Size and rate dependent grain boundary motion mediated by disconnection nucleation

The synthetic driving force method is a widely-used technique in molecular dynamics simulations to investigate the migration of grain boundaries. Its physical essence, however, has been under debate for quite some time for generating the driving force by artificially introducing some energy into the crystals. In this study, the elementary process governing the grain boundary motion under the driven motion method was explored by applying a varying synthetic driving force that increases from zero at a constant rate, which is in contrast to a constant driving force that is usually applied in past studies. With this method, it was found that a rate-controlling process, i.e., disconnection nucleation that has been reported before to dominate the physical grain boundary motion coupled to an applied shear, also operated for grain boundary motion caused by the synthetic driving force. Furthermore, the disconnection nucleation mediated process was also found to cause a strong size dependence and transitions of grain boundary motion modes at different temperatures. It is hoped that with this study, the synthetic driving force method in studying grain boundary motion can be used with more confidence in its physical essence and a universal mechanism can be proposed to explain grain boundary motion in materials despite how it is caused.

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