Nanoindentation twin-sensitive measurements and strengthening model of HPT OFHC 99.99% purity copper

Severe plastic deformation (SPD) techniques are among the most effective deformation modes of introducing a high rate and density of dislocations in metallic materials and alloys. The newly introduced dislocations have different characters. These were classified after Hansen and the Risœ group as statistically stored (SSD), also called incidental dislocations (ID), and geometrically necessary (GND) dislocations. As the strain cumulates some of these dislocations, namely the GNDs, are promoted to form very-low, low, and eventually high-angle boundaries. That is, new cell and grain structured are formed as the plastic deformation accumulate. Studies of the early stages of plastic deformation inducing microstructure modifications are properly carried out on pure metals as they strengthen only by the effect of dislocation, crystallite boundaries, and texturing of the metallic matrix. On this basis, the present work focuses on an electron microscopy study of the early plastic deformation stages induced in an OFHC 99.99% pure copper by high-pressure torsion (HPT). A threshold stress for the initiation of twinning formation within the Cu-grains was identified. Nanoindentation measurements were performed at different penetration depths. Thus, a correlation between the tip size-sensitive hardness evaluation (known as indentation size effect, ISE), occurring at the lower penetration depths, and the twinning formation during the early stages of HPT was found.