Hardening rate under reverse loading in 316 L : Back stress and effective stress evolutions

Abstract
The Bauschinger effect has been investigated in a low stacking fault energy f.c.c. alloy. Reverse hardening rate is discussed in terms of internal stresses evolutions as a function of reverse straining. A distinction is made between short-range (effective stress) and long-range (back stresses) elastic interactions. Moreover the back stress is partitioned into intra-granular and inter-granular components. The evolutions of al1 stress components are directly related to the modification of dislocation densities and distributions in grains (over 50 grains). Back stress evolution is explained by heterogeneous dislocation patterns dissolution and formation of new ones with opposite polarization. This dissolution is promoted by both components of the back stress which enhance reverse slip and cross-slip under reverse straining. But only the cross-slip can induce plastic strain irreversibility and then permanent softening. The effective stress evolution results from a competition between latent hardening, dislocations emission at wall/channel interface and annihilation process. After stress reversal, the observed transient softening results from the evolution of dislocation distribution and density, in particular from the dissolution of the polarized structures (walls, cells) developed during pre-straining. The permanent softening occurs at reverse plastic strains higher than 2% when the initial heterogeneous dislocation structures are re-established by reverse loading. This permanent softening is mainly associated with the intra-granular back stress.