James F. Belak

Void nucleation and associated plasticity in dynamic fracture of polycrystalline copper: an atomistic simulation

We investigate the microscopic mechanisms of void growth in polycrystalline copper undergoing triaxial expansion at a large, constant strain rate: a process central to the initial phase of dynamic fracture. Void nucleation, growth, interaction and coalescence are studied using atomistic simulations. The influence of pre-existing microstructure on the void growth is characterized both for nucleation and for growth. These processes are found to be in agreement with the general features of void distributions observed in experiment. We also examine some of the microscopic mechanisms of plasticity associated with void growth.

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.

On the nucleation and growth of voids at high strain-rates

The nucleation and growth of voids at high strain-rate is studied in copper as a model face centered cubic (fcc) material using large scale molecular dynamics (MD) methods. After a brief introduction to dynamic fracture, results are presented for the homogeneous nucleation of voids in single crystal copper and the heterogeneous nucleation in nanoscale polycrystalline copper. The simulations suggest void growth occurs through anisotropic dislocation nucleation and emission in agreement with experiment and the observed anisotropy of the tensile flow stress in fcc crystals. A phenomenological model for the transition from intergranular to transgranular fracture at high strain-rate is presented.

Onset of void coalescence during dynamic fracture of ductile metals.

Molecular dynamics simulations in three-dimensional copper are performed to quantify the void coalescence process leading to fracture. The correlated growth of the voids during their linking is investigated both in terms of the onset of coalescence and the ensuing dynamical interactions through the rate of reduction of the distance between the voids and the directional growth of the voids. The critical intervoid ligament distance marking the onset of coalescence is shown to be approximately one void radius in both measures.

Three-dimensional molecular dynamics simulations of void coalescence during dynamic fracture of ductile metals

Void coalescence and interaction in dynamic fracture of ductile metals have been investigated using three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based on the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. The interaction of the voids is not reflected in the volumetric asymptotic growth rate of the voids, as demonstrated here. Finally, the practice of using a single void and periodic boundary conditions to study coalescence is examined critically and shown to produce results markedly different than the coalescence of a pair of isolated voids.

Void growth by dislocation-loop emission

Experimental results from spall tests on aluminum reveal the presence of a dense dislocation structure in an annulus around a void that grew under the tensile pulse when a shock wave was reflected at the free surface of the specimen. The proposition is that dislocation emission from the void surface under load is a viable mechanism for void growth. To understand void growth in the absence of diffusive effects, the interstitial-loop emission mechanism under tensile hydrostatic stress is investigated. First, the micromechanics of pile-up formation when interstitial loops are emitted from a void under applied macroscopic loading is reviewed. Demand for surface energy expenditure upon void-surface change is taken into consideration. It is demonstrated that in face-centered cubic metals loop emission from voids with a radius of ∼10 nm is indeed energetically possible in the hydrostatic stress environment generated by shock loading. On the other hand, the levels of hydrostatic stress prevalent in common structu...

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).

Symmetrical tilt grain boundaries in bcc transition metals: Comparison of semiempirical with ab-initio total-energy calculations

Abstract Five different semiempirical total-energy methods, provided in the literature and applicable for atomistic simulations of extended defects in bcc transition metals, are investigated in a comparative study. The comparison is made with recent theoretical ab-initio (local-density-functional theory) and experimental (high-resolution transmission electron microscopy) studies for the specific case of the Σ = 5, (310)[001] symmetrical tilt grain boundaries (Σ = 5 STGBs) in Nb and Mo. The considered semiempirical real-space approaches based on different approximations of the tight-binding and related methods are the Finnis–Sinclair central-force potentials, non-central-force bond-order potentials recently advanced by Pettifor and co-workers, and non-central-force potentials based on the model-generalized pseudopotential theory of Moriarty. As semiempirical reciprocal-space methods, a very simple d-basis tight-binding model by Paxton and an elaborate environment dependent spd-basis orthogonal tight-binding model by Haas et al. are included in the analysis. The virtues and deficiencies of these models in their ability to predict the translation states and interfacial energies of the ∑ = 5 STGB are discussed.

Atomistic Simulations for Multiscale Modeling in bcc Metals

Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted C(capital Sigma)5 (310)[100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, (small gamma) surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2(111) screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.

A Monte Carlo study of the α–β order–disorder transition in solid nitrogen

The α–β phase transition in solid N2 has been investigated using the constant pressure Monte Carlo method. This is accomplished by examining both phases in the temperature range 25≤T≤50 K, where they are everywhere at least metastable. It is found that the cubic α phase undergoes an orientational order–disorder transition into a disordered cubic phase as the temperature is increased to T=41 K, and remains in this state until melting. Similarly the orientationally disordered hexagonal phase persists from melting down to 33 K, where it undergoes a transition into a hexagonal structure with short‐range orientational order.