Invited Lecture
Mobility of screw dislocations in hexagonal close-packed zirconium and titanium

Titanium and zirconium have a close plastic behaviour arising from their hexagonal close-packed crystallography and from their similar electronic structure. In particular, plasticity in these two transition metals is controlled by screw dislocations gliding in the prism planes, with cross-slip in the first-order pyramidal planes or in the basal planes activated at high enough temperature and a strong hardening associated with interstitial solute addition. We use atomistic simulations relying on ab initio calculations to study core properties of the screw dislocations and their mobility in both metals. These calculations show that screw dislocations may adopt different cores that are dissociated either in a prism or in a pyramidal plane. The prismatic core easily glides in its habit plane, whereas the pyramidal core needs to overcome an important energy barrier to glide. The prismatic glissile core is the most stable in Zr, but the dislocation ground state in Ti corresponds to the pyramidal core. As a consequence, dislocation glide is easy and confined in the prismatic planes at low temperature in pure Zr, whereas a locking-unlocking mechanism operates in Ti where the locked periods correspond to a slow and limited glide in pyramidal planes and the unlocked periods to a rapid and extended glide in prismatic planes, in agreement with in situ TEM straining experiments. Finally, we study the interaction of interstitial solute atoms, in particular oxygen and sulfur, with these different configurations of the screw dislocation. Ab initio calculations evidence a strong repulsion with both solutes repelling the stacking fault ribbon, thus inducing dislocation cross-slip and leading to the production of jogs on the screw dislocation. This short-range interaction mechanism is responsible for the strong hardening associated with oxygen addition.