Department of Physics, Stanford University, Stanford, CA 94305, USA.
Proc Natl Acad Sci U S A. 2011 Mar 29;108(13):5174-8. doi: 10.1073/pnas.1017171108. Epub 2011 Mar 14.
Dislocation nucleation is essential to our understanding of plastic deformation, ductility, and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significantly larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviors are expected to occur in other nucleation processes in solids.
位错成核对于我们理解晶体材料的塑性变形、延展性和机械强度至关重要。分子动力学模拟在揭示位错成核的基本机制方面发挥了重要作用，但在研究实验相关条件下的成核时，其有限的时间尺度仍然是一个重大挑战。在这里，我们通过从伞形采样中确定激活自由能来证明，可以在很宽的条件范围内准确预测位错成核速率。我们的数据揭示了非常大的激活熵，这对位错成核速率的贡献是数量级的乘法因子。在恒应变下的激活熵是由热膨胀引起的，振动熵的贡献可以忽略不计。在恒应力下的激活熵明显大于在恒应变下的激活熵，这是由于热软化。大的激活熵是由非谐效应引起的，这表明在广泛用于固体中速率估计的谐波近似中存在局限性。在其他固体成核过程中也可能出现类似的行为。
