the different energy contributions (chemical free energy, interfacial energy, lattice parameters, elastic constants) were obtained from either computational thermodynamics databases or from first-principles density functional theory and molecular statics simulations. From this information, the evolution and equilibrium morphology of the θ′ precipitates is simulated in 3D using the phase-field model. The model was able to reproduce the evolution of the different orientation variants of plate-like shaped θ′ precipitates with orientation relationship (001) θ' //(001) α and [100] θ' //[100] α during homogeneous nucleation as well as the heterogeneous nucleation on dislocations, leading to the formation of precipitate arrays. Heterogeneous nucleation on pre-existing dislocation(s) was triggered by the interaction energy between the dislocation stress field and the stress-free transformation strain associated to the nucleation of the θ′ precipitates. Moreover, the mechanisms controlling the evolution of the morphology and the equilibrium aspect ratio of the precipitates were ascertained. All the predictions of the multiscale model were in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Density functional theory is used to provide theoretical insights into the ternary nanolaminated and layered transition metal boride (MAB phase) of MoAlB, with calculations of crystal structure, electronic structure, lattice dynamics and elastic properties, including a corresponding hypothetical MAX phase compound Mo 2 AlC for comparison. The calculated atomic configuration matches well with experiment. The metal-like electronic structure contributes to the physical origin of the high electrical conductivity of MoAlB. Strong covalent bonding is present between the B atoms, as well as between the Mo and B atoms, and significantly the much weaker Al Al bonds are consistent with the high fracture toughness and damage tolerance seen in MoAlB. With increasing pressure, the shrinkage is highest along the b axis, and lowest along the c axis. From the calculated second-order elastic constants, the bulk moduli B , shear moduli G , Young's moduli E and Poisson ratio μ are 207 GPa, 137 GPa, 336 GPa and 0.23, respectively. The G / B ratio of 0.66—similar in magnitude to values in MAX phases –demonstrate similarities in properties between MAB and MAX phases. Lattice dynamics are examined in detail, with 9 Raman-active modes and 6 infrared-active modes identified and analyzed in terms of their atomic motion and wavenumbers. [ABSTRACT FROM AUTHOR]

Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO 2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO 2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb 5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb 2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO 2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb 5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition. [ABSTRACT FROM AUTHOR]

Plasticity in zirconium alloys is mainly controlled by the interaction of 1 / 3 〈 1 2 ¯ 10 〉 screw dislocations with oxygen atoms in interstitial octahedral sites of the hexagonal close-packed lattice. This process is studied here using ab initio calculations based on the density functional theory. The atomic simulations show that a strong repulsion exists only when the O atoms lie in the dislocation core and belong to the prismatic dislocation habit plane. This is a consequence of the destruction of the octahedral sites by the stacking fault arising from the dislocation dissociation. Because of the repulsion, the dislocation partially cross-slips to an adjacent prismatic plane, in agreement with experiments where the lattice friction on screw dislocations in Zr-O alloys has been attributed to the presence of jogs on the dislocations due to local cross-slip. [ABSTRACT FROM AUTHOR]

Using first-principles density functional theory (DFT), we calculate the diffusivities of 32 different solute elements—all transition metals, together with Al and Si—in fcc cobalt within the formalism of the five-frequency model. For self-diffusion in fcc cobalt, we compare the accuracy of various approximations to the exchange-correlation energy functional of DFT in estimating the activation energy, and find that only the Perdew-Burke-Ernzerhof (PBE) approximation agrees well with experimental reports and all other functionals largely overestimate it. Our calculations also show that an accurate estimation of the self-diffusion coefficient requires explicit calculation of the effective jump frequency and vacancy formation entropy via phonons. Using accurate self-diffusion data and scaling all solute-related attempt frequencies with respect to the attempt frequency for self-diffusion using a simple relation involving the atomic mass and melting temperature of the solute yields solute diffusivities in excellent agreement with experiments, where such data is available. We find that large solutes spontaneously relax toward the nearest neighbor vacancy to relieve the misfit strain, and the extent of this relaxation correlates negatively with the migration energy. Thus, in general, larger solutes have lower migration energies and diffuse faster than smaller solutes in fcc cobalt. However, extremely large solutes, e.g., group III elements Sc, Y, Lu, tend to be trapped in an energy valley located halfway toward the vacancy, and monovacancy mediated diffusion may no longer be valid in such cases. Finally, for all the solutes considered, we systematically tabulate the diffusion-related quantities calculated—diffusion prefactors, migration and activation energies—constructing an extensive and accurate first-principles database for solute diffusion in fcc cobalt. [ABSTRACT FROM AUTHOR]

It is well-known that the stability of Pd-Si amorphous alloys can be improved substantially by minor additions of alloying elements such as Cu and Ag. Such improvement in stability is explained herein, whereby microscopic models based on efficient atomic packing and electronic structure were applied to the results of first principles simulations of Pd 82 Si 18 , Pd 77.5 Si 16.5 Cu 8 and Pd 75 Si 15 Cu 7 Ag 3 metallic glasses. It was revealed that while the atomic packing model fails to unequivocally explain the stabilizing effect of the binary Pd-Si alloy due to minor additions of Cu and/or Ag, the contribution of electronic states with lower energies to the stability of the amorphous structure is increased markedly. Further, the observed enhancement in the compressive ductility as a result of the addition of Ag to the Pd 77.5 Si 16.5 Cu 8 alloy can be correlated to the combined effects of an increased heterogeneity in the local topology, weakened covalency and, hence, reduced directionality of Pd-Pd bonds as well as enhanced metallicity in the Pd 75 Si 15 Cu 7 Ag 3 amorphous alloy. The analysis was further expanded to amorphous alloys where their characteristic binary prototypes are synthesised from late transition metals and non-metals as well as those comprising early transition metals and late transition metals. The findings shed light on the effects of minor alloying on the cooperative and competitive relationship between the topological and electronic structure of amorphous alloys. [ABSTRACT FROM AUTHOR]

An in-depth understanding of the origin of low lattice thermal conductivity κ lat is considered to be the prerequisite for developing and designing high thermoelectric performance materials. However, the microscopic mechanism of low κ lat for α-MgAgSb has not been fully elaborated. Here we first thoroughly unveil the underlying mechanism using density functional theory (DFT) calculations. It is demonstrated that the unique lattice dynamic properties of α-MgAgSb contribute to the low κ lat , including a large number of optical branches, weak chemical bonding, the strong anharmonic coupling between longitudinal acoustic (LA) and longitudinal optical (LO) branches, high anharmonic behavior of LO branch. Nanostructuring has an effect in decreasing the lattice thermal conductivity due to the high density of boundaries and defects. Most importantly, Yb doping on the Mg site is chosen to optimize carrier concentration and simultaneously create strong point-defect phonon scattering. Collectively, a peak ZT ∼1.4 at 550 K and average ZT ∼1.2 from 300 K to 550 K have been achieved, which further demonstrates the realistic prospect of MgAgSb based thermoelectric materials for low-temperature waste heat harvesting. [ABSTRACT FROM AUTHOR]

We extended and updated Mott's two-band model for the composition-dependence of thermal and electrical conductivity in binary metal alloys based on high-throughput time-domain thermoreflectance (TDTR) measurements on diffusion multiples and scatterer-density calculations from first principles. Examining Au-Cu, Au-Ag, Pd-Ag, Pd-Cu, Pd-Pt, Pt-Rh, and Ni-Rh binary alloys, we found that the nature of the two dominant scatterer-bands considered in the Mott model changes with the alloys, and should be interpreted as a combination of the dominant element-specific s - and/or d -orbitals. Using orbital and element-resolved density-of-states values calculated with density functional theory as input, we determined the correct orbital mix that dominates electron scattering for all examined alloys and found excellent agreement between fitted models and experimental results. This general model of the composition dependence of the thermal and electrical resistivity can be readily implemented into the CALPHAD framework. [ABSTRACT FROM AUTHOR]

A roadblock in utilizing InGaAs for scaled-down electronic devices is its anomalous dopant diffusion behavior; specifically, existing models are not able to explain available experimental data on beryllium diffusion consistently. In this paper, we propose a more comprehensive model, taking self-interstitial migration and Be interaction with Ga and In into account. Density functional theory (DFT) calculations are first used to calculate the energy parameters and charge states of possible diffusion mechanisms. Based on the DFT results, continuum modeling and kinetic Monte Carlo simulations are then performed. The model is able to reproduce experimental Be concentration profiles. Our results suggest that the Frank-Turnbull mechanism is not likely, instead, kick-out reactions are the dominant mechanism. Due to a large reaction energy difference, the Ga interstitial and the In interstitial play different roles in the kick-out reactions, contrary to what is usually assumed. The DFT calculations also suggest that the influence of As on Be diffusion may not be negligible. [ABSTRACT FROM AUTHOR]

We present theoretical prediction and experimental evidence of a new MAX phase alloy, Mo 2 ScAlC 2 , with out-of-plane chemical order. Evaluation of phase stability was performed by ab initio calculations based on Density Functional Theory, suggesting that chemical order in the alloy promotes a stable phase, with a formation enthalpy of −24 meV/atom, as opposed to the predicted unstable Mo 3 AlC 2 and Sc 3 AlC 2 . Bulk synthesis of Mo 2 ScAlC 2 is achieved by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 °C. High resolution transmission electron microscopy reveals a chemically ordered structure consistent with theoretical predictions with one Sc layer sandwiched between two Mo C layers. The two-dimensional derivative, the MXene, is produced by selective etching of the Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo 2 ScC 2 , i.e. the first Sc-containing MXene. The here presented results expands the attainable range of MXene compositions and widens the prospects for property tuning. [ABSTRACT FROM AUTHOR]

Predicting the influence of the surface on the effective elastic properties of nanoscale structures and nanomaterials remains a challenge, which we here address on both levels, continuum and atomic. Density Functional Theory (DFT) computation at the atomic level yields the first reliable surface excess elastic parameters for the (111) and (001) surfaces of gold. At the continuum level, we derive closed-form expressions for the effective elastic behavior that can be combined with the DFT-derived excess elastic parameters to obtain the effective axial, torsion, and bending stiffness of circular nanowires with surface excess elasticity. The two approaches use different reference frames, and we emphasize the need for consistent stress definitions and for conversion between the separate stress measures when transferring results between the approaches. We present excess elastic parameters separately for Cauchy and 2 nd Piola-Kirchhoff stresses, demonstrating that the conversion substantially modifies their numerical value and may even invert their sign. The results afford an assessment of the contribution of the surface excess elastic parameters to the effective elastic response of nanoscale beams or wires. This assessment sheds doubt on earlier suggestions relating experimental observations of an effective stiffening or softening at small size to the excess elasticity of clean surfaces. [ABSTRACT FROM AUTHOR]

Using extensive first-principles density functional calculations, we calculate thermodynamic properties of binary fcc cobalt-based alloys with 25 different solute elements. For each solute element X , we calculate its (a) nearest- and next-nearest-neighbor solute–vacancy binding energy, (b) dilute impurity mixing energy with respect to the equilibrium and hypothetical fcc-based reference states of the solute, (c) enthalpy of formation of cobalt-rich binary ordered compounds in the Co– X system, (d) solubility enthalpy, and other derived quantities. We find that the solute–vacancy binding energies of all the studied solutes in fcc cobalt are positive (indicating favorable binding), in contrast to nickel, aluminum, magnesium, and copper-based alloys, where mid 3 d transition metal solutes have unfavorable solute–vacancy binding. We study the physical and chemical effects influencing solute–vacancy binding energy, and find that (a) it correlates broadly with solute size—larger solute atoms possess stronger binding with vacancies—and this is understood in terms of strain relief and secondary next-nearest-neighbor interactions, and (b) it follows a parabolic trend as a function of d -occupancy across the transition metal series—falling to a minimum at the middle of the series—and this is understood in terms of a d -band filling effect. We also find this d -band filling effect in other calculated quantities such as the dilute impurity volumes and dilute impurity mixing energies. To aid in building thermodynamic databases, we tabulate all the calculated thermodynamic quantities, and compare them with experimental phase diagrams data and previous literature, and find good agreement where such data is available. [ABSTRACT FROM AUTHOR]

Dislocation cores, the regions in the immediate vicinity of dislocation lines, control a number of properties such as dislocation mobility, cross-slip and short-range interactions with other defects. The quantitative modeling of dislocation cores requires an electronic-level description of atomic bonding. Ab initio quantum mechanical calculations of dislocation cores based on the density functional theory have progressed rapidly thanks to the steady increase in computing capacities and the development of dedicated numerical methods and codes. Our aim in this overview paper is, after a description of the methodology regarding in particular the boundary conditions, to review the new and unexpected results obtained on dislocation cores from first principles, including the identification of unforeseen stable and metastable cores and the quantitative evaluation of both interaction energies and energy pathways, in pure metals and alloys of different crystallography (FCC, BCC, HCP) as well as semiconductors. We also identify key challenges to be explored in this rapidly growing field. [ABSTRACT FROM AUTHOR]

This work demonstrates that room temperature (RT) ferromagnetism might be triggered in pure hafnium (Hf), a classic paramagnet, by severe plastic deformation (SPD) via high pressure torsion (HPT). The origin of this phenomenon is elucidated by a combined approach including density functional theory (DFT) calculations and transmission electron microscopy (TEM). In particular, it is shown that the elastic lattice distorsions induced in pure Hf as a consequence of grain refinement down to the nanocrystalline regime by HPT lead to the development of a new monoclinic crystalline structure that exhibits a spontaneous magnetization at RT. DFT calculations are utilized to prove that local stretching of the original pure Hf hexagonal close packed (hcp) lattice along specific pyramidal directions, due to the presence of internal stresses in the deformed nanostructure, may give rise to the emergence of the monoclinic phase, which is endowed with a net magnetic moment. An excellent agreement is found between DFT calculations and experimental TEM observations, which provide a first evidence of the presence of the pure Hf monoclinic crystal lattice. This work shows that SPD may constitute a viable, yet widely unexplored, strategy to tune the magnetic properties and, in particular, to induce RT ferromagnetism in bulk non-magnetic metals. [ABSTRACT FROM AUTHOR]