Dynamic overshoot in -iron by atomistic simulations

Abstract

We present large-scale molecular dynamic simulations of microcrack growth in α-iron based on an N -body potential model which gives a good description of defect energetics, anisotropic elasticity and phonon frequency spectra. We demonstrate dynamic overshoot of a pre-existing microcrack under impact loading. We show that the basic behaviour of the simulations is in agreement with the predictions of continuum models. Dynamic phenomena, such as scattering of stress waves at crack faces, acoustic emission (due to bond breakage) and reflections of the stress waves from sample borders, are studied here in detail. The results on microcrack growth, unimpeded by the wave reflections from external sample borders, indicate that under fast loading or at high crack velocities, i.e. under high strain rates, transient twin formation is possible from the crack tip with later detwinning at free crack faces as the crack advances: a twinning equivalent to virtual Knott dislocations.

Superelasticity in bcc nanowires by a reversible twinning mechanism

“…Previously, work related to fracture have reported twinning as a mechanism at high strain rates in iron but with slower rates a dislocation mechanism takes over. 26,27 To verify whether the observed superelasticity mediated by the twinning mechanism in our simulations is dependent on the strain rate, we further checked the deformation behavior of Mo, W, and Fe nanowires under quasistatic conditions by using an annealing MD method. To perform this calculation, we first applied a prescribed strain ͑0.014%͒ to the relaxed ͓100͔-oriented nanowires through the fixed atomic layers at the ends at 0 K and then minimized the energy of the nanowire using the conjugate-gradient ͑CG͒ method.…”

Slip Processes and Fracture in Iron Crystals

“…The 3D codes have been tested in perfect samples under simple uni-axial tension and in thermal simulations. The simulated thermal expansion in bcc iron agrees well with experimental data (see Machová [13]), as well as the phonon frequency spectra [11,12].…”

“…The 3D codes have been tested in perfect samples under simple uni-axial tension and in thermal simulations. The simulated thermal expansion in bcc iron agrees well with experimental data (see Machová [13]), as well as the phonon frequency spectra [11,12].In crack simulations we consider a central pre-existing Griffith (through) crack loaded in tension mode I. The relatively long crack is embedded in a thin bcc iron crystal having the basic cubic {100} orientation.…”

“…We use our new MD code for parallel processing in MPI. Interatomic interactions in bcc iron are described using N-body potentials of Finnis-Sinclair type (Ackland et al [11,12]). The 3D codes have been tested in perfect samples under simple uni-axial tension and in thermal simulations.…”

“…The interatomic interactions across the free crack faces are not allowed. Surface relaxation has been performed before a loading to avoid its influence on crack tip processes.We performed the 3D crack simulations at temperatures of 0 K and 300 K. At these temperatures, the samples were loaded symmetrically in the <001> directions by prescribing external forces F ext distributed homogeneously at individual atoms lying in several surface layers, similar to [11]. The samples were loaded slowly, gradually in time.…”