ON THE NUCLEATION OF DISLOCATIONS AT A CRYSTAL-SURFACE

An exact expression for the elastic energy associated with a semicircular shear dislocation loop emanating from a free surface is derived (within continuum dislocation theory) and compared with an earlier approximation. The energy required to activate a semicircular dislocation loop into its unstable 'saddle-point'' configuration is then re-calculated, 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 temperature, is almost 50% less than earlier estimates. The principal drawback to this type of calculation is also discussed, namely, that the critical radius of an incipient dislocation loop is on the order of one atomic spacing, which is too small for a continuum theory to be valid.