  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 abinitio 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.
