dislocation glide

Here is the central magic trick of metals. To shear a perfect crystal you would have to slide a whole plane of atoms over another all at once, snapping every bond across that plane simultaneously — an enormous force. But a crystal with a dislocation cheats: it moves the dislocation instead, and the dislocation shifts the slip one narrow row of atoms at a time. Dislocation glide is this motion of a dislocation through its slip plane, and it is how real crystals shear. The same reason you move a heavy rug not by dragging it but by kicking a small ruck across it — one fold's worth of bonds at a time, each easy.

Step through the mechanism. An edge dislocation is the edge of an extra half-plane. Under a shear stress, the atoms just ahead of the core snap their bond to the current half-plane and rebond to make the NEXT plane the new half-plane; the dislocation has advanced by one Burgers vector, but only a handful of atoms moved, only a little. Repeat, repeat, repeat, and the dislocation sweeps clear across the crystal; when it exits at the far surface, the whole top block has slipped by exactly one b, leaving a one-atom step on the surface. Glide is called CONSERVATIVE motion because it moves no atoms into or out of the line — it needs no diffusion, so it works even at low temperature, and it is fast.

This one-row-at-a-time trick is the honest headline of the whole field: it is WHY real metals are ten to a hundred times weaker than a perfect lattice. A flawless crystal would need a stress near G/10 (G is the shear modulus) to shear; a real annealed metal yields at something like G/1000 to G/100000, because it is riddled with dislocations that glide at a small fraction of that stress. Strengthening a metal, in the end, is nearly always about making dislocation glide harder — pinning the dislocations so they cannot glide so freely.