a screw dislocation
Think of a multi-storey car park with a single spiral ramp that joins every level. Drive round and round and you climb smoothly from the ground floor to the roof without ever meeting a step — the floors are not really separate at all, they are one continuous helical sheet. A screw dislocation does this to the atomic planes of a crystal: instead of neat, separate, parallel layers, the planes are sheared so that circling once around a central line lifts you up by exactly one layer. What should have been a stack of pancakes becomes a single spiral ramp.
The precise picture: imagine cutting partway into a crystal along a plane, then sliding the two lips of the cut past each other by one atomic spacing PARALLEL to the cut edge, and letting them rebond. The atoms no longer lie in flat sheets; they wind around the line at the end of the cut like a spiral staircase. That line is the screw dislocation. Its defining feature is that the Burgers vector b — the amount and direction of the slip — is PARALLEL to the dislocation line, not perpendicular as for an edge. There is no extra half-plane here; the disregistry is a twist, not an insertion.
Because b lies along the line, a screw dislocation has a special freedom: any plane that contains the line also contains b, so the screw can glide on MANY planes and can switch from one to another — a move called cross-slip that lets a screw dislocation dodge obstacles an edge could not. Most real dislocations are neither pure edge nor pure screw but a smooth mixture, yet the pure screw is one of the two clean end-members every dislocation is built from. Its stress field is pure shear with no squeeze or stretch, which is why a screw, unlike an edge, cannot climb.
Grip a block of atoms above a horizontal cut and shear it one lattice spacing toward yourself, but only over the front half of the cut. The atoms along the line where the sheared region ends now spiral around that line: start on the top plane, walk one loop around the line, and you arrive on the plane below — a helix of pitch equal to b. Here b points along the line, the hallmark of screw character.
A screw dislocation is a spiral ramp joining stacked atomic planes; its Burgers vector runs parallel to the line, and it can cross-slip.
There is no extra half-plane in a screw dislocation — that mental image belongs only to the edge. The most common slip is to picture a screw as an inserted plane; it is a shear twist, and its Burgers vector lies along, not across, the line.