A Model for the Change in Elastic Modulus of a Solid Caused by Point Defects

Point defects introduced into copper at low temperatures by irradiation are known to produce bulk expansion. We suggest here that an external stress might cause the defect (represented as oversized spherical inclusions) to relax. This would change the expansion produced by the defects so that the deformation due to the stress would be greater and the elastic modulus would thus appear smaller. This relaxation is represented by a small deformation of the spherical inclusions, and elastic energy considerations are used to calculate the change in the elastic modulus of Cu due to such defects. If the radius of the inclusions is chosen to be two lattice constants, the experimentally observed value of 18% for the decrease in the elastic modulus at saturation density of defects is obtained.