Vacancy dipoles in fatigued copper

Abstract

The weak beam technique was used to perform an electron-microscopical study of dislocation dipoles in single crystals of copper fatigued beyond saturation at a resolved plastic shear strain amplitude of 0·3%. It has been shown previously that both primary edge dipoles and narrower faulted primary dipoles occur in these specimens (Antonopoulos and Winter 1976).

An experiment designed to detect dipole flipping among the unfaulted dipoles gave a negative result and a sign determination revealed a two to one preponderance of the vacancy type. The faulted dipoles were found to be of vacancy type exclusively. It therefore seems likely that, after saturation, fatigue produces dislocation dipoles of vacancy type only.

As an explanation of this it is suggested that non-linear elastic effects cause interstitial dipoles to have higher energy than vacancy dipoles. The practical consequence is that the persistent slip bands into which fatigue deformation concentrates (Winter 1974, Finney and Laird 1975) are gradually ‘fibre-loaded’ in tension during the later stages of fatigue testing. The stresses produced in this way could cause the secondary slip which is known to occur late in the test but which is not observed in the early parts (say the first 20 000 cycles) of a test at low strain amplitude. In addition, the fibre-loading gives some insight into the nucleation and early propagation of fatigue cracks.