Molecular dynamics simulation of ⟨c+a⟩ dislocation core structure in hexagonal-close-packed metals

The core structures of <c + a> dislocations in hexagonal-close-packed (hcp) metals have been investigated by molecular dynamics (MD) simulation using a Lennard-Jones-type pair potential. The <c + a> edge dislocation has two types of core at 0 K; one is a perfect dislocation (type A), and the other has two 1/2 <c + a> partials (type B). Type A transforms to type B by abruptly increasing temperature from 0 K to 293 K, while type B is stable in temperature range from 0 K to 293 K. In contrast, type A extends parallel to (0001) at 30 K, and this extended core is still stable at 293 K. These results suggest that the <c + a> edge dislocation glides on the {11 (2) over bar2} as two 1/2 <c + a> partial dislocations and becomes sessile due to changes of the core structure. The <c + a> screw dislocation spreads over two {10 (1) over bar1} planes at 0 K. The core transforms into a unsymmetrical structure at 293 K, which is spread over {11 (2) over bar2} and {10 (1) over bar1}, and core spreading occurs parallel to {11 (2) over bar2} at 1000 K. A critical strain to move screw dislocations depends on the sense of shear strain. The dependence of the yield stress on the shear direction can be explained in terms of these core structures.