Session E18 - Materials Theory and Computation for Industrial Problems I: Materials.
FOCUS session, Monday afternoon, March 12
Room 307-308, Washington State Convention Center

[E18.001] Dielectric Properties of ZrO_2

The low dielectric constant of silicon dioxide has been recognized as one of the fundamental limits on current SiO_2/Si semiconductor technology. Much recent effort on alternate gate dielectrics has been focused on high-\epsilon metal oxides, which might be used to provide physically thicker dielectrics, thus reducing leakage and enhancing gate capacitance. Zirconia (ZrO_2) is a promising candidate because of its good dielectric properties and thermal stability in direct contact with Si. Zirconia has three low-pressure structural phases, namely cubic, tetragonal, and monoclinic. We have studied the dielectric properties of these three polymorphs, including the lattice response as well as the purely electronic response, in the framework of a first-principles ultrasoft-pseudopotential approach.

[E18.002] Defect ordering in aliovalently-doped cubic zirconia from first principles

Defect ordering in aliovalently-doped cubic zirconia is studied using gradient corrected density-functional calculations. Intra- and inter-sublattice ordering interactions have been investigated for both the cation (Zr and dopant ions) and anion (oxygen ions and vacancies) species. For yttria-stabilized zirconia, the crystal structure of the experimentally identified ordered compound Zr_3Y_4O_12 (\delta) is established, and we predict the existence of new low-temperature zirconia-rich ordered phases. Anion vacancies repel each other at short separations, but show an energetic tendency to align as third nearest neighbors along \langle 111 \rangle directions. Doping with divalent (Be, Mg, Ca, Sr, Ba, Pt) and trivalent (Y, Sc, B, Al, Ga, In) oxides shows that anion vacancies prefer to be close to undersized cations, and away from oversized cations. With intermediately sized cations, the vacancies show no particular preference, and are thus less prone to be tied up by the dopants when traversing such oxides. This offers some simple insight into the high conductivity of Y and Sc doped zirconia, as well the recent success using, e.g., lanthanide oxides. Our calculations highlight In as a particularly promising dopant/co-dopant for high ionic conductivity.

[E18.003] Structural Form of Cubic BC_2N

Super-hard cubic boron-carbonitides (c-BC_2N) are studied with the use of the ab initio psuedopotential density functional method. Total energy, lattice constant, bulk and shear moduli are calculated for all possible c-BC_2N structures starting from the eight-atom cubic zinc-blende structured unit cell. The results obtained provide a plausible explanation for recent experimental observations as well as a possible path to the synthesis of the materials.

[E18.004] First-principles phonon dispersion and structural energetics of NiTi

NiTi undergoes a martensitic transition from a cubic B2 structure at high temperature to a low temperature monoclinic B19' structure. We have investigated the low energy surface of NiTi using first-principles total-energy and density-functional-perturbation theory calculations in the ultrasoft pseudopotential implementation of Baroni et al. For cubic NiTi, we have computed the phonon dispersion relation throughout the first Brillouin zone. The most important features are instability for a small range of wavevectors along the \Gamma-M line, and a more pronounced instability at M. In a four atom supercell, we have computed the energy for freezing in the M phonons, with and without strain relaxation. In addition, the symmetry-breaking strain which produces the observed ground state structure lowers the energy still further, as expected. Our results for the structural energetics for distortions of the cubic structure are summarized in a polynomial energy expression, which forms the foundation for effective Hamiltonian studies of the martensitic phase transition and properties of the high-temperature cubic structure.

[E18.005] Materials by Design: Cybersteels

A systems approach to the computational design of materials as dynamic hierarchical structures has been pioneered by the multi-institutional Steel Research Group centered at Northwestern University. Integrating process/structure/property/performance relations, the approach combines materials science, applied mechanics and quantum physics. Materials and design services based on the methods, tools and databases developed are being commercialized by QuesTek, employing QuesTek's Computational Materials Dynamics (CMD) system. Predictive control of nanostructures provides efficient strengthening, optimal control of transformation plasticity achieves record toughness, and quantum mechanical predictions provide Quantum Steels with enhanced intergranular cohesion and hydrogen resistance. A new class of ultrahard case-hardening steels with a 50surface hardness is entering a range of applications demanding high fatigue and wear resistance for greatly improved component life.

[E18.006] Automating First-Principles Phase Diagram Calculations

Although the formalism that allows the calculation of solid state phase diagrams from first principles is well established, its practical implementation remains a tedious process. In order to maximize the technological impact of this powerful tool, we have automated the process of performing such calculations. An overview of the parameters the algorithm requires as an input as well as its output is given. The accuracy of the method is assessed through the calculation of a variety of phase diagrams while its usefulness in technological applications is illustrated through the following example. It is well known that disordered metallic alloys exhibit hardening in the presence of short-range order. However, quantifying the level of the short-range order from experimental measurements is notoriously challenging, making it difficult to estimate the correlation between short-range order and hardness. In contrast, first-principles phase diagram calculations directly provide very accurate short-range order parameters at a fraction of the time and cost of accurate experimental determinations.

[E18.007] Incorporating first-principles energetics in computational thermodynamics approaches

Computational thermodynamic approaches (e.g., those pioneered by the CALPHAD community) have become a valuable tool in the calculation of complex, multicomponent phase equilibria often found in industrial alloys. These methods rely on databases of free energies, which are often obtained from an optimization process involving experimental thermodynamic data and phase diagrams. However, many phases of practical interest, such a precipitate phases, are metastable, and the metastable phase boundaries are often not well characterized. Consequently, these important phases are often absent from computational thermodynamics databases. We demonstrate that first-principles, density functional calculations provide a means to obtain thermodynamic functions of phases absent from current databases. We illustrate this approach with the famous metastable Cu-containing precipitate phases (GP zones and Al_2Cu-\theta') often found in age-hardened aluminum alloys. We discuss issues of the accuracy of first-principles energetics and the incorporation of absolute vs. relative energies into thermodynamics databases.

[E18.008] Chemical Order of HCP Ag_2Al: resolving the discrepancy between X-ray and TEM data.

Structural formation energies from electronic-structure calculations are used to construct an effective alloy Hamiltonian and then Monte Carlo simulations are performed to study chemical ordering in HCP bulk and nano-precipitated Ag-Al alloy. We find that the ground-state structure of bulk HCP Ag_2Al is that proposed by Neumann(J.P. Neumann, Acta Metall. 14), 505 (1966) from x-ray data, while the structure of small HCP Ag_2Al precipitates in Al matrix looks like those observed by TEM(J.M. Howe et al.), Phil. Mag. A 56, 31 (1997) (with Ag wetting the interface between matrix and precipitate). These results resolve a long-standing apparent discrepancy between X-ray diffuse scattering and TEM data and their interpretation.

[E18.009] First-principles investigation of the rhenium effect in Cr-based alloys.

While chromium exhibits attractive features like high melting temperature, high modulus to density ratio, and good oxidation resistance, the principle problem which limits its application is room temperature brittleness. The observed improvment of its ductility by alloying with Re was investigated with full-potential LMTO calculations including GGA corrections. The electronic structure and the ground-state properties (lattice parameters, elastic constants, cohesive energies) were calculated for bcc Cr and Cr-Re alloys within a wide range of Re concentration. The crystal parameters of assumed A15 and L1_2-type Cr_3Re compounds were optimized and their elecronic structure was compared with that for Cr-Re 25 % -alloy. The appearance of a metastable A15-phase for Cr-Re system was predicted and the conditions for its stabilization were analyzed. The mechanism of a possible ductilizing Re-effect appears to be the formation of small close-packed clusters or precipitates with A15-type structure which scavenge carbon impurities and avoid the formation of brittle chromium carbides.

[E18.010] Multiscale Modeling of Precipitate Microstructure: A Combined First-Principles/Cluster-Expansion/Phase-Field Approach

Although highly accurate for predicting alloy properties, current computational resources limit density functional calculations to relatively small systems with a few hundred atoms. Therefore, the direct application of these techniques to problems of alloy microstructure (involving billions of atoms or more) is clearly impossible. On the other hand, continuum phase-field models have been successful at accurately modeling alloy microstructure evolution; however, these methods are formulated in terms of empirical or difficult-to-measure thermodynamic input: 1) bulk free energies, 2) interfacial energies, and 3) elastic strain energies. Using the industrially-important problem of Al_2Cu-\theta' evolution in aluminum alloys, we demonstrate how first-principles atomistic calculations may be combined with a mixed-space cluster expansion approach to yield all of the necessary thermodynamic input for a phase field model. The incorporation of these energetic properties, obtained from atomistics, into a continuum microstructural model represents a real breakthrough in modeling capabilities: the first-ever ``first-principles'' model of precipitate microstructure evolution.

[E18.011] Effects of anisotropy on microstructural evolution in polycrystalline systems

The important role of anisotropy in determining the structure and properties of systems as diverse as IC interconnects, magnetic recording materials, and nanocrystalline structures has led to increased interest recently in understanding how interface energy and mobility anisotropy control the formation of structures at the micro and nano scales. Advances in computer simulation of microstructural evolution, which allow incorporation of grain boundary anisotropies and rigid body motion of grains are reviewed*. Recent extensions of these models show that these properties can dramatically change behavior of polycrystalline systems and can be successfully used for the design of materials with unique structural and physical properties.

*A. Kazaryan, Y. Wang and B.R. Patton, Scripta Mater.,vol.41,p.487 (1999); A. Kazaryan, Y. Wang S.A. Dregia and B.R. Patton, Phys. Rev. B, vol.61, p.14275

[E18.012] Simulations of Dry Sliding Friction in a Model Lennard-Jones Solid

We report on two-dimensional large-scale Molecular Dynamics simulations for Lennard-Jones solids, with initially flat commensurate and incommensurate interfaces, as a function of sliding velocity for various compressions. The high velocity behavior is similar for both commensurations, exhibiting an inverse power law for the frictional force when the velocity exceeds approximately one tenth the sound speed. At lower velocities, the frictional behavior differs, with the incommensurate case exhibiting a frictional force linear in velocity at low velocities followed by a maximum related to a phase transformation with a phase front propagating normally to the original interface. The frictional force for the commensurate interface approaches a finite value at low velocities.

[E18.013] Grain Boundary Dislocations and Sliding

We describe modeling of grain boundary sliding and partial dislocations at an aluminum sigma 3112 boundary. A striking result of these first principles calculations is that sliding the boundary results in a net ongoing translation of the boundary in a direction perpendicular to the boundary. First principles calculations were used to determine the two stable symmetry-equivalent configurations of this boundary which exist far from the partial grain boundary dislocations. First principles nudged elastic band calculations were then used to determine the Peierls barriers associated with displacing the grain boundary between these two symmetry-equivalent configurations. Finally, a generalized Peierls-Nabarro Model was used to obtain the partial dislocation widths from the calculated Peierl's barriers. The dislocation core widths are wide due to the small Peierls barriers. The theory is in good agreement with new TEM observations of dislocation widths presented here.

[E18.014] First principles study of dislocations in bcc metals

The microscopic origins of plasticity in bcc structural metals are far more complex and less well understood, as compared to that of fcc and hcp metals. BCC metals have many active slip planes and violate the Schmid law. Understanding the microscopic origins of plastic flow in these metals are therefore crucial. We present new results on the structure and dynamics of the <111> screw dislocation in tantalum, which controls the low temperature plastic behavior of the material. Results in larger cells confirm our previous ab initio result[1] which cast doubt on the prevailing broken symmetry model for the dislocation core used to explain the non-Schmidt plastic behavior of bcc metals. We shall present these results, and the results of a new study of the critical yield stress for plastic flow in bcc tantalum.

[1] ``Ab initio study of screw dislocations in Mo and Ta: a new picture of plasticity in bcc transition metals,'' by Sohrab Ismail-Beigi and T.A.~Arias, Physical Review Letters 84:7, 1499--1502 (14 February 2000). Preprint: http://xxx.lanl.gov/abs/cond-mat/9908110~. http://xxx.lanl.gov/abs/cond-mat/9908110

Part E of program listing