John A. Moriarty

The equation of state of platinum to 660 GPa (6.6 Mbar)

Platinum metal was shock compressed to 660 GPa using a two‐stage light‐gas gun to qualify this material as an ultrahigh‐pressure standard for both dynamic and static experiments. The shock velocity data are consistent with most of the previously measured low‐pressure data, and an overall linear us−up relationship is found over the range 32–660 GPa. As a part of this work, we have also extended the Hugoniot of the tantalum standard we use to 560 GPa; we have included these data into a new linear fit of the tantalum Hugoniot between 55–560 GPa. We also present the results of a first‐principles theoretical treatment of compressed platinum. The fcc phase is predicted to remain stable to beyond 550 GPa. In addition, we have calculated the 300‐K pressure‐volume isotherm and the Hugoniot. The latter is in excellent agreement with experimental results and qualifies the former to at least 10% accuracy.

Equation of state of Al, Cu, Mo, and Pb at shock pressures up to 2.4 TPa (24 Mbar)

Equation‐of‐state data and corresponding first‐principles theory for the metals Al, Cu, Mo, and Pb are reported over the shock pressure range 0.4–2.4 TPa (4–24 Mbar). Strong shock waves were generated by nuclear explosions and a two‐stage light‐gas gun. The experimental data occur in the hot liquid‐metal regime, where condensed‐matter theory applies but with unusually large thermal components to the equation of state.

Quantum-based atomistic simulation of materials properties in transition metals

We present an overview of recent work on quantum-based atomistic simulation of materials properties in transition metals performed in the Metals and Alloys Group at Lawrence Livermore National Laboratory. Central to much of this effort has been the development, from fundamental quantum mechanics, of robust many-body interatomic potentials for bcc transition metals via model generalized pseudopotential theory (MGPT), providing close linkage between ab?initio electronic-structure calculations and large-scale static and dynamic atomistic simulations. In the case of tantalum (Ta), accurate MGPT potentials have been so obtained that are applicable to structural, thermodynamic, defect, and mechanical properties over wide ranges of pressure and temperature. Successful application areas discussed include structural phase stability, equation of state, melting, rapid resolidification, high-pressure elastic moduli, ideal shear strength, vacancy and self-interstitial formation and migration, grain-boundary atomic structure, and dislocation core structure and mobility. A number of the simulated properties allow detailed validation of the Ta potentials through comparisons with experiment and/or parallel electronic-structure calculations. Elastic and dislocation properties provide direct input into higher-length-scale multiscale simulations of plasticity and strength. Corresponding effort has also been initiated on the multiscale materials modelling of fracture and failure. Here large-scale atomistic simulations and novel real-time characterization techniques are being used to study void nucleation, growth, interaction, and coalescence in series-end fcc transition metals. We have so investigated the microscopic mechanisms of void nucleation in polycrystalline copper (Cu), and void growth in single-crystal and polycrystalline Cu, undergoing triaxial expansion at a large, constant strain rate - a process central to the initial phase of dynamic fracture. The influence of pre-existing microstructure on the void growth has been characterized both for nucleation and for growth, and these processes are found to be in agreement with the general features of void distributions observed in experiment. We have also examined some of the microscopic mechanisms of plasticity associated with void growth.

Accurate atomistic simulation of (a/2) 〈111〉 screw dislocations and other defects in bcc tantalum

Abstract The fundamental atomic-level properties of (a/2)(111) screw dislocations and other defects in bcc Ta have been simulated by means of new quantum-based multi-ion interatomic potentials derived from the model generalized pseudopotential theory (MGPT). The potentials have been validated in detail using a combination of experimental data and ab-initio electronic structure calculations on ideal shear strength, vacancy and self-interstitial formation and migration energies, grain-boundary atomic structure and generalized stacking-fault energy (γ) surfaces. Robust and accurate two- and three-dimensional Greens function (GF) techniques have been used to relax dynamically the boundary forces during the dislocation simulations. The GF techniques have been implemented in combination with a spatial domain decomposition strategy, resulting in a parallel MGPT atomistic simulation code that increases computational performance by two orders of magnitude. Our dislocation simulations predict a degenerate core structure with threefold symmetry for Ta, but one that is nearly isotropic and only weakly polarized at ambient pressure. The degenerate nature of the core structure leads to possible antiphase defects (APDs) on the dislocation line as well as multiple possible dislocation kinks and kink pairs. The APD and kink energetics are elaborated in detail in the low-stress limit. In this limit, the calculated stress-dependent activation enthalpy for the lowest-energy kink pair agrees well with that currently used in mesoscale dislocation dynamics simulations to model the temperature-dependent single-crystal yield stress. In the high-stress limit, the calculated Peierls stress displays a strong orientation dependence under pure shear and uniaxial loading conditions, with an antitwinning-twinning ratio of 2.29 for pure shear {211}-(111) loading.

Total-energy-based prediction of a quasicrystal structure

Quasicrystals are metal alloys whose noncrystallographic symmetries challenge traditional methods of structure determination. We employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first-principles considerations. Our Monte Carlo simulations take as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition and use a limited amount of experimental structural data. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of t, the golden mean.

Accurate atomistic simulations of the Peierls barrier and kink-pair formation energy for 〈111〉 screw dislocations in bcc Mo☆

Abstract Using multi-ion MGPT interatomic potentials derived from first-principles generalized pseudopotential theory, we have performed accurate atomistic simulations on the energetics of dislocation motion in the bcc transition metal Mo. Our calculated results include the (110) and (211) generalized stacking fault (γ) energy surfaces, the Peierls stress required to move an ideal straight 〈111〉 screw dislocation, and the kink-pair formation energy for nonstraight screw dislocations. Many-body angular forces, which are accounted for in the present theory through explicit three- and four-ion potentials, are quantitatively important to such properties for the bcc transition metals. This is demonstrated explicitly through the calculated y surfaces, which are found to be 10–50% higher in energy than those obtained with pure radial-force models. The Peierls stress for an applied 〈111〉 {112} shear is computed to be about 0.025μ, where μ is the bulk shear modulus. For zero applied stress, stable kink-pairs are predicted to form for kink lengths greater than 4b , where b is the magnitude of the Burgers vector. For long kinks greater than 15b , the calculated asymptotic value of the kink-pair formation energy is 2.0 eV.

Atomistic simulations of dislocations and defects

This paper reviews selected recent research on the atomistic simulation of dislocation and defect properties of materials relevant to the multiscale modeling of plasticity and strength, with special emphasis on bcc metals and including work at extreme conditions. Current topics discussed include elasticity and ideal strength, dislocation structure and mobility, grain boundaries, point defects, and rapid resolidification, as well as noteworthy examples of research that directly impacts the issue of linking of length and/or time scales, as required in multiscale materials modeling. The work reviewed has been inspired by the recent international Workshop on Multiscale Modeling of Materials Strength and Failure held in October 2001 at Bodega Bay, California.

Atomic structure of the Σ5 (310)/[001] symmetric tilt grain boundary in molybdenum

Abstract Atomistic simulations offer an important route towards understanding and modeling materials behavior. Incorporating the essential physics into the models of interatomic interactions is increasingly difficult as materials with more complex electronic structures than f.c.c. transition metals are addressed. For b.c.c. metals, interatomic potentials have been developed that incorporate angularly dependent interactions to accommodate the physics of partially filled d-bands. A good test of these new models is to predict the structure of crystal defects and compare them with experimentally observed defect structures. To that end, the Σ5 (310)/[001] symmetric tilt grain boundary in Mo has been fabricated and characterized by HREM. The experimentally observed structure is found to agree with predictions based on atomistic simulations using angular-force interatomic potentials developed from model generalized pseudopotential theory (MGPT), but disagrees with predictions based on radial-force potentials, such as those obtained from the Finnis–Sinclair method or the embedded atom method (EAM).

Equation of state of beryllium at shock pressures of 0.4–1.1 TPa (4–11 Mbar)

High-pressure shock Hugoniot data were measured for Be using strong shock waves generated by underground nuclear explosions. These data and a preliminary theoretical analysis are reported.

High-pressure tailored compression: Controlled thermodynamic paths

We have recently carried out exploratory dynamic experiments where the samples were subjected to prescribed thermodynamic paths. In typical dynamic compression experiments, the samples are thermodynamically limited to the principal Hugoniot or quasi-isentrope. With recent developments in a functionally graded material impactor, we can prescribe and shape the applied pressure profile with similarly shaped, nonmonotonic impedance profile in the impactor. Previously inaccessible thermodynamic states beyond the quasi-isentropes and Hugoniot can now be reached in dynamic experiments with these impactors. In the light gas gun experiments on copper reported here, we recorded the particle velocities of the Cu–LiF interfaces and have employed hydrodynamic simulations to relate them to the thermodynamic phase diagram. Peak pressures for these experiments are on the order of megabars, and the time scales range from nanoseconds to several microseconds. The strain rates of these quasi-isentropic experiments are approx...