The Impact of the Stacking Fault Energy of Nanostructured Metals on Phenomena During Annealing at the High Hydrostatic Pressure

The present study investigates the impact of stacking fault energy on the microstructure evolution and mechanical properties of nanostructured metals that differ in stacking fault energy, annealed under high hydrostatic and atmospheric pressure. Ag and Ni were selected as materials of low and high stacking fault energy, respectively. To this end, nanostructured metals were obtained by high pressure torsion and subsequently annealed by high hydrostatic pressure annealing, performed under 2 GPa at 0.4 homologous temperature for 1h. For comparison, similar experiments at the same temperature and time were performed under atmospheric pressure. After deformation and annealing, the microstructures were examined using transmission and scanning electron microscopy, and further analysed in terms of grain size, coefficient of grain size variation, and twinning frequency. The stored energy and peak temperatures were measured by differential scanning calorimetry. The mechanical properties were evaluated from microhardness measurements and tensile tests. It is demonstrated that the pressure applied during annealing leads to a more profound retardation of microstructure evolution in the low stacking fault energy material, mainly due to a higher deformation nanotwin density. The twinning deformation mechanism generates a higher dislocation density and a lower grain size than those achieved by dislocation slip.