Abstract
Many specimens at the nanoscale are pristine of dislocations, line defects which are the main carriers of plasticity. As a result, they exhibit extremely high strengths which are dislocation-nucleation controlled. Since nucleation is a thermally activated process, it is essential to quantify the stress-dependent activation parameters for dislocation nucleation in order to study the strength of specimens at the nanoscale and its distribution. In this work, we calculate the strength of Mo nanoparticles in molecular dynamics simulations and we propose a method to extract the activation free-energy barrier for dislocation nucleation from the distribution of the results. We show that by deforming the nanoparticles at a constant strain rate, their strength distribution can be approximated by a normal distribution, from which the activation volumes at different stresses and temperatures are calculated directly. We found that the activation energy dependency on the stress near spontaneous nucleation conditions obeys a power-law with a critical exponent of approximately 3/2, which is in accordance with critical exponents found in other thermally activated processes but never for dislocation nucleation. Additionally, significant activation entropies were calculated. Finally, we generalize the approach to calculate the activation parameters for other driving-force dependent thermally activated processes.
Highlights
Dislocation nucleation is described naturally in atomistic simulations and direct calculation of the activation parameters for dislocation nucleation has been addressed computationally in recent years
We apply the model to the problem of heterogeneous dislocation nucleation and we study the activation parameters for dislocation nucleation in Mo nanoparticles under compression using classical molecular dynamics (MD) simulations
We suggest that the different values of Tm fitted for the lowest and highest temperatures is indicative of a non-linear relation between the activation entropies and energies, in the range of activation energies calculated from the MD simulations; the slope of the activation energy-entropy relation is smaller at lower temperatures than at higher temperatures, to the results in Cu nanorods in constant-strain ensemble[19]
Summary
Many specimens at the nanoscale are pristine of dislocations, line defects which are the main carriers of plasticity. Since nucleation is a thermally activated process, it is essential to quantify the stressdependent activation parameters for dislocation nucleation in order to study the strength of specimens at the nanoscale and its distribution. We calculate the strength of Mo nanoparticles in molecular dynamics simulations and we propose a method to extract the activation free-energy barrier for dislocation nucleation from the distribution of the results. To control the strength of pristine specimen at the nanoscale, estimating the activation parameters for dislocation nucleation under different external driving forces, from which the nucleation rates can be estimated, is imperative. This varying driving force technique allows us to calculate the activation parameters for dislocation nucleation in Mo nanoparticles and, in particular, to study the stress-dependency of the free-energy barrier
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