Twinning and dislocation activity in silver processed by severe plastic deformation

Severe plastic deformation (SPD) provides attractive procedures for producing ultrafine-grained (UFG) metals in bulk form [1, 2]. One of the most frequently used methods is equal-channel angular pressing (ECAP) where it is possible to produce relatively large bulk UFG metals having dimensions of several centimeters in all directions [3]. It is now well established that the high strength of metals processed by ECAP is due to their high dislocation densities and small grain sizes [4]. There have been extensive reports documenting the evolution of the microstructure during the processing by ECAP of pure face-centered cubic (fcc) metals having medium or high stacking fault energies (SFE). Thus, it was reported that in metals such as Al and Cu, the grain size reaches a minimum value and the dislocation density saturates after about 4 passes of ECAP [4]. Nevertheless, very little information is available at present describing microstructural evolution during ECAP in pure fcc metals where the SFE is very low. It was shown recently that in Ag, where the SFE is only *16 mJ m-2 [5], the dislocation density was *46 ± 5 9 1014 m-2 after 8 passes of ECAP [6, 7] where this density is exceptionally high by comparison with other fcc metals, such as Au or Cu [6, 8–10]. The high dislocation density in Ag is a consequence of the very low SFE because the annihilation of dislocations is hindered by their high degree of dissociation into partials. The present investigation was therefore initiated to provide a comprehensive report on the evolution of grain structure and dislocation and twin densities when Ag is processed to very high strains up to a maximum of 16 passes through the ECAP die. As will be demonstrated, the experimental results are consistent with, and provide the first direct support for, a theoretical model of deformation in fcc metals with low SFE developed earlier by Mu¨llner and Solenthaler [11].