[SY-C7] Scrutinizing screw dislocation glide initiation at finite temperatures in BCC metals
Plasticity of body-centered-cubic (BCC) metals at low temperatures is determined by screw dislocation kinetics. Because the core of screw dislocation in BCC metals has non-planar structure, its motion is complex and unpredictable. For example, although density functional theory (DFT) predicts slip on a {110} plane, the actual slip plane departs from the prediction at elevated temperatures, its mechanism having been a mystery for decades. In the current study, we conducted molecular dynamics simulations at finite temperatures using a recently-developed empirical potential that has the single-hump Peierls barrier for 1/2<111> the screw dislocation jump and examined the dislocation jump process.
In our molecular dynamics, the dislocation glides on the {110} plane with the highest Schmidt's factor, as predicted by DFT at low temperatures, while the dislocation approximately glides on a {112} plane at higher temperatures. Thus, our molecular dynamics simulations successfully reproduced the transition of the slip plane observed in experiments. To examine how the glide-plane transition takes place, we particularly developed a post-analysis tool to scrutinize the initiation of screw dislocation jumps from one Peierls valley to the next with high spatiotemporal resolution. The results indicated that heaved motion of screw dislocation line inside a Peierls valley sometimes causes a double-kink nucleation on a {110} plane with lower Schmidt's factor. Because of the twinning/anti-twinning asymmetry of the core structure, the average slip plane over the long kinetics departed from the original {110} plane and approximately becomes {112}. This suggests that thermal fluctuation of screw dislocation line are responsible for the transition of the slip plane.
We also observed a jerky dislocation motion at low temperatures similar to experiments. Close investigation of this motion using the new post-analysis tool indicates that inertial effect causes the jerky motion, i.e., a previous jump beyond the Peierls barrier can directly cause the double-kink nucleation for the next jump.
In our molecular dynamics, the dislocation glides on the {110} plane with the highest Schmidt's factor, as predicted by DFT at low temperatures, while the dislocation approximately glides on a {112} plane at higher temperatures. Thus, our molecular dynamics simulations successfully reproduced the transition of the slip plane observed in experiments. To examine how the glide-plane transition takes place, we particularly developed a post-analysis tool to scrutinize the initiation of screw dislocation jumps from one Peierls valley to the next with high spatiotemporal resolution. The results indicated that heaved motion of screw dislocation line inside a Peierls valley sometimes causes a double-kink nucleation on a {110} plane with lower Schmidt's factor. Because of the twinning/anti-twinning asymmetry of the core structure, the average slip plane over the long kinetics departed from the original {110} plane and approximately becomes {112}. This suggests that thermal fluctuation of screw dislocation line are responsible for the transition of the slip plane.
We also observed a jerky dislocation motion at low temperatures similar to experiments. Close investigation of this motion using the new post-analysis tool indicates that inertial effect causes the jerky motion, i.e., a previous jump beyond the Peierls barrier can directly cause the double-kink nucleation for the next jump.