The activation energy for dislocation nucleation at a crack

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The detailed atomic-scale mechanisms of dislocation nucleation and motion are investigated as a function of the external load. Analysis of the atomic configurations around the crack tip demonstrates an intimate coupling of the nucleating dislocation with a step formed at the crack surface. Displacement and stress fields around both nucleating and moving dislocations are compared to the predictions of the Peierls-Nabarro continuum-elastic model by Rice.

The size of a nucleating incipient dislocation is found to be larger than that of a fully-formed dislocation. Also, the authors elucidate the reasons why the value of the unstable-stacking energy estimated by means of the rigid-block sliding concept, a feature common to several continuum-elastic models, overestimates the activation energy for dislocation nucleation. This allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack.

An exact solution for the loading at that nucleation instability is developed via the J -integral for the case when the crack and slip planes coincide, and an approximate solution is given when they do not. Solutions are also given for emission of dissociated dislocations, especially partial dislocation pairs in fcc crystals.