Grain refinement and hardness saturation in pure Nickel subjected to a sequence of ECAP and HPT.

A detailed investigation was carried out to describe the microstructure grain refinement process and hardness evolution of pure nickel processed using a combination of two severe plastic deformation (SPD) techniques: equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). Two different shear deformation modes were used and compared: a co-operating shear deformation in which the same sense of sample rotation between ECAP and HPT was maintained; a reversing shear deformation in which the sense of rotation was inverted between ECAP and HPT. ECAP was conducted using the route Bcin which the sample was rotated counter clockwise by 90° between passes for up to 4 passes. Hardness was measured using nanoindentation techniques. It was found that the combination of ECAP + HPT allowed an effective grain refinement and hardening of nickel. Microstructure cell and grain refinement to sub-micrometer levels, and a significant metal hardening was achieved already at low ECAP + HPT strain regimes, thus demonstrating the beneficial synergic effect of the two plastic shear deformation techniques. Saturation limits in terms of cell size reduction (∼280 nm) and of hardness rise with cumulative straining (εtrue; ECAP+HPT > 7.5) were reached at ECAP-B/4 + HPT for P = 6 GPa and N=1 counter clockwise revolution, and it was anticipated to P = 3 GPa as the number of revolutions doubled.