Cyclic creep process in AISI 316L stainless steel in terms of dislocation patterns and internal stresses

Abstract

In order to understand the ratchetting process in polycrystalline stainless steel, a consistent mechanical data base was provided and associated dislocation features were explored by qualitative and quantitative TEM observations. Particular attention was paid to the effect of peak stress ( σ max) and mean stress ( σ m) on the ratchet strain rate. The effect of tensile plastic strain history (peak stress) on cyclic creep was rationalized under the form of three peak stress phases ( R 0, R I and R II) in which the cyclic deformation mechanisms are different. Planar slip, during the cyclic test in phase R 0, led to a high plastic strain reversibility which inhibited cyclic creep. The cyclic creep threshold stress corresponds to a cross-slip activity promoted by a specific long-range internal stress state. Peak stresses which were higher than a threshold stress σ th equal to 230 MPa (stage R I and stage R II) induced the formation of two types of dislocation trapping, namely in dipolar walls and in polarized walls. Only the last type induced cyclic creep. Special attention was paid to the dependence of ratchetting process on the fluctuations of the intergranular and intragranular back stress and their evolution as a function of the number of cycles. Both types of internal stress fluctuations acted to enhance cyclic creep.