Asymmetry of slip and shape changes during cyclic deformation of α-iron single crystals

Abstract

During cyclic deformation at prescribed plastic strain amplitude (net zero strain) initially round α-iron single crystals became elliptical. These shape changes could be related satisfactorily to the asymmetry of slip (of screw dislocations) in tension and compression by a simple model which, furthermore, permitted an estimate of the relative contributions of screw and non-screw dislocations to dislocation glide. Systematic investigations showed that the shape changes were most pronounced at high strain amplitudes and cyclic strain rates. The shape changes were found to depend on the strain amplitude in much the same way as the mechanical saturation stresses. An interpretation was proposed in terms of cyclic micro- and macro-strain ranges corresponding to the motion of predominantly non-screw dislocations and screw and non-screw dislocations, respectively. The possible significance of the discussed effects with regard to fatigue failure of b.c.c. metals was pointed out and supported by experimental evidence.