Dislocations, line defects in lattice crystals, are the main factors to influence the strength and ductility of crystalline materials. A material in which defects are easy to move and multiply will normally be soft and malleable, whereas a material in which dislocation can't move and be easily created will be hard and brittle. Understanding how a new dislocation generates is an essential aspect to know the mechanism of crystal defect physics and to improve the mechanical properties of engineering material. But some commonly held notions of a nucleation criterion, e.g., an extension of Schmid's Law (whereby the dislocation will nucleate when the resolved stress acting on it reaches a critical value) are incorrect to predict dislocation nucleation. Here, we provide and test a new nucleation criterion based on stress gradients of a crystalline solid undergoing deformation, and demonstrate the criterion's ability to correctly predict dislocation nucleation via direct atomistic simulations with three-dimensional MD (Molecular Dynamics) simulations.
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