Form of the displacement
We now need to modify the stationary dislocation model to estimate the change in misfit energy of a dislocation as it moves.
The half-plane of atoms moves by much less than the dislocation. The half-plane of atoms was used as the origin, so we think of the dislocations as a moving origin from which we can estimate the displacements. We use the parameter alpha to describe the fraction across the unit cell across which the dislocation has moved.
As shown in the following animation, when the dislocation has moved through b (where α = 1) the extra half plane of atoms is now moved to the plane adjacent to the new half plane.
The dislocation has moved about twice as far as the extra half plane.