Role of dislocation pile-ups in nucleation-controlled size-dependent strength of Fe nanowires

We studied the strength of Fe faceted nanowires under diametrical compression using molecular dynamics simulations. We show that a series of consecutive events of dislocation nucleations occurs at the vertices of the nanowire, and two edge dislocation pile-ups are formed before a catastrophic event occurs, previously named a cross-split of edge dislocations. The compressive stress at which dislocations are nucleated depends on the specimen's size and obeys a power-law, with an exponent that depends on the number of dislocations already nucleated into the pile-up. The role of the dislocation pile-up is discussed and, on the basis of classical nucleation theory, we examine the contribution of the dislocation pile-up to the nucleation conditions at the vertices of the nanowire. We found that three contributions need to be accounted for: the stress gradient at the vertices, the back stress from the nucleated pile-up, and the image stresses due to the confined volume of the nanowire. In addition, the proposed model agrees with the distribution of the dislocations inside the pile-up. We ratify that cross-splitting occurs at the head of the pile-up when the distance between the two leading dislocations is about twice their Burgers vectors. Considering this value as the limit for dislocation core coalescence, the results of our model and molecular dynamic simulations for the stresses needed for cross-splitting were in excellent agreement. Based on these results, we found the dependency of the critical stress needed for cross-splitting on the size. Finally, we extend the discussion to the general role of dislocation pile-ups in nucleation-controlled plasticity.

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