Softening and Microstructural Coarsening without Twin Formation in FCC Metals with Low Stacking Fault Energy after Processing by High-Pressure Torsion

Abstract

Gold (Au), silver (Ag) and copper (Cu) were severely deformed through the process of high pressure torsion (HPT). The grain sizes were reduced to the submicrometer range and the hardness increased significantly by the HPT process. However, after holding at room temperature, Au and Ag exhibited grain growth and thus softening without heating. The softening and microstrcutural coarsening occurred rather quickly in Ag and moderately in Au but nothing changed in Cu even after keeping for prolonged time. No twins were formed along with the grain growth in the HPT-processed Au and Ag. High density lattice defects and enhanced diffusivity in the metals are responsible for such an unusual softening behavior.