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) an d effective stress (Sigma (ef)) evolutions. Irrespective of the material st udied, tensile hardening behaviour before necking is divided into three sta ges (I, II, and III). These stages were previously discussed using qualitat ive and semiquantitative TEM observations [1]. In particular, we have shown that intergranular back stress evolution relates the hardening rate in sta ge 1, where single and planar slip are observed in most of the grains. In t he other stages, latent hardening and intragranular back stress are the mai n parts of the hardening rate in relation with the formation of heterogeneo us dislocation structures. An increase of grain size and/or a decrease of s tacking fault energy favour planar slip and then stage 1, in terms of plast ic strain. The transition between stage If and stage III seems to be less d ependent 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 relat ion, however, effective stress is less dependent on grain size than back st ress, This last dependence increases in stage 1, where intergranular back s tress is the main part of hardening and decreases in the other stages where this component decreases and intragranular back stress increases. The grai n size effect on effective stress is well explained in terms of mean length path using dislocation modelling.