Per Söderlind

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.

Shear-induced anisotropic plastic flow from body-centred-cubic tantalum before melting

There are many structural and optical similarities between a liquid and a plastic flow. Thus, it is non-trivial to distinguish between them at high pressures and temperatures, and a detailed description of the transformation between these phenomena is crucial to our understanding of the melting of metals at high pressures. Here we report a shear-induced, partially disordered viscous plastic flow from body-centred-cubic tantalum under heating before it melts into a liquid. This thermally activated structural transformation produces a unique, one-dimensional structure analogous to a liquid crystal with the rheological characteristics of Bingham plastics. This mechanism is not specific to Ta and is expected to hold more generally for other metals. Remarkably, this transition is fully consistent with the previously reported anomalously low-temperature melting curve and thus offers a plausible resolution to a long-standing controversy about melting of metals under high pressures.

Fermi surface nesting and pre-martensitic softening in V and Nb at high pressures

First-principles total-energy calculations were performed for the trigonal shear elastic constant (C44) of body-centred cubic (bcc) V and Nb. A mechanical instability in C44 is found for V at pressures of ~2 Mbar which also shows a softening in Nb at pressures of ~0.5 Mbar. We argue that the pressure-induced shear instability (softening) of V (Nb) is due to the intra-band nesting of the Fermi surface.

On the electronic configuration in Pu: spectroscopy and theory

Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory, have been used to study the electron configuration in Pu. These methods suggest a 5fn occupation for Pu of 5≤n<6, with , contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission, x-ray absorption, and electron energy loss spectra. Instead, these spectra support the traditional conjecture of a 5f5 occupation in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.

Geometry and electronic structure of α-Pu: A theoretical study

The highly complex ground-state structure of Pu has been fully relaxed using first-principles forces and the obtained geometry compares very well with experimental data. Ab initio molecular-dynamics ~MD! simulations at 300 K further confirm the stability of the relaxed structure, and reveal the nature of the vibrations in this system. In addition, magnetic ordering in a-Pu is studied in detail, showing a strong tendency in Pu to develop magnetic moments that vary considerably in magnitude depending on the atomic position in the lattice, with an overall antiparallel alignment. These spin-polarized ~SP! calculations reproduce, the experimental bulk modulus of a-Pu. Combining the bulk modulus from the SP calculations, and the vibrational contribution to the thermal expansion from the MD simulations, we can account for the anomalous thermal expansion of a-Pu.

dhcp as a possible new ϵ′ phase of iron at high pressures and temperatures☆

Abstract Using in-situ X-ray diffraction and ab initio theoretical calculations, we have identified a possible new phase of iron, ϵ′-Fe, in the stability field of what was previously known for ϵ(hcp)- and γ(fcc)-Fe. The crystal structure of ϵ′-Fe is indexed to a dhcp structure with an ABAC stacking sequence. The ϵ′-phase exists at temperatures lower than the γ-phase and at pressures between 15 and 40 GPa, which differs from the stability field previously suggested for β-Fe above 40 GPa.

The phase diagram of cobalt at high pressure and temperature: the stability of -cobalt and new -cobalt

A metastable dhcp phase of cobalt, , has been discovered below 60 GPa by using in situ x-ray diffraction and a laser-heated diamond-anvil cell. The volume at 34 GPa is with = 3.190. First-principles theory shows that and are close in energy below 60 GPa, and that temperature and magnetic effects can make dhcp-Co more stable than hcp-Co. The -phase is found to be stable and quenchable over a wide range of pressure and temperature. New constraints for the phase diagram of Co are presented.

Thermodynamic modeling of chromium: strong and weak magnetic coupling

As chromium is a decisive ingredient for stainless steels, a reliable understanding of its thermodynamic properties is indispensable. Parameter-free first-principles methods have nowadays evolved to a state allowing such thermodynamic predictions. For materials such as Cr, however, the inclusion of magnetic entropy and higher order contributions such as anharmonic entropy is still a formidable task. Employing state-of-the-art ab initio molecular dynamics simulations and statistical concepts, we compute a set of thermodynamic properties based on quasiharmonic, anharmonic, electronic and magnetic free energy contributions from first principles. The magnetic contribution is modeled by an effective nearest-neighbor Heisenberg model, which itself is solved numerically exactly by means of a quantum Monte Carlo method. We investigate two different scenarios: a weak magnetic coupling scenario for Cr, as usually presumed in empirical thermodynamic models, turns out to be in clear disagreement with experimental observations. We show that instead a mixed Hamiltonian including weak and strong magnetic coupling provides a consistent picture with good agreement to experimental thermodynamic data.

An Alternative Model for Electron Correlation in Pu

Using a density functional theory based approach that treats the 5f electrons relativistically, a Pu electronic structure with zero net magnetic moment is obtained, where the 5f orbital and 5f spin moments cancel each other. By combining the spin and orbital specific densities of states with state, spin and polarization specific transition moments, it is possible to reconstruct the experimentally observed photoemission spectra from Pu. Extrapolating to a spin-resolving Fano configuration, it is shown how this would resolve the extant controversy over Pu electronic structure.