Development of microstructure in silver subjected to severe plastic deformation [Sviluppo di microstrutture in campioni di argento sottoposti a deformazione plastica severa]

Commercially pure silver has been subjected to severe plastic deformation (SPD) by ECAP and by asymmetric cold rolling (ASR). Microstructure evolution as a function of imparted equivalent strain (up to about 8) has been investigated mainly by SEM and TEM. Mechanical properties have also been measured by room-temperature tensile testing. Silver is here selected as a representative FCC metal with a low stacking fault energy featuring high probability of deformation by twinning. Limited information exists on its structure evolution in the ultrafine grain-size range after SPD. Due to its ductility, the silver samples could be successfully deformed by the two processing techniques up to strain values exceeding 8. The ECAP processed materials featured a submicrometer-size equiaxed grain structure with sharp grain boundaries. On the contrary, both symmetric and asymmetric rolling led to a subgrain structure with a higher dislocation density at grain interiors and less defined grain boundaries. The tensile properties achieved after the different processing routes consistently differed. In ECAP samples the strength improved at first passes and then showed a plateau for the whole range of imposed strain here considered. In asymmetrically and symmetrically rolled silver, the achieved strength almost continuously improved even at larger strains. An estimate of the work-hardening behaviour over the whole strain range of the silver samples was computed by adding the experienced strain supplied during processing to that experienced during tensile testing. It was confirmed that in the asymmetrically rolled samples, the overall work-hardening behaviour was higher with respect to ECAP samples, at least up to equivalent strains of around 8.