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The composite β ′/ β ″ precipitates rather than well-defined β ′ phase form in trace-Sn containing Al-Mg-Si alloy peak-aged at 250 °C, leading to considerable morphological refinement. The preferential occupation of 2~6 adjacent Si sites of the tiny β ′ phase by Sn and typical β ″ substructure units covering the precipitates are frequently observed. The substructures grow upon further ageing into differently oriented β ″ with 2~3 complete structural unit cells. It is inferred that the β ′ (Mg 9 Si 5-x Sn x ) forms initially on the Sn/vacancy complexes and then acts as heterogeneous nucleus for β ″. These findings demonstrate Sn could fundamentally alter the pre−/precipitation behaviors in Al-Mg-Si alloys. [ABSTRACT FROM AUTHOR]

The VH 1 defect in diamond (one hydrogen atom saturating one of the four dangling bonds of the carbon atoms surrounding a vacancy) is investigated at the quantum mechanical level by using the periodic supercell approach, an all electron Gaussian type basis set, “hybrid” functionals and two supercells. Both spin states, characterized by S z = 1/2 and S z = 3/2 and resulting from the unpaired electrons on the three unsaturated carbon atoms around the vacancy, have been investigated. The doublet state VH 1 d turns out to be more stable by 0.34 eV than the quadruplet VH 1 q . The band structure, charge and spin density distributions are analyzed. The two states present very different IR spectra, with peaks that can be used for their characterization. In particular the CH stretching peak can vary by as much as 400 cm −1 : from about 2800 cm −1 for VH 1 q to about 3200 cm −1 for VH 1 d in the harmonic approximation. Anharmonicity is quite important but very different in the two cases: for the C-H stretching, about 330 cm −1 for VH 1 q and only 190 cm −1 for VH 1 d , so that the final anharmonic wavenumbers differ by as much as 500 cm −1 . [ABSTRACT FROM AUTHOR]

Ab initio calculations of electronic, magnetic and optical properties of defected (beryllium (Be) doped or void induced) buckled free standing (FS) germanene have been explored. Concentrations of doping as well as vacancy (keeping a fixed low amount of Be) are increased thoroughly, in order to study the modifications of different physical properties critically. Our study reveals that, incorporation of doping and void destroy Dirac cone in band structure of germanene. Finite bandgap for the requirement of field effect transistor (FET) applications is obtained in case of semiconducting configuration with 15.62% doping concentration. Magnetism is also induced for doping of Be with high concentration only, which is supported from projected density of states (PDOS) and charge density analysis. Anisotropic effects are prominent in optical properties like dielectric functions, absorption spectra, reflectivity and its modulation and conductivity. Static real part of dielectric constant increases linearly with increase in doping concentration but decreases with increase in vacancy concentration considering parallel polarization of electro-magnetic (EM) wave. Predicted computational results of plasma frequencies are in well agreement with expected analytical data. Peak corresponding to maximum intensity of electron energy loss spectra (EELS) appears at the position of plasma frequency in case of every structure. We expect, this study may help for better understanding of next generation germanene based nano-technology. [ABSTRACT FROM AUTHOR]

Point defects, particularly vacancies, exist abundantly in the structure of a variety of clay minerals (e.g., kaolinite). Such defects significantly alter the physical/chemical characteristics of the minerals. Dissolution of kaolinite in an alkali medium is a critical step in the geopolymerization process and it determines the properties of the final product. In this work, a series of molecular dynamics (MD) simulations were carried out in the isothermal-isobaric (NPT) ensemble at 298 K and 1 atm to study the influence of structural vacancies (Al and Si vacancies in particular) on the interaction/dissolution of the two basal surfaces of kaolinite (partially deprotonated octahedral and tetrahedral surfaces) exposed to alkali media. Two different alkali media were used. One contained sodium cation (Na + ) only while the other potassium cation (K + ) and their concentrations were 3 M and 5 M. The MD results showed that regardless of the type of cation and cation concentration, Al vacancies on the octahedral surface promoted the dissolution of Al into the solution compared to the surface without Al vacancies. However, there existed a vacancy concentration (2 Al vacancies per 576 Al atoms) at which the dissolution amount was the maximum and above which the dissolution decreased with increasing Al vacancy concentration. The presence of Si vacancies in the tetrahedral surface did not show significant effect on the dissociation of Si. Radial distribution function (RDF) analyses indicated that the degree of crystallinity of the octahedral surface decreased as the number of Al vacancies increased but this was not the case for the tetrahedral surface. [ABSTRACT FROM AUTHOR]

The effects of defects on the fracture resistance of materials have attracted considerable attention recently. In the present work, the vacancy effects on the spallation in single-crystalline Ni are studied by nonequilibrium molecular dynamics simulations. The vacancy concentration ranges from 0% to 2.0%, and the spallation in shock wave loading along three low-index directions ([0 0 1], [1 1 0], and [1 1 1]) is investigated. We found that vacancies provide the sites of nucleation for compression-induce plasticity, and tension stress-induced plasticity plays the key role in void nucleation. Along the [0 0 1] direction, the degree of spall damage does not increase with the increase in vacancy concentration; however, along the [1 1 0] and [1 1 1] directions, it decreases with the increase in vacancy concentration when the vacancy concentration is higher than the threshold value. [ABSTRACT FROM AUTHOR]

The core structures and mobility of ⟨c⟩ dislocations in magnesium were predicted using both density functional theory and molecular dynamics simulations. The pure edge and screw cores are compact at 0 K. With increasing temperature up to 700 K, the edge dislocation dissociates into two ⟨c⟩/2 partials on the basal plane, but remains immobile. The screw dislocation remains compact and continuously cross-slips between the three prismatic planes. At room temperature, the screw dislocation only glides after emitting a vacancy at 420 MPa. The Peierls stress of the dislocation after vacancy emission is 50 MPa. [ABSTRACT FROM AUTHOR]

The hydrogen segregation at divacancies and impurity-vacancy clusters in nickel is studied using ab-initio calculations. Binding energy for up to 6 H atoms segregated at impurity-vacancy cluster and up to 10 atoms segregated at a divacancy is presented. The binding energy varies from −0.396 eV for H atom at the octahedral site “shared” by two vacancies to ≈ −0.27 eV for other sites. For clusters with more than 4 segregated H atoms the average binding energy is close to −0.3 eV. The effect of a soluble impurity on H segregation to a vacancy depends on the solute-vacancy interaction. The Mo atom is energetically repelled from vacancies, while Al and Li have weak binding energies. For the (Li, Al or Mo)-vacancy clusters the H segregation energy is increased for Li and decreased for Al and Mo. Li is predicted to have the largest effect on H segregation. The impurity effects and H binding correlate with variation of the defect formation volume. [ABSTRACT FROM AUTHOR]

The environment dependent interatomic potential (EDIP) including Ziegler–Biersack–Littermark (ZBL) interactions for close encounters is applied to cascades starting from a host atom and from an interstitial atom. We find the room temperature displacement threshold to be 25 eV, increasing to 30 eV at 900 K. The latter correlates well with the measured threshold for vacancy production. Additionally, divacancy production is found to occur, including interlayer divacancies from around 60 eV. The data suggest a new, continuous damage function applies, where the threshold region depends on the square root of the primary knock-on atom (PKA) energy in excess of the threshold, evolving to a linear dependence on PKA energy. [ABSTRACT FROM AUTHOR]

By using molecular dynamics and cluster dynamics simulations, we probed the role of hydrogen-vacancy complexes on nucleation and growth of proto nano-voids upon dislocation plasticity in α-Fe. Our atomistic simulations reveal that, unlike a lattice vacancy, a hydrogen-vacancy complex is not absorbed by dislocations sweeping through the lattice. Additionally, this complex has lower lattice diffusivity; therefore, it has a lower probability of encountering and being absorbed by various lattice sinks. Hence, it can exist metastably for a rather long time. Our large-scale molecular dynamics simulations show that when metals undergo plastic deformation in the presence of hydrogen at low homologous temperatures, the mechanically driven out-of-equilibrium dislocation processes can produce extremely high concentrations of hydrogen-vacancy complex (10 −5 ∼ 10 −3 ). Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. The current work provides one possible route for the experimentally observed nano-void formation in hydrogen embrittlement of steels and bridges atomic-scale events and damage with macroscopic failure. [ABSTRACT FROM AUTHOR]

The behavior of atomic hydrogen in osmium dioxide and ruthenium dioxide is investigated through ab initio calculations using density functional theory. Interstitial hydrogen behaves as a donor in both metal oxides and only has one stable interstitial position, bonding with an O atom to form an OH O defect. The rates of diffusion parallel to the c -axis are estimated to be 3.95 × 10 − 7 exp(−0.793/ k B T ) m 2 /s and 3.64 × 10 − 7 exp(−0.659/ k B T ) m 2 /s in OsO 2 and RuO 2 , respectively. The rates of diffusion perpendicular to the c -axis are estimated to be 1.80 × 10 − 6 exp(−0.941/ k B T ) m 2 /s and 1.10 × 10 − 6 exp(−1.21/ k B T ) m 2 /s in OsO 2 and RuO 2 , respectively. Additionally, the binding energy of multiple H atoms in (O, Os, Ru) vacancies have been calculated. High binding energies of hydrogen in metal vacancies result in multiple H atoms associating with a single metal vacancy. In RuO 2 , the binding energy of multiple H atoms outweighs the formation energy of a single Ru vacancy as well as that of a Ru–O divacancy. [ABSTRACT FROM AUTHOR]

A theoretical model is suggested to determine the critical conditions for generation of circular prismatic misfit dislocation loops in hollow core–shell nanoparticles. Based on a strict solution of the linear elasticity boundary-value problem for a circular prismatic dislocation loop in a free-surface shell, we examine the loop formation energy in the nanoparticle and show a potential opportunity to fabricate hollow, coherently bonded, i.e. dislocation-free, core–shell nanoparticles by using thin-wall shells with inner-to-outer radii ratio >0.8 as supporting cores. [Copyright &y& Elsevier]

Abstract: Addition of trace amounts of Al, Si and Y into V–4Cr–4Ti alloy is beneficial for the mechanical properties under irradiation. It is thus important to investigate the influence of solute/trace elements on stabilities, energetics and diffusion behaviors of vacancy defects. We performed first-principles calculations to evaluate vacancy–solute/trace interaction inside dilute V–X (X =Ti, Cr, Al, Si, Y) and V–4Cr–4Ti–(Al, Si, Y) alloys. With addition of Si and Y, vacancy-based complexes tend to form near Ti–Si and Ti–Y pairs, while the effect of Al is negligible. Moreover, diffusion coefficients of solute/trace element in vanadium were derived using nine-frequency model. With high binding energy and low diffusion coefficient, Si atom is strongly attractive to vacancy in vanadium matrix. Our theoretical results suggest that the interactions between vacancy and solute/trace elements play some role in the evolution of microstructures inside vanadium alloys. [Copyright &y& Elsevier]

Highlights: [•] OVP site is the most stable site for CH3OH adsorption. [•] OVP site is not the most active site for CH3OH dissociation. [•] CH3OH on OVP vs. single OV is different from O2 on OVP vs. single OV. [•] Different 5-fold Ti sites show similar behaviors. [•] 5-Fold Ti sites are not suitable sites for CH3OH adsorption. [ABSTRACT FROM AUTHOR]