First-principles Energetics Research Articles

First-principles insights into solute partition among various nano-phases in Al−Cu−Li−Mg alloys

First-principles energetics calculations were performed over a group of 25 candidate elements, to investigate their partition energies and behaviors among major nano-phases (δʹ-Al3Li, θʹ-Al2Cu, T1-Al6Cu4Li3 and S-Al2CuMg) in most commercial Al−Cu−Li−Mg alloys. It was revealed that principal alloying element Li cannot directly affect θʹ and S, Cu cannot directly affect δʹ, Mg cannot directly affect θʹ and δʹ but can weakly partition into T1. As for micro-alloying elements, Si is the only element that can partition into all the four nano-phases. Au can partition into δʹ, θʹ and S. Cd can partition into δʹ, T1 and S. Zr can partition into δʹ and T1. Ni substitution can weakly partition into θʹ and S. Ti, V, Zn, Ag and Mo can partition into δʹ only. Cr, Mn, Fe and Co cannot directly affect any of the four nano-phases. Almost all RE-elements can strongly partition into T1, δʹ and S, but would be expelled by θʹ. Only Sc is somehow particular to S, showing no partition preference in S. The newly obtained fundamental results could constitute a simple prototype database in support for the accelerated design of Al−Li based alloys and other related Al alloys.

Ab Initio-Based Modeling of Thermodynamic Cyclic Voltammograms: A Benchmark Study on Ag(100) in Bromide Solutions.

Experimental cyclic voltammograms (CVs) measured in the slow scan rate limit can be entirely described in terms of the thermodynamic equilibrium quantities of the electrified solid-liquid interface. They correspondingly serve as an important benchmark for the quality of first-principles calculations of interfacial thermodynamics. Here, we investigate the partially drastic approximations made presently in computationally efficient calculations for the well-defined showcase of an Ag(100) model electrode in Br-containing electrolytes, where the nontrivial part of the CV stems from the electrosorption of Br ions. We specifically study the entanglement of common approximations in the treatment of solvation and field effects, as well as in the way macroscopic averages of the two key quantities, namely, the potential-dependent adsorbate coverage and electrosorption valency, are derived from the first-principles energetics. We demonstrate that the combination of energetics obtained within an implicit solvation model and a perturbative second order account of capacitive double layer effects with a constant-potential grand-canonical Monte Carlo sampling of the adsorbate layer provides an accurate description of the experimental CV. However, our analysis also shows that error cancellation at lower levels of theory may equally lead to good descriptions even though key underlying physics such as the disorder-order transition of the Br adlayer at increasing coverages is inadequately treated.

Environment-dependent phase stabilities of titanium hydrides: First-principles prediction

Various phases of TiHx (x=1−2) were constructed, with all possible H-atom configurations being considered. The first-principles energetics calculations were performed to evaluate their formation preference and mechanical and thermodynamic stabilities. All the results were then combined to construct the stable and metastable phase stability diagrams of TiHx. It is suggested that many stable and metastable TiHx phases have very comparable formation energies and thus are likely to co-exist. Their relative stabilities are strongly depended on temperature (T) and the partial pressure of hydrogen (p(H2)). Over the entire T and p(H2) range of interest, γ-TiH and γ-TiH2 are the only two stable phases. ɛ-TiH, γ-TiH1.5, ɛ- and δ-TiH1.75, and ɛ-TiH2 can possibly present as metastable phases. These metastable phases are all mechanically stable, and have a thermodynamic tendency to continuously absorb or release H until reaching the equilibrium phases of γ-TiH2 or γ-TiH.

Hydrogen trapping energetics at BCC iron-helium interfaces

Density functional theory (DFT) calculations have been performed to assess the trapping and segregation strength of hydrogen (H) to helium (He) interfaces in BCC (Fe) as a function of He density and surface orientation. The He density ranges from 0 to 2 He/V which corresponds to equilibrium bubbles that are expected to form in the structural components in a fusion reactor environment. The DFT calculations consist of a slab of BCC metal and an initial lattice of FCC He. We report on the binding energy of H to these interfaces as well as the migration barriers into and away from the surface, which together provide information on the de-trapping energy for H at He bubbles. It was found that the binding energy of H to He-Fe interfaces decreases with increasing He density. Migration barriers are also modified due to the He density, which are sensitive to the surface orientation as well as the diffusion pathway. These results provide valuable first-principles energetics that are necessary for higher-order, mesoscale models, and provide insight into the extent to which He bubbles may trap tritium in fusion structural materials.

Design of Fe2B-based ductile high temperature ceramics: First-principles calculations and experimental validation

Iron Boride (Fe2B) has significant potential to be used as a wear- and corrosion-resistant ceramic because of its high hardness, good corrosion resistance, and low cost. However, the toughness and elastic heat resistance of Fe2B can be further improved under high-temperature wear conditions. First-principles energetics and phonon calculations showed that Ti, Cr, Mn, Co, Ni, Nb, Mo, Ta, and W can dissolve in Fe2B. Among these, Cr, Mn, and Ni can form a continuous solid solution in (Fe, M)2B. Furthermore, Ti, Cr, Mn, Co, Ni, Cu, Y, Nb, Mo, Ta, and W improved the toughness of Fe2B. The results were validated by the experimental results. It was found that the experimental fracture toughness of Fe2B doped with Cr and Ni reached 4.16 and 4.27 MPa m1/2, respectively, which are higher than 3.64 MPa m1/2 of pure Fe2B. Chemical bond analyses showed that the toughness was improved owing to the stiffening of the B–B bond and the invariability of the Fe–B bond in Fe2B by doping Cr and Ni with increasing pressure. The elastic heat resistance of alloyed Fe2B was related to the degree to which the B–B and Fe–B bonds weakened with temperature. The less these bonds strength weakened with increasing temperature, the better the elastic heat resistance. The bond strength of Fe–B and B–B bonds in Fe2B doped with Cr showed a minimum decrease with increasing temperature, whereas the bond strength of Fe–B and B–B bonds in Fe2B doped with Ta showed a maximum decrease with increasing temperature. This indicates that the alloying element Cr can improve the elastic heat resistance of Fe2B, while Ta decreases the elastic heat resistance of Fe2B.

Theoretical modeling of edge-controlled growth kinetics and structural engineering of 2D-MoSe2

We introducethe firstreactive force field (ReaxFF) for Mo/Se/H interactions, which enables large-scale molecular dynamics simulations of the synthesis, processing, and characterization of 2D-MoSe2and whose parameters are trained primarily on first-principles energetics data includingboth periodic and non-periodic calculations. This new potential elucidates thestructural transition frommetallic to semiconducting phases, the energetics of variousdefects, and the Se-vacancy migration barrier.A theoretical model developed based on this potential and the Wulff constructionalso describes an observed morphology evolution of 2D-MoSe2domains during growth.Since controllable edge-mediated growth kinetics of 2D-MoSe2are of great interest to the 2D community, we believe that this new ReaxFF potentialtrained against the edge formation energies of MoSe2nanoribbons with different Se coverageswill be a powerful complementary tool to experimental studies by simulating the edge-growth kinetics of 2D-MoSe2at high speed and low cost.

Nucleation of Y–Si–O Nano-clusters in Multi-microalloyed Nano-structured Ferritic Alloys: a First-principles Study

First-principles energetics analyses were performed to investigate the nucleation of (Y–Si–O) nano-clusters (NCs) in nano-structured ferritic alloys (NFAs). It was predicted that the nucleation of (Y-Si–O) NCs follows the same general pathway as previously found for (Y–Ti/Al/Zr–O) NCs in NFAs: they all tend to begin with the (O–O) pairs and grow to (O–Y)-cores and further to larger NCs by attracting Y and other solute elements nearby. Nucleation of a hexa-atomic –[2Y–Si–3O]– cluster can reduce the total energy by 4.71 eV. Among various microalloying-induced NCs, the nucleation preference ordering was predicted as (Y–Zr–O) > (Y–Ti–O) > (Y–Al–O) > (Y–Si–O). The number densities and chemical compositions of NCs in multi-microalloyed NFAs would largely depend on the number availability of (O,Y)-cores, as well as the relative abundance and atomic diffusivities of microalloying solute elements nearby.

An ab-initio study of hydrogen trapping energetics at BCC tungsten metal-noble gas interfaces

Density functional theory (DFT) calculations have been performed to assess the trapping and segregation strength of hydrogen (H) to noble gas interfaces in tungsten (W). These calculations consist of a supercell containing a slab of BCC W and an initial lattice of FCC noble gas atoms. The interfaces included noble gases of helium, neon, and argon, with densities in the range of 1-4 atoms-per-vacancy (V), and W surface orientations of (100), (110), and (111). We report on the binding energy of H to these interfaces as well as the modification to the migration barriers in the W slab, which together provide information on the segregation strength and de-trapping energy for H at noble gas bubbles. These calculations indicate that the binding energy of H to W-noble gas interfaces varies with surface orientation and decreases with increasing gas density; whereas the H migration energy is sensitive to the noble gas density, surface orientation, and diffusion pathway, and typically increases with gas density. Together, the de-trapping energy of H to these interfaces is shown to depend less significantly on noble gas density. These DFT calculations provide valuable first-principles energetics necessary for mesoscale models, and provide insight into the temperatures required to de-trap tritium from He bubbles in fusion plasma facing components.

Defective GP-zones and their evolution in an Al-Cu-Mg alloy during high-temperature aging

Nano-precipitates at the two-stage hardening peaks of an Al-5.0Cu-0.4 Mg (wt%) alloy under isothermal aging at 210 °C were thoroughly characterized using aberration-corrected scanning transmission electron microscopy and first-principles energetics calculations. It was observed, for the first time, that individual one-dimensional GPB1 units can form with a high number density in the matrix, due to the stabilization of substitutional defects. The GPII zones at the early-stage peak aging, as well as the θ′ nano-precipitates at the second-stage peak aging, are also frequently found to be defective in structure. Further analyses suggest that the high-temperature age hardening involves a complex association and evolution of the defective GP-zones.

Effect of the modulation ratio on the interface structure of TiAlN/TiN and TiAlN/ZrN multilayers: First-principles and experimental investigations

The interface structure of the multilayered transition metal nitride coatings, which are widely used for various industrial applications, plays a crucial role in their properties. Here, first-principles calculations combined with key experiments are used to investigate the interface structures of TiAlN/TiN and TiAlN/ZrN multilayers with varied modulation ratios. Based on first-principles energetics calculations, it was found that TiAlN/TiN multilayers favour the coherent growth, independent of the modulation ratio of TiAlN to TiN, whereas the modulation ratio of TiAlN to ZrN has an important effect on the interface structure of TiAlN/ZrN multilayers. This prediction is in agreement with our experimental investigations. For TiAlN/TiN and TiAlN/ZrN multilayers with modulation periods of 4–6 nm and varied modulation ratios, the coherent interfaces between the TiAlN and TiN sublayers are observed in all TiAlN/TiN multilayers, while only the TiAlN/ZrN multilayer with a lower modulation ratio of TiAlN to ZrN exhibits the overall metastable epitaxial growth. The formation of metastable coherent interfaces in TiAlN/ZrN multilayers can be attributed to the nonequilibrium deposition process with the presence of defects, such as interface dislocations.

First-principles energetics of rare gases incorporation into uranium dioxide

First-principles density functional theory–generalized gradient approximation methods have been used to calculate the energetics (incorporation energy, formation energy and binding energy) of rare gases (He, Ne, Ar, Kr and Xe) at the three incorporation sites (octahedral interstitial, uranium and oxygen vacancies) of uranium dioxide. The Hubbard parameter U and van der Waals corrections have been used to describe the strongly correlated electronic behavior of uranium 5f electrons and the weak interactions of rare gases, respectively. The results indicate that the energetics of rare gases depend significantly on the incorporation sites and on the atomic properties such as atomic radius. All rare gases considered here are energetically unfavorable at the three incorporation sites. However, rare gases exhibit significant binding ability to both U and O vacancies. The main trends of relative stability of rare gases generally reflect a size effect: the rare gases become more unstable with increasing atomic number. Electronic structures of these systems containing rare gases also exhibit general trends in their relative stability and charge-transfer character.

Carbon dioxide sorption in a nanoporous octahedral molecular sieve

We have performed first-principles density functional theory calculations, incorporated with van der Waals interactions, to study CO2 adsorption and diffusion in nanoporous solid—OMS-2 (Octahedral Molecular Sieve). We found the charge, type, and mobility of a cation, accommodated in a porous OMS-2 material for structural stability, can affect not only the OMS-2 structural features but also CO2 sorption performance. This paper targets K+, Na+, and Ba2+ cations. First-principles energetics and electronic structure calculations indicate that Ba2+ has the strongest interaction with the OMS-2 porous surface due to valence electrons donation to the OMS-2 and molecular orbital hybridization. However, the Ba-doped OMS-2 has the worst CO2 uptake capacity. We also found evidence of sorption hysteresis in the K- and Na-doped OMS-2 materials.

First-Principles Energetics of Some Nonmetallic Impurity Atoms in Plutonium Dioxide

The energetics of some typical nonmetallic impurity atoms (H, He, B, C, N, O, F, Ne, Cl, Ar, Kr, and Xe) in PuO2 are calculated using a projector augmented-wave method under the framework of density functional theory. The Hubbard parameter U and van der Waals corrections are used to describe the strongly correlated electronic behavior of f electrons in Pu and weak interactions of rare gases, respectively. Three incorporation sites of impurity atoms, that is, octahedral interstitial, O vacancy, and Pu vacancy sites, are considered. The results indicate that the energetics of impurity atoms depend significantly on the incorporation sites and on atomic properties such as atomic radius and electron affinity. Almost all impurity atoms considered here are energetically unfavorable at the three incorporation sites, with the exception of the F atom at the octahedral interstitial and O vacancy sites. The trends of incorporation energies of rare gas atoms generally reflect a size effect. Furthermore, charge-transfer analysis reveals that the valence electrons can be polarized more easily with increasing atomic number of rare gas elements. Finally, electronic structures of these systems containing impurity atoms also exhibit general trends in their relative stability and chemical bonding character.

First-principles energetics of water clusters and ice: A many-body analysis

Standard forms of density-functional theory (DFT) have good predictive power for many materials, but are not yet fully satisfactory for cluster, solid, and liquid forms of water. Recent work has stressed the importance of DFT errors in describing dispersion, but we note that errors in other parts of the energy may also contribute. We obtain information about the nature of DFT errors by using a many-body separation of the total energy into its 1-body, 2-body, and beyond-2-body components to analyze the deficiencies of the popular PBE and BLYP approximations for the energetics of water clusters and ice structures. The errors of these approximations are computed by using accurate benchmark energies from the coupled-cluster technique of molecular quantum chemistry and from quantum Monte Carlo calculations. The systems studied are isomers of the water hexamer cluster, the crystal structures Ih, II, XV, and VIII of ice, and two clusters extracted from ice VIII. For the binding energies of these systems, we use the machine-learning technique of Gaussian Approximation Potentials to correct successively for 1-body and 2-body errors of the DFT approximations. We find that even after correction for these errors, substantial beyond-2-body errors remain. The characteristics of the 2-body and beyond-2-body errors of PBE are completely different from those of BLYP, but the errors of both approximations disfavor the close approach of non-hydrogen-bonded monomers. We note the possible relevance of our findings to the understanding of liquid water.

Structure and energetics of the coherent interface between the θ′ precipitate phase and aluminium in Al–Cu

The θ′ phase is a well-known intermediate phase that forms as part of the classic decomposition sequence of an aluminium–copper solid solution. It is also the archetypal precipitate phase of high-strength aluminium alloys. Here we use atomic-resolution electron microscopy imaging to reveal that the coherent interface of θ′ with aluminium can adopt a structure that differs from that previously assumed based on the bulk crystal structures. These observations, combined with first-principles energetics calculations for realistic models of embedded θ′ precipitates, show that this new interfacial structure constitutes an intermediate state in the atomistics of precipitate thickening. These findings will require revision of theoretical models of θ′ precipitate growth and nucleation.

Type II detwinning in NiTi

Shape memory effect in nickel-titanium (NiTi) alloys depends on phase transformation between two phases and growth of twin variants in martensite called detwinning. An outstanding issue regarding detwinning in NiTi has been the lack of fundamental understanding of its mechanism at the atomistic level. The present article resolves this issue via first-principles energetics calculations of twin nucleation and growth. Our results based on ion relaxation and valence charge distribution point to a distinct energy barrier during detwinning process and the mechanism is mediated by a complex conjunction of shear and shuffle.

First-principles energetics of hydrogen traps in α-Fe: Point defects

We performed a series of density functional theory (DFT) calculations to quantify the binding energy of hydrogen to a number of point defects in body-centered cubic Fe, e.g. vacancies, interstitial carbon and substitutional solutes. We found the following: (i) Vacancies are the strongest H trap, with a binding energy of 0.57 eV. (ii) The binding energy of H to C is 0.09 eV. (iii) Most substitutional solutes (except Si, Cr, Mn, Co, and Mo) trap H with positive binding energies up to 0.25 eV. The maximum H binding energy for Si, Cr, Mn, Co and Mo is essentially zero, meaning they do not interact with H. (iv) The H–substitutional solute binding energies are roughly correlated with the solute atom’s electronegativity and size. (v) The presence of a solute atom near a vacancy does not affect hydrogen binding to the vacancy provided the solute atom is not significantly larger than Fe.

First-principles density functional calculations for Mg alloys: A tool to aid in alloy development

In studying the thermodynamics and phase stability of Mg alloys, one is often confronted with the lack of accurate, quantitative experimental data. This deficiency can be partially rectified via first-principles calculations based on density functional theory. In this paper, we will illustrate the utility of first-principles energetics for Mg alloys using three case studies (i) formation energies of ordered/disordered solid phases; (ii) solute–vacancy binding energies in Mg; (iii) point defect formation energies of β-Mg 17Al 12. These first-principles calculations can provide highly accurate thermodynamic and kinetic information for Mg alloys.

First-principles energetics and structural relaxation of antigorite

We have investigated the antigorite m = 17 structure [Mg 48 Si 34 O 85 (OH) 62 ] by density functional theory (DFT), with the aim to probe the method on such a large and low symmetry (Pm) system. We found a satisfactory match with the experiments using both LDA and GGA approximations to exchangecorrelation, although the former performs slightly better here. Predicted cell constants are within 0.3% for LDA, and 0.5% for GGA, of the experimental values. Average atomic displacements after relaxation are within ~0.06 Å. All the fine structural details of antigorite are reproduced: apical Si-O bonds shorter than basal Si-O bonds; external Mg-O distances shorter than internal Mg-O distances; pronounced tetrahedral ditrigonal distortion. Where palpably biased bond distances were present in the experimental data because of disorder, theoretical methods promptly recover the structure into more reliable bond geometry. These findings let one envisage the employment of DFT calculations as a promising tool for refining or validating complex structures, whenever the experiments suffer of limitations due to the poor quality of the material being investigated. As an additional benefit, we also compare the total energies of two competing models for the m = 17 antigorite structure, the one refined by Capitani and Mellini (2004) and that proposed by Dódony et al. (2002). We found the former more stable, both as published (ΔE = 1.1 kJ/mol·atom -1 ) and after full cell relaxation at constant volume (ΔE = 0.5 kJ/mol·atom -1 ).

Thermodynamic modeling of the Cr–Pt binary system using the cluster/site approximation coupling with first-principles energetics calculation

A thermodynamic description of Cr–Pt was developed in this study by combining first-principles calculation with the Calphad approach. The 0 K enthalpies of formation of Cr 3Pt (A15), the ordered Cr x Pt 1− x face-centered cubic (fcc) L1 2 compounds at x(Pt) = 0.25 and 0.75 and L1 0 compound at x(Pt) = 0.5 were obtained from first-principles calculation. They were used for optimizing the Gibbs energies of the corresponding phases in the Cr–Pt system. The cluster/site approximation (CSA) model was employed to model the phases in the fcc family: ordered L1 2, L1 0 and disordered A1 (they are also referred to as the three states of fcc phase). The phase boundaries and thermodynamic properties calculated from the current thermodynamic description are in good agreement with the experimental data as well as the first-principles calculation. The stability of the three fcc states and the order–disorder transition of L1 2/A1 and L1 0/A1 are reasonably described by the current description.