The enumeration and transformation of dislocation dipoles II. The transformation of interstitial dipoles into vacancy dipoles in an open dislocation array

If the presence of crystal surfaces permits some of the dipole content of a dislocation array to leave the crystal to form intrusions or extrusions on the surface, then the dipole strength of the array, as defined in part I, can change by glide processes. It is confirmed that the energy difference between vacancy and interstitial dipoles of equal height due to nonlinear elasticity is a possible cause of the conversion of interstitial dipoles to vacancy dipoles by slip, with the consequent production of extrusions at the crystal surface. The energy difference increases with decreasing dipole height and may account for the observed preponderance of small vacancy dipoles. However, the gain in energy resulting from the conversion of interstitial dipoles to vacancy dipoles is offset by long-range elastic energy stored in the field of the ‘fibre stress’—the average tensile stress in the band resulting from the bias. This leads to an equilibrium bias in dipole population and causes the fibre stress to increase as cyclic straining reduces the height of the dipoles.