Adaptive strain-boost hyperdynamics simulations of stress-driven atomic processes

Abstract

The deformation and failure phenomena of materials are the results of stress-driven, thermally activated processes at the atomic scale. Molecular-dynamics (MD) simulations can only span a very limited time range which hinders one from gaining full view of the deformation physics. Inspired by the Eshelby transformation formalism, we present here a transformation "strain-boost" method for accelerating atomistic simulations, which is often more efficient and robust than the bond-boost hyperdynamics method [R. A. Miron and K. A. Fichthorn, J. Chem. Phys. 119, 6210 (2003)] for exploring collective stress-driven processes such as dislocation nucleation, that have characteristic activation volumes larger than one atomic volume. By introducing an adaptive algorithm that safely maximizes the boost factor, we directly access the finitetemperature dynamical process of dislocation nucleation in compressed Cu nanopillar over time scale comparable to laboratory experiments. Our method provides stress-and temperature-dependent activation enthalpy, activation entropy and activation volume for surface-dislocation nucleation with no human guidance about crystallography or deformation physics. Remarkably, the accelerated MD results indicate that harmonic transition-state theory and the empirical Meyer-Neldel compensation rule give reasonable approximations of the dislocation nucleation rate. The deformation and failure phenomena of materials are the results of stress-driven, thermally activated processes at the atomic scale. Molecular-dynamics ͑MD͒ simulations can only span a very limited time range which hinders one from gaining full view of the deformation physics. Inspired by the Eshelby transformation formalism, we present here a transformation "strain-boost" method for accelerating atomistic simulations, which is often more efficient and robust than the bond-boost hyperdynamics method ͓R. A. Miron and K. A. Fichthorn, J. Chem. Phys. 119, 6210 ͑2003͔͒ for exploring collective stress-driven processes such as dislocation nucleation, that have characteristic activation volumes larger than one atomic volume. By introducing an adaptive algorithm that safely maximizes the boost factor, we directly access the finite-temperature dynamical process of dislocation nucleation in compressed Cu nanopillar over time scale comparable to laboratory experiments. Our method provides stress-and temperature-dependent activation enthalpy, activation entropy and activation volume for surface-dislocation nucleation with no human guidance about crystallography or deformation physics. Remarkably, the accelerated MD results indicate that harmonic transition-state theory and the empirical Meyer-Neldel compensation rule give reasonable approximations of the dislocation nucleation rate. Disciplines Engineering | Materials Science and Engineering Comments

Adaptive-boost molecular dynamics simulation of carbon diffusion in iron

“…Table I shows the average jump period at each temperature, as calculated using (8). At 200K, the time-acceleration factor we obtained is 1.45 × 10 17 .…”

“…If a sufficient number of adaptive boost steps are implemented successively without transition, −∆V L total (A) will become approximately a replica of the free-energy surface F (A). In the present method, it is not necessary to define the shape of the boost potential in advance 5,7,8 .…”

“…Here, we have developed a general adaptive boost formalism based on previous developments [2][3][4][5][6][7][8] . Given a uncharacterized V (r), we first evaluate the smooth histogram ρ(A) of collective variables (CVs) 9,10 A ≡ {A 1 (r), · · · , A M (r)} by MD, and then utilize the smooth histogram (probability density) of A as the boost potential ∆V (r) ∝ k B T ln ρ(A) (1) in subsequent accelerated MD simulations.…”

How to identify dislocations in molecular dynamics simulations?

“…For the purpose of quantifying plastic deformation and visualizing defect evolution at the atomic level, the atomic local shear strain [33,34] [32] easy to extend and able identify a wide range of coordination structures expensive complex system dislocation extraction algorithm (DXA) [48] advance and with Burgers vectors can be calculated automatically complex and need an ideal reference configuration arbitrary lattices For each neighbor j of atom i, their current separation vector is…”

PECASE - First Principles Modeling of Mechanics and Chemistry of Materials

“…Shear stress relaxation is calculated from a master-equation description in which the system follows a transition-state pathway trajectory of hopping among local energy minima separated by activation barriers, which is in turn sampled by accerlated MD-based algorithms 6 . Quantitative connection is established between the temperature variation of the calculated viscosity and the underlying potential energy and inherent stress landscape of several glasses 11,12 , showing a different landscape topography or 'terrain' is needed for low-temperature viscosity from that associated with high-temperature viscosity 11 .…”