S. P. Chen

Investigation of the effects of boron on Ni sub 3 Al grain boundaries by atomistic simulations

A series of simulations has been performed on grain boundaries in Ni and Ni{sub 3}Al with and without boron doping using embedded atom-style potentials. A new procedure of obtaining reference data for boron related properties from electronic band structure calculations has been employed. Good agreement with existing experimental structural and energetic determinations was obtained. Boron is found to segregate more strongly to grain boundaries than to free surfaces. Adding boron to grain boundaries in Ni and Ni{sub 3}Al increases their cohesive strength and the work required to pull apart the boundary. This effect is much more dramatic for Ni-rich boundaries than for stoichiometric or Al-rich boundaries. In some Ni-rich cases, adding boron increases the cohesive strength of the boundary to such an extent that the boundaries become stronger than the bulk. Bulk Ni{sub 3}Al samples that are Ni-rich produce Ni-rich grain boundaries. The best cohesive properties of Ni{sub 3}Al grain boundaries are obtained when the boundary is Ni saturated and also with boron present. Boron and nickel are found to co-segregate to the grain boundaries.

Theoretical studies of grain boundaries in Ni3Al with boron or sulfur

It is well known that grain boundaries (GB) can have pronounced effects on the physical properties of materials (mechanical properties, corrosion resistance, fracture path, resistivity, etc.). Accordingly, a great deal of effort has been devoted to trying to understand the structure, energetics, and properties of grain boundaries. Significant experimental and theoretical progress has been made in understanding grain boundaries in pure systems, while the understanding of grain boundaries in alloy systems is much less developed. Also the mechanical properties of the grain boundaries are not well understood. In the present report, the authors summarize recent results on atomistic simulations of grain boundaries in the Ll/sub 2/ ordered alloy Ni/sub 3/Al. Understanding grain boundaries in this material is of particular importance since intergranular fracture limits the applicability of this otherwise promising material. To put these results into perspective, additional simulations were performed on grain boundaries in pure Ni and Al. Many features of grain boundaries in the ordered alloy may be understood in terms of the results on pure Ni and Al grain boundaries. The authors also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni and Ni/sub 3/Al.

Theoretical studies of ultrathin film-induced faceting on W(111) surfaces

Abstract We have used local volume (or EAM) potentials to study the pyramidal faceting (or reconstruction) of a W(111) surface induced by face center cubic (fcc) metals Pd, Pt, Au, and a body center cubic (bcc) metal Mo. We found that the surface-energy differences of (211) and (111) surfaces of bcc W increases as one or few monolayers of Pd, Pt, Au, and Mo films are deposited. We found that the lateral relaxation which is allowed on the (211) surface further increases the surface energy anisotropy as the thickness of the fcc metal film increases. Our calculated results are consistent with the argument that the surface energy anisotropy is the driving force for the faceting, but do not rule out three-dimensional (3D) island growth as another possible mechanism for the (211) faceting. We also found that there is a possible bilayer growth mode in W(211) surfaces with Pt and Pd films.

Reconstruction of the (310), (210) and (110) surfaces in fcc metals

Abstract We have investigated the 1 × 2 missing-row reconstruction for the (310), (210), and (110) surfaces of Ni, Pd, Pt, Cu, Ag, Au, and Al using embedded atom potentials. The embedded method (EAM) predicts that on the (310) and (110) surfaces, only Au and Pt reconstruct, but that all seven metals reconstruct on the (210) face. We also introduce a simple model, based on the second moment of the missing bond distribution, that predicts 1 × 2 and possibly higher-order reconstruction for all (210) surfaces and for (110) and (310) surfaces of metals with a large Cauchy pressure (C12–C44). On the (110) surfaces, the EAM and the simple model predictions agree with experiment, but not on the (210) surface in the one case studied.

Stacking-fault energy and yield stress asymmetry in molybdenum disilicide

Abstract Stacking-fault energies in MoSi2 due to shear along ⟨331⟩ have been calculated by ab-initio and modified embedded-atom method (MEAM) calculations. The results are used to investigate the configurations of ½⟨331⟩ dislocations and their mobility. Shear of ⅙⟨331⟩ in the {103} plane of MoSi2 produces an antiphase boundary (APB) whose geometry. called APB(1), is different from that produced by ⅙⟨331⟩ in the opposite direction, APB(2). MEAM calculations show that APB(1) is stable while both types of calculation show that APB(2) is unstable. Both ab-initio and MEAM calculations show that there is a stable fault close to APB(2) with a displacement of about ⅛⟨331⟩ in the same direction. The calculations also show that there is a stable fault in the {110} plane with a displacement of ¼⟨111⟩. The identical fault is produced by a shear of ¼⟨331⟩. There is good agreement between the fault energies calculated by the two methods and also with the experimental value (200-370 mJ m−2). The agreement between the calculated fault energies in the {013} plane is not so good. One factor is that the relaxation procedures are different; the MEAM method has more flexibility as well as a larger number of atoms, possibly explaining why it gives lower stable fault energies. The {103} planes have an unusual five-layer ABCDE stacking sequence with successive planes offset by ⅕⟨301⟩. Shear of 1/10⟨351⟩ in the correct direction gives a low-energy fault with Mo atoms surrounded by the correct number (ten) of Si nearest neighbours. This vector is close to the ⅛⟨331⟩ shear that produces a stable fault and may explain its low calculated energy. Various dissociated configurations of ½⟨331⟩ dislocations are considered on the basis of ⅙⟨331⟩, ⅛⟨331⟩, ¼⟨331⟩ and 1/10⟨351⟩ partials. All can have asymmetrical arrangements which will respond differently to the direction of the applied stress, explaining why {103}⟨331⟩ slip is much easier for crystals compressed along [100] than along [001].

Compositional and physical changes on perovskite crystal surfaces

The surface composition of BaTiO{sub 3}, SrTiO{sub 3}, and CaTiO{sub 3} perovskite (100) surface is determined by shell-model calculations. The TiO{sub 2}-terminated surface is energetically favorable for BaTiO{sub 3} and SrTiO{sub 3}, which is consistent with experimental observations on SrTiO{sub 3}. On the other hand, the CaO-terminated surface is preferred for CaTiO{sub 3} where Ca{sup 2+} is the smallest 2+ cation in these titanates. Ions on (100) surface rumple and induce surface dipoles. The surface ferroelectric polarization stabilizes the surface and changes its sign as the surface composition changes from TiO{sub 2} to CaO. This phenomenon is expected to affect the stability and properties of epitaxial films on perovskite substrates.

Anomalous relaxations of (0001) and (1010) surfaces in hcp metals

Abstract We have used local-volume potentials (LVP) to study the (0001) and (10 1 0) surfaces of 13 hcp metals. The calculated structures and energies of (0001) and (1010) surfaces show some agreement with experiment, particularly as to the sign of the relaxation. All the metals show deep changes in interlayer spacings. On (0001) surfaces Co, Gd, Hf, Mg, Re, Ru, Ti and Tl show first layer-spacing contraction while Be, Dy, Er, Sc and Zr show anomalous first layer-spacing expansion. Similarly, on (10 1 0) surfaces Co, Dy, Gd, Mg, Re, Ru, Ti and Tl show first layer-spacing contraction while Be, Er, Hf, Sc and Zr show anomalous first layer-spacing expansions. The prediction of first layer-spacing expansion of the Be(0001) surface agrees with a recent low-energy electron diffraction (LEED) experiment. The results on hcp (0001) surfaces contrast with relaxation on fcc (111) surfaces, also reported here, where the model always give contractions.

Structural and dynamical behavior of Al trimer on Al(111) surface

Abstract Trimer is the smallest cluster that can have a one-dimensional or two-dimensional structure on surfaces, and it can diffuse and transform between these structures. Using first-principles density-functional theory (DFT) calculations, the structural and dynamical behaviors of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has three different kinds of structure conformations (groups with similar configurations): close-packed (compact) triangular trimers, non-compact triangular trimers, and linear trimers. The close-packed triangular trimers are more stable than the non-compact triangular trimers and the linear trimers, while most of the non-compact triangular trimers are as stable as the linear trimers. For the dynamics of Al trimer on Al(111) surface, there are three different kinds of diffusion mechanisms: (1) concerted translations and rotation of compact triangular trimers (the highest energy barrier by DFT calculation, Ed=0.24xa0eV); (2) back-and-forth transformation between compact triangular trimers and linear trimers (Ed=0.21xa0eV); and (3) translation of linear trimers (Ed=0.28xa0eV). Among these different mechanisms with similar height of diffusion barriers, the concerted translations of the compact triangular trimers have the longest displacement of the center of mass in the least steps. Therefore, we expect the long-range diffusion of Al trimer on Al(111) surface is dominated by the concerted motion process of the compact triangular trimers. The concerted translations and concerted rotations of Al trimer on Al(111) surface have also been observed in the molecular dynamics simulations using the embedded atom method.

Z methodology for phase diagram studies: platinum and tantalum as examples

The Z methodology is a novel technique for phase diagram studies. It combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. In the direct Z method, the solid phases along the melting curve are determined by comparing the solid-liquid equilibrium boundaries of candidate crystal structures. The inverse Z method involves quenching the liquid into the most stable solid phase at various temperatures and pressures to locate a solid-solid boundary. The direct and inverse Z methods in conjunction with the VASP ab initio molecular dynamics package are used to investigate the phase diagrams of tantalum and platinum. We compare our results to the most recent experimental data.

Atomistic Modeling of the Negative Thermal Expansion in δ- Plutonium Based on the Two-State Description

The δ phase of plutonium with the fcc structure exhibits an unusual negative thermal expansion (NTE) over its narrow temperature range of stability, 593–736 K. An accurate description of the anomalous high-temperature volume effect of plutonium goes beyond the current capability of electronic-structure calculations. We propose an atomistic scheme to model the thermodynamic properties of δ-Pu based on the two-state model of Weiss for the Invar alloys, inspired by the simple free-energy analysis previously conducted by Lawson et al. The two-state mechanism is incorporated into the atomistic description of a many-body interacting system. Two modified embedded atom method potentials are employed to represent the binding energies of two competing electronic states in δ-Pu. We demonstrate how the NTE takes place in δ-Pu by means of Monte Carlo simulations implemented with the two-state mechanism.