dislocation climb

Glide lets an edge dislocation slide sideways within its slip plane, cheaply and at any temperature. But there is a second, quite different way an edge dislocation can move: it can step UP or DOWN, out of its slip plane entirely. This perpendicular motion is called climb, and it works by making the extra half-plane grow or shrink. Since you cannot move the edge of a half-plane out of plane without adding or removing atoms from it, climb is fundamentally different from glide — it requires matter to be shuffled in and out.

Here is the mechanism, step by step. To climb UP by one row, the bottom edge of the extra half-plane must lose its row of atoms: each atom leaves the edge and hops away as the crystal absorbs it, which is the same as a vacancy arriving at the edge and the atom filling it elsewhere. To climb DOWN, the half-plane must GAIN a row, fed by atoms diffusing in (or vacancies leaving). Either way, climb consumes or emits vacancies, and vacancies only move by diffusion, which is thermally activated — so climb needs heat. That is why climb is called NON-conservative motion (atoms are not conserved on the line) and why it is negligible at room temperature but important at high temperature, above roughly half the melting point.

Only edge (and the edge component of mixed) dislocations can climb, because a pure screw has no half-plane to add to or subtract from. Climb matters enormously in the slow, hot regimes of engineering: it is the key step in creep, the gradual deformation of metals under load at high temperature (turbine blades, steam pipes), because it lets dislocations escape obstacles they cannot glide past by climbing over them. It also drives recovery and the shrinking of dislocation loops, and it lets dislocation networks coarsen. Where glide is fast and cold, climb is slow and hot — the diffusion-limited sibling of dislocation motion.