a screw dislocation

The screw dislocation is harder to see but worth the effort, because it is genuinely strange and elegant. Picture a multi-storey parking garage where, instead of flat floors stacked with stairs between them, the floor is one continuous spiral ramp — you drive round and round and rise a level each loop without ever climbing a step. A screw dislocation twists the atomic planes of a crystal into exactly that kind of helical ramp: what looked like separate flat layers become a single spiral sheet winding around a central line.

It forms by shearing the crystal partway. Imagine cutting halfway into a block, then sliding the two faces of the cut past each other by one atomic spacing, parallel to the cut's edge, and letting them re-bond. The line at the tip of the cut, where the slip ran out, is the screw dislocation. Its defining feature is the opposite of the edge type: the Burgers vector runs parallel to the dislocation line, not perpendicular. That is why, following the atoms around the line, you spiral up like the thread of a screw — hence the name.

Screw dislocations glide too, moving sideways under shear to carry slip, and they have one freedom the edge type lacks: because the Burgers vector is parallel to the line, a screw can cross-slip, switching onto a different slip plane that shares its slip direction to dodge an obstacle. That extra mobility matters in real deformation and in how metals recover. Screw dislocations are also famous in crystal growth: the never-ending step where the spiral meets a surface gives new atoms a perpetual ledge to attach to, so many crystals grow up that self-perpetuating screw ramp.