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Predicting the dislocation nucleation rate as a function of temperature and stress

Published online by Cambridge University Press:  05 October 2011

Seunghwa Ryu
Affiliation:
Department of Physics, Stanford University, Stanford, California 94305
Keonwook Kang
Affiliation:
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Maxico 87545
Wei Cai*
Affiliation:
Department of Mechanical Engineering, Stanford University, Stanford, California 94305
*
a)Address all correspondence to this author. e-mail: caiwei@stanford.edu
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Abstract

Predicting the dislocation nucleation rate as a function of temperature and stress is crucial for understanding the plastic deformation of nanoscale crystalline materials. However, the limited time scale of molecular dynamics simulations makes it very difficult to predict the dislocation nucleation rate at experimentally relevant conditions. We recently develop an approach to predict the dislocation nucleation rate based on the Becker–Döring theory of nucleation and umbrella sampling simulations. The results reveal very large activation entropies, which originated from the anharmonic effects, that can alter the nucleation rate by many orders of magnitude. Here we discuss the thermodynamics and algorithms underlying these calculations in greater detail. In particular, we prove that the activation Helmholtz free energy equals the activation Gibbs free energy in the thermodynamic limit and explain the large difference in the activation entropies in the constant stress and constant strain ensembles. We also discuss the origin of the large activation entropies for dislocation nucleation, along with previous theoretical estimates of the activation entropy.

Type
Invited Feature Paper
Copyright
Copyright © Materials Research Society 2011

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