Stacking fault energy (s.f.e.) and grain size effects (d) on the tensile behaviour of f.c.c. polycrystalline alloys at 300 K: Back stress and effective stress evolutions

The aim of this work is to provide experimental results to understand grain size and stacking fault energy effects (gamma/mub) on tensile hardening f.c.c. alloys. The hardening rate is discussed in terms of back stress (X) and effective stress (Sigma (ef)) evolutions. Irrespective of the material studied, tensile hardening behaviour before necking is divided into three stages (I, II, and III). These stages were previously discussed using qualitative and semiquantitative TEM observations [1]. In particular, we have shown that intergranular back stress evolution relates the hardening rate in stage 1, where single and planar slip are observed in most of the grains. In the other stages, latent hardening and intragranular back stress are the main parts of the hardening rate in relation with the formation of heterogeneous dislocation structures. An increase of grain size and/or a decrease of stacking fault energy favour planar slip and then stage 1, in terms of plastic strain. The transition between stage If and stage III seems to be less dependent on grain sizes irrespectively of s.f.e.. The classical Hall-Petch relation is discussed in terms of back and effective stresses for different plastic strain levels. If these two components verify the Hall-Petch relation, however, effective stress is less dependent on grain size than back stress, This last dependence increases in stage 1, where intergranular back stress is the main part of hardening and decreases in the other stages where this component decreases and intragranular back stress increases. The grain size effect on effective stress is well explained in terms of mean length path using dislocation modelling.