Mechanical behavior and deformation kinetics of gradient structured Cu-Al alloys with varying stacking fault energy

In this paper, a series of Cu-Al alloys (2.2, 4.5, and 6.9 wt pct Al) with decreasing stacking fault energy (SFE) were processed by surface mechanical attrition treatment (SMAT) to obtain a gradient structure (GS). The yield strength of SMAT-ed samples is attributed to the volume fraction of GS layer and the ductility is associated with the dynamic recovery during deformation. Kocks-Mecking model was utilized to describe the storage and annihilation of dislocations in these samples. The results show that the minimal K2, which represents dynamic recovering, appears in Cu-4.5 wt%Al alloy with a medium SFE, indicating a suppressed dislocation annihilation and a delayed plastic instability during plastic deformation. Repeated stress relaxation tests were applied to characterize the activation volume and the evolution of mobile dislocation. It turns out that the highest relative mobile dislocation density (ρm/ρm0) emerges in Cu-4.5 wt%Al sample. This could be ascribed to the sufficient twin boundaries (TBs) which can keep the accumulated dislocations slipping easily and lower the exhaustion of mobile dislocation. The high ρm/ρm0 demonstrates that more mobile dislocation is preserved and the dislocation annihilation is effectively prohibited in the sample, in accordance with the low K2 obtained from Kocks-Mecking model, providing a reasonable interpretation for the good ductility.