S.P. Chen

Accurate Interatomic Potentials for Ni, Al and Ni3Al

To obtain meaningful results from atomistic simulations of materials, the interatomic potentials must be capable of reproducing the thermodynamic properties of the system of interest. Pairwise potentials have known deficiencies that make them unsuitable for quantitative investigations of defective regions such as crack tips and free surfaces. Daw and Baskes [Phys. Rev. B 29, 6443 (1984)] have shown that including a local “volume” term for each atom gives the necessary many-body character without the severe computational dependence of explicit n-body potential terms. Using a similar approach, we have fit an interatomic potential to the Ni 3 Al alloy system. This potential can treat diatomic Ni 2 , diatomic Al 2 , fcc Ni, fcc Al and L1 2 Ni 3 Al on an equal footing. Details of the fitting procedure are presented, along with the calculation of some properties not included in the fit.

Many-body central force potentials and properties of grain boundaries in NiAl

Abstract Empirical many-body central force potentials of the Finnis-Sinclair type have been constructed for B2 NiAl by fitting a number of equilibrium properties of this alloy and reproducing the asymmetric behaviour of constitutional point defects in off-stoichiometric NiAl. At the same time these potentials ensure the structural and mechanical stability of the B2 lattice and reproduce quite adequately the equilibrium properties of Ni3Al. Using these potentials, grain boundaries in NiAl have been studied by computer simulation. It was found that in stoichiometric NiAl alloy boundaries with a surplus of aluminium have appreciably lower cohesive strength than the stoichiometric boundaries or boundaries with a surplus of nickel. From the structural point of view, boundaries with a surplus of aluminium possess the largest expansions and large ‘holes’ usually occur in the boundary regions. On the other hand, boundaries with the stoichiometric configuration or with a surplus of nickel have more compact structures. The interaction of the antisite defects and vacancies with grain boundaries was also studied and segregation of nickel and aluminium in off-stoichiometric alloys discussed with the help of these results.

Stacking fault energy of the NbCr2 Laves phase

Abstract A total energy study has been performed on the NbCr2 Laves phase using first-principles electronic structure calculations based on the full-potential linear muffin-tin orbital method. For the two Laves phase structures, C15 and C14, cohesive energies and heats of formation were obtained. A method was developed to calculate the stacking fault energy, γ, in C15 Laves phases, using only two quantities: the cohesive energy difference between C15 and C14 Laves phases, and the C15 lattice constant. For C15 NbCr2, the calculated stacking fault energy is 90mJ m−2. The calculated result is in good agreement with an experimental result deduced from an extended dislocation node.

Phase stability and elasticity of C15 transition-metal intermetallic compounds

First-principle quantum mechanical calculations based on the local-density-functional theory have been performed to study the electronic, physical and metallurgical properties of C15 intermetallics MV{sub 2} (M = Zr, Hf, or Ta). The elastic constants of C15 HfV{sub 2} + Nb were measured by the resonant ultrasound spectroscopy technique. The phase stability of C15 HfV{sub 2} + Nb was studied by specific heat measurements and by transmission electron microscopy in a low temperature specimen holder. The total energies and their lattice volume dependence were used to obtain the equilibrium lattice constants and bulk modulus. The band structures at the X-point near the Fermi level were employed to understand the anomalous temperature dependence of shear modulus of the C15 intermetallics. It was found that the double degeneracy with a linear dispersion relation of electronic levels at the X-point near the Fermi surface is mainly responsible for the C15 anomalous elasticity. The density of states at the Fermi level, N(E{sub F}), and the Fermi surface geometry were obtained to understand the low temperature phase instability of C15 HfV{sub 2} and ZrV{sub 2} and the stability of C15 TaV{sub 2}. It was proposed that the large N(E{sub F}) and Fermi surface nesting are the physical reasons for the structural instability of the C15 HfV{sub 2} and ZrV{sub 2} at low temperatures. The relation between anomalous elasticity and structural instability of C15 HfV{sub 2} and ZrV{sub 2} is also discussed.

Modelling the growth of NiAl epilayer on zinc-blende substrate

Abstract Interface structure and epitaxial growth of NiAl on III-V or 11-VI zinc-blende substrate have been studied by using the embedded atom method. Simulation showed that the {001} NiAl on {001} zinc-blende substrate forms a stable epitaxy only if a Ni monolayer is predeposited. A new model for the NiAl-zinc-blende substrate interface is proposed. The investigation of d-spacing changes in the intermetallic overlayer showed anomalies near the interface and free surface, and this behaviour is expected to affect the properties of the epilayer.

Theoretical studies of Ni/sub 3/Al and NiAl with impurities

Intermetallic compound has been extensively studied because of their superior properties in strength, low creep rate, and high melting point. But most of the systems have room temperature ductility problems, like Ll/sub 2/ and B2 compounds. Both Ll/sub 2/ Ni/sub 3/Al and B2 NiAl exhibit intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising material. In an effort trying to understand the fracture mechanism, we have used embedded atom potentials to study the properties of Ni/sub 3/Al and NiAl. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni/sub 3/Al and NiAl. 22 refs., 2 figs.

AB Initio Calculations of Point Defects in Silicon

The authors use the ab initio plane wave pseudopotential method and the density functional theory (DFT) to study the arsenic(As)-vacancy interactions in silicon. The detailed lattice distortions surrounding the As-vacancy defect and the energetics of As-vacancy reaction around the six-fold ring are investigated. The authors find that the As displaces its neighboring silicon atoms outward while the vacancy attracts its neighboring atoms inward. The binding energy and the formation energy of an As-vacancy pair are 1.21 eV and 2.37 eV, respectively. Once the vacancy and As binds together, the highest migration barrier for the whole complex is 1.19 eV, which is in good agreement with the experimental measurement of 1.07 eV. The calculated activation energy for the vacancy mediated diffusion of the neutral As in silicon is 3.56 eV. The nature of the binding between As and vacancy is explained from the lattice distortions introduced by the As-vacancy complex.

Elastic constants of a Laves phase compound: C15 NbCr{sub 2}

The single-crystal elastic constants of C15 NbCr{sub 2} have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropic elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. The implications of the results are discussed with regards to Poisson`s ratio and the direction dependence of Young`s modulus.

Atomistic study of energy and structure of surfaces in NiO

The structure and energy of surfaces in NiO have been studied by atomistic calculations employing short range Buckingham potentials fitted to properties of NiO. The polarizability of lattice anions is included by using the shell model. The results show that the surface energy depends strongly on surface orientation, and the 100 surface has the lowest energy. Surfaces with higher energy prefer to reconstruct into 100 facets to lower their energy and to stabilize their structure.

Interface relaxations in epitaxial NiAl intermetallics

Abstract Computer simulations using molecular statics and dynamics techniques, and employing the embedded atom method were utilized to study interfacial relaxations in NiAl intermetallics. The interface structure revealed anomolous atomic relaxations, which cannot be predicted from continuum elasticity. It was also observed that the coherency between NiAl and AlX (where X is a group V element) substrates substantially changes the strain perpendicular to the interface. These strains cannot be explained by linear elasticity. Surface relaxations were consistent with reported values.