Entropic effect on the rate of dislocation nucleation.

Area of Science:

• Materials Science
• Solid Mechanics
• Computational Materials Science

Background:

• Dislocation nucleation is crucial for understanding material properties like plastic deformation and strength.
• Molecular dynamics simulations are vital for studying dislocation nucleation but are limited by timescale.
• Accurate prediction of nucleation rates under experimental conditions remains a challenge.

Purpose of the Study:

• To accurately predict dislocation nucleation rates over a wide range of conditions.
• To investigate the contribution of activation entropy to nucleation rates.
• To explore the underlying causes of large activation entropies and their implications.

Main Methods:

• Utilized umbrella sampling to determine activation free energy.
• Performed molecular dynamics simulations.
• Analyzed activation entropy contributions under constant strain and constant stress conditions.

Main Results:

• Dislocation nucleation rates were accurately predicted by calculating activation free energy.
• Very large activation entropies were observed, significantly increasing nucleation rates.
• Activation entropy at constant stress was notably larger than at constant strain due to thermal softening.
• Anharmonic effects were identified as the primary cause of large activation entropies.

Conclusions:

• Umbrella sampling provides an accurate method for predicting dislocation nucleation rates.
• Large activation entropies, stemming from anharmonic effects, are critical and often overlooked factors in nucleation processes.
• The harmonic approximation is insufficient for accurate rate estimation in solids due to these anharmonic effects.
• Similar entropic effects are expected in other solid-state nucleation phenomena.