A Multiple Site Type Nucleation Model and Its Application to the Probabilistic Strength of Pd Nanowires

Pristine specimens yield plastically under high loads by nucleating dislocations. Since dislocation nucleation is a thermally activated process, the so-called nucleation-controlled plasticity is probabilistic rather than deterministic, and the distribution of the yield strengths depends on the activation parameters to nucleate. In this work, we develop a model to predict the strength distribution in nucleation-controlled plasticity when there are multiple nucleation site types. We then apply the model to molecular dynamics (MD) simulations of Pd nanowires under tension. We found that in Pd nanowires with a rhombic cross-section, nucleation starts from the edges, either with the acute or the obtuse cross-section angles, with a probability that is temperature-dependent. We show that the distribution of the nucleation strain is approximately normal for tensile loading at a constant strain rate. We apply the proposed model and extract the activation parameters for site types from both site types. With additional nudged elastic bands simulations, we propose that the activation entropy, in this case, has a negligible contribution. Additionally, the free-energy barriers obey a power-law with strain, with different exponents, which corresponds to the non-linear elastic deformation of the nanowires. This multiple site type nucleation model is not subjected only to two site types and can be extended to a more complex scenario like specimen with rough surfaces which has a distribution of nucleation sites with different conditions to nucleate dislocations.

Automated summary

In this study, a model is developed to predict the strength distribution in nucleation-controlled plasticity with multiple nucleation site types. Molecular dynamics simulations on Pd nanowires reveal temperature-dependent nucleation probabilities. The distribution of nucleation strain is found to approximate a normal distribution under constant strain rate. Activation entropy is shown to have negligible contribution. The free-energy barriers follow a power-law with strain, indicating nonlinear elastic deformation of the nanowires. This model can be extended to more complex scenarios, such as rough surfaces with a distribution of nucleation sites under different conditions.