Irreversibility of cyclic slip in persistent slip bands of fatigued pure f.c.c. metals

Parallel screw dislocations performing cyclic slip are considered. Plastic resolved-shear-strain amplitudes γpl below a certain limit γpl,s can be accomplished by a static dislocation density, i.e. by the reversible to-and-fro motion of the screw dislocations. If γpl ≥ γpl,s, steady-state cyclic deformation requires that the dislocation density is maintained constant by a dynamical equilibrium between dislocation multiplication and annihilation. The latter is associated with a certain slip irreversibility χ which is calculated. The results of the calculation are compared with a model of cyclic slip which has been proposed previously for persistent slip bands (PSBs) in pure f.c.c. metals. It is concluded that with respect to the screw dislocations γpl,PSB ≥ γpl,s. Accordingly the slip mechanism in PSBs becomes irreversible. Taking into account the behaviour of both the screw and edge dislocations the irreversible fraction of the strain amplitude p = γpl,irr/γpl is calculated. One obtains 0.1 < p < 0.2. The relation between p and the parameters which determine the evolution of extrusions and the rate of development of the surface topography of PSBs is pointed out.