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.

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One critical issue in heteroepitaxial, lattice mismatched growth is the inevitable appearance of threading dislocations which reside in the relaxing film and degrade its semiconducting properties. It has been shown in previous work that threading dislocations interact with each other through a series of annihilation and fusion reactions to decrease their density as the film thickness increases and follow a l/h decay, where h is the film thickness. A characteristic reaction radius is associated with these interactions. In previous simulations, the reaction radius was taken to be a constant value estimated using a simple approximation based on infinite, parallel dislocation lines. Here, a continuum-based elasticity approach is taken to more accurately quantify the reaction radius by comparing the Peach-Koehler force of one dislocation acting on another at a free surface with the lattice resistance to dislocation motion. The presence of the free surface gives rise to a moderate reduction of the interaction force. Results are compared with preliminary experimental data for GaAs films grown on InP. INTRODUCTION AND BACKGROUND

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.