In order to understand the ratcheting process in polycrystalline stainless steel, the associated dislocation features were explored by means of qualitative and quantitative transmission electron microscopy. Particular attention was paid to the effect of the peak stress (σmax) and the mean stress (σm) upon the ratcheting strain-rate. The effect of the tensile plastic strain history (peak stress) upon cyclic creep was expressed in the form of peak stress phases (R0, RI, RII) in which the cyclic deformation mechanisms were different. Planar slip during cyclic testing in phase R0 led to a high plastic strain reversibility which inhibited cyclic creep. The cyclic creep threshold stress corresponded to cross-slip activity which was promoted by a specific long-range internal stress state. Peak stresses which were higher than a threshold stress, σth, of 230MPa (stages RI and RII) led to the formation of 2 types of dislocation trapping: in dipolar walls and in polarized walls. Only the latter type led to cyclic creep. Especial attention was paid to the dependence of the ratcheting process upon fluctuations in the intergranular and intragranular back-stress, and their evolution as a function of the number of cycles. Both types of internal stress fluctuation tended to enhance cyclic creep.

Cyclic Creep Process in AISI316L Stainless Steel in Terms of Dislocation Patterns and Internal Stresses. C.Gaudin, X.Feaugas: Acta Materialia, 2004, 52[10], 3097-110