Atomistic study of crack-tip plasticity in precipitation hardened monocrystalline aluminum

Understanding the atomistic mechanisms of dislocation-based plasticity ahead of a crack-tip in precipitation hardened alloys is a challenging problem due to the complexity of the interactions between the precipitates in the microstructure and the variety of defects nucleated at the crack-tip, such as dislocations, stacking faults and micro-twins. In this paper, we use classical molecular dynamics simulations to perform a comprehensive atomistic analysis of the factors that influence the motion of dislocations ahead of a crack-tip in precipitation hardened aluminum. Specifically, the effects of planar copper GPII zones on the motion of dislocations emitted at the crack-tip of an aluminum crystal in four different crystal orientations under constant strain-rate loading were investigated. By placing the precipitates close to the crack-tip, it was found that they did not affect the nucleation of the first dislocation significantly unless they were located immediately ahead. Moreover, in some crystal orientations, subsequent nucleations were appreciably delayed due to the shielding effect of the first dislocation interacting with the precipitate. Following emission, the interaction between the emitted dislocations and the precipitates consisted of different mechanisms, including shear cutting, Orowan looping, and cross-slip, depending on the crystal orientation. The resistance to dislocation motion caused by the precipitates was quantified by determining the interaction time between each dislocation and the precipitates. It was found that although the applied load in each unit cell was high, the dislocations could be significantly slowed down in some of the crystals. This resulted in less dislocation activity ahead of the crack-tip, especially in the crystals for which micro-twinning was the dominant driver of plasticity. The results of this work pave the way for the development of accurate models to predict the evolution of plasticity in metallic materials by providing a quantified assessment of dislocation motion in complex alloy microstructures.