Diffusion-controlled creep: A tensile stress increases the separation of atoms on grain boundaries that are normal to the stress axis, and the Poisson contraction decreases the separation of atoms on grain boundaries that are parallel to the stress axis. The result is a driving force for the diffusional transport of atoms from grain boundaries parallel to the tensile stress to boundaries normal to the tensile stress. Such diffusion produces a plastic elongation. Th specimen elongates as atoms are added to grain boundaries perpendicular to the stress.