Role of strain path change in grain refinement by severe plastic deformation: A case study of equal channel angular extrusion

Equal-channel angular extrusion (ECAE) provides exciting opportunities to explore the role of strain path change (SPC) in grain refinement by severe plastic deformation (SPD). In this study, crystal plasticity simulations were carried out using a viscoplastic self-consistent model for a face-centered cubic model material processed via an extended range of processing routes and with two die angles (90° and 120°). Each processing route was defined according to the interpass billet rotation angle (χ), which varied from 0° to 180° at intervals of 15°. Based on a statistical analysis of the simulated slip activities, it is proposed that differences in grain refinement among these cases can be best correlated to key differences in the slip activities, i.e. the significance of newly activated slip systems at pass-to-pass transitions corresponding to macroscopic SPCs. Accordingly, grain refinement is anticipated to be most efficient for routes with χ near 75° for the 90° die or 0–45° for the 120° die, and least efficient with χ near 180° for both dies. The relative grain refinement efficiencies thus predicted are in good overall agreement with those indicated by the generation of high-angle boundaries and reduction of grain size in pure copper measured by electron back-scatter diffraction. It is suggested that the effect of SPC and the resulting characteristic slip activities should be incorporated in understanding the effectiveness of grain refinement and unpinning the underlying grain subdivision mechanisms in SPD with different SPCs.