Anna Machová

Atomistic simulation of stacking fault formation in bcc iron

We present large scale atomistic simulations of crack growth in iron under quasistatic loading in mode I. We show that long cracks display a brittle character of extension, while the growth of smaller cracks is accompanied by emission of partial dislocations from the crack tip and subsequent transformation of the stacking faults behind the dislocations to multilayer twins. The competing shear processes at a crack tip are characterized in terms of the relative sliding of up to four adjacent atomic planes emanating from the crack tip region. The results are in agreement with a global energy balance derived from perfect samples, and with experimental observations that twinning and fracture are cooperating processes under sufficiently large quasistatic loading at low temperatures.

Reconciliation of continuum and atomistic models for the ductile versus brittle response of iron

We present a detailed analysis of the ductile versus brittle response of bcc iron containing a sharp crack and show that a continuum model based on the Peierls– Nabarro model for dislocation formation is consistent with our atomistic results. Specifically, we compare continuum predictions for dislocation emission from a crack tip loaded in mode I under plane strain, tensile conditions with atomistic results for a (¯ 1 0)[1 1 0] crack emitting full edge dislocations in the � 111 � { 112 } slip system. The simulations are based on an N -body potential of the Finnis–Sinclair type for iron at 0 K, and the continuum dislocation model incorporates recent improvements that account for tension–shear coupling on a prescribed slip plane, as well as the T-stress, in an anisotropic solid. We show that the critical load for dislocation nucleation is influenced by the T-stress (modulated by the level of external stress applied parallel to the crack plane in a biaxially-loaded plate), which possesses a critical value associated with a change of mechanism between dislocation emission and crack extension. The results are consistent with recent preliminary analyses that address the effect of crack size and the role of the T-stress in the ductile versus brittle response of crystals.

Stress calculations on the atomistic level

The paper examines different definitions of the local atomic stress at zero temperature for homogeneous and inhomogeneous strain across an interface. It is shown that if inhomogeneous straining occurs within the range of interatomic interaction, then the interplanar concept (based on force balance) describes better the local stress in comparison with a definition of the volume stress derived from the energy density around an atom.

Crack Orientation versus Ductile-Brittle Behavior in 3D Atomistic Simulations

The paper presents results of molecular dynamic (MD) simulations in 3D bcc iron crystals with edge pre-existing cracks (001)[110] and (110) [110] (crack plane/crack front) loaded uni-axially in tension mode I at temperature of 300 K. The iron crystals in MD have the same orientation and similar geometry as in our recent fracture tests performed at room temperature on iron (3wt.%Si) single crystals [1].

Crack growth in single crystals of α-iron (3 wt.% Si)

Experimental and theoretical investigations by the method of acoustic emission and atomistic simulations by a molecular dynamics technique show that brittle-ductile transition in α-iron is very sensitive to loading rate and that the character of acoustic emission is different when different processes operate at the crack tip. A self-similar concept for comparison of experimental and atomistic results is proposed for fracture tensile tests.

A Model for Crack-Induced Nucleation of Dislocations, Complex Stacking Faults and Twins

This paper addresses a class of deformation mechanisms involving the coordinated shear of multiple, parallel slip planes. Relevant phenomena include complex stacking faults, deformation twins, and dislocation nucleation ahead of cracks in metals. As part of the theory, we revisit the notion of a multi-plane slip potential, we develop a criterion for the emergence of microtwins, and we discern the conditions that favor microtwin versus dislocation nucleation. The model is constructed using concepts from the continuum-based Peierls-Nabarro framework for extended dislocation cores.

Ductile-Brittle Behavior of Microcracks in 3D

Results of several parallel molecular dynamics crack simulations in bcc iron crystals with up to 128 million atoms are presented. The crack (001)[010] of Griffith type is loaded in Mode I. We observe dislocation emission and twinning near the free sample surfaces and later plastically induced crack initiation.

Sample Geometry and the Brittle-Ductile Behavior of Edge Cracks in 3D Atomistic Simulations by Molecular Dynamics

We present new results of molecular dynamic (MD) simulations in 3D bcc iron crystals with edge cracks (001)[010] and (-110)[110] loaded in mode I. Different sample geometries of SEN type were tested with negative and positive values of T-stress according to continuum prediction by Fett.

Ductile-Brittle Behavior along Crack Front and T-Stress

We present new results of molecular dynamic (MD) simulations in 3D bcc iron crystals with embedded central through crack (001)[110] of Griffith type loaded in mode I. Two different sample geometries of the same crystallographic orientation were tested with negative and positive values of the T-stress, which change the ductile-brittle behavior along the crack front in 3D. This phenomenon is explained in the framework of stress analysis, both on the continuum and atomistic level.