The ease of a metal to deform plastically in selected crystallographic planes depends on the core structure of its dislocations. As the latter is controlled by electronic interactions, metals with the same valence electron configuration usually exhibit a similar plastic behaviour. For this reason, titanium and zirconium, two transition metals of technological importance from the same column of the periodic table, have so far been assumed to deform in a similar fashion. However, we show here, using in situ transmission electron microscopy straining experiments, that plasticity proceeds very differently in these two metals, being intermittent in Ti and continuous in Zr. This observation is rationalized using first-principles calculations, which reveal that, in both metals, dislocations may adopt the same set of different cores that are either glissile or sessile. An inversion of stability of these cores between Zr and Ti is shown to be at the origin of the profoundly different plastic behaviours.

Although the favored glide planes in hexagonal close-packed Zr are prismatic, screw dislocations can escape their habit plane to glide in either pyramidal or basal planes. Using ab initio calculations within the nudged elastic band method, we show that, surprisingly, both events share the same thermally activated process with an unusual conservative motion of the prismatic stacking fault perpendicularly to itself. Halfway through the migration, the screw dislocation adopts a nonplanar metastable configuration with stacking faults in adjacent prismatic planes joined by a two-layer pyramidal twin.Plastic deformation in metals results mainly from the motion of line defects called dislocations. Many dislocation properties derive from the atomic-scale structure of their core, the region in the immediate vicinity of the dislocation line where crystallinity is disrupted. One such property is cross slip, i.e., the ability for screw dislocations to change glide plane, a stress-releaving process central to strain hardening and fatigue resistance [1,2].Thus far, cross slip has mostly been studied in facecentered cubic (fcc) metals for the conventional planar dissociated 1/2 110 {111} dislocations. Elasticity models [2,3] confirmed by atomic-scale simulations [4,5] showed that the dominant cross-slip mechanism involves a local constriction of the dislocation in its initial glide plane followed by redissociation in the crossslip plane (Friedel-Escaig mechanism). Another mechanism, which occurs under higher stresses met, for instance, in nanocrystalline plasticity [6], involves the successive change of glide plane of both partial dislocations [7]. Mechanisms involving a metastable configuration of the screw dislocation spread over several planes are also possible, as found in iridium [8].Cross slip is also observed in hexagonal close-packed (hcp) metals. In zirconium and titanium, 1/3 1210 dislocations are dissociated and glissile in prismatic {1010} planes [9], as confirmed by first-principles calculations [10,11]. But screw dislocations have also been reported experimentally to glide at high temperatures in both first-order pyramidal π 1 {1011} planes [12][13][14] and basal {0001} planes [12, 15] (see Fig. 1 for a graphical description of these planes). These secondary-slip processes do not correspond to cross slip as understood in fcc metals because the parent and the cross-slipped glide planes are not equivalent. Rather, this secondary slip in hcp metals is related to the fundamental question: how can a dislocation dissociated in a plane glide in another plane?In this Letter, we employ a combination of firstprinciples and empirical potential atomic-scale calculations to study secondary slip in hcp metals. We focus mainly on zirconium, although we checked that the present findings also apply to titanium. We show that unexpectedly, both basal and pyramidal slips are limited by the same thermally activated process, which involves an unusual motion of the prismatic stacking fault perpendicularly to itself, and results in a no...

Atomistic simulations, based either on an empirical interatomic potential or on ab initio calculations, are used to study the pyramidal glide of a 1/3 1210 screw dislocation in hexagonal close-packed zirconium. Generalized stacking fault calculations reveal a metastable stacking fault in the first order pyramidal {1011} plane, which corresponds to an elementary pyramidal twin. This fault is at the origin of a metastable configuration of the screw dislocation in zirconium, which spontaneously appears when the dislocation glides in the pyramidal plane.