ON THE NUCLEATION OF DISLOCATIONS AT A CRYSTAL-SURFACE

An exact expression for the elastic energy associated with a semicircu lar shear dislocation loop emanating from a free surface is derived (w ithin continuum dislocation theory) and compared with an earlier appro ximation. The energy required to activate a semicircular dislocation l oop into its unstable 'saddle-point'' configuration is then re-calcula ted, based on the modified expression for the self-energy. It is found that the shear stress necessary to emit the loop, as a function of te mperature, is almost 50% less than earlier estimates. The principal dr awback to this type of calculation is also discussed, namely, that the critical radius of an incipient dislocation loop is on the order of o ne atomic spacing, which is too small for a continuum theory to be val id.