Observation of the critical thickness phenomenon in dislocation dynamics simulation of microbeam bending

Three-dimensional discrete dislocation dynamics simulations of thin aluminium beams in flexure give data in good agreement with critical thickness theory. The yield points (onset of significant plastic deformation) occur near the stress predicted by the theory. In the plastic regime, dislocation densities increase with plastic deformation within the bulk of the beam thickness, while the surface regions are relatively denuded of dislocations. The stress within the beam relaxes in the central part, then rises linearly to the free surfaces. This behaviour is quantitatively in agreement with critical thickness theory with a strain-thickness product for relaxation (plastic deformation) of 0.57 nm.