Dynamic high pressure torsion : a novel technique for dynamic severe plastic deformation

Severe plastic deformation (SPD) processing of polycrystalline metals is often used to obtain ultra-fine grained and nano-crystalline microstructures. Increasing the strain rate during the SPD process is known to have a favorable effect on both grain fragmentation and mechanical properties, mainly through its effect on dislocation densities and twinning. However, dynamic SPD is barely explored, mainly because of the lack of suitable experimental techniques. In the present paper, a novel dynamic high pressure torsion facility is presented which imposes a dynamic, torsional deformation to a sample subjected to high hydrostatic pressure. The setup combines basic features of both split Hopkinson torsion bar (SHTB) and static high pressure torsion (HPT) devices. Torsional speeds up to 30,000 rpm can be reached, giving rise to strain rates in the sample up to 10,000/s. The first series of tests have been performed on commercially pure aluminum. The effect of DHPT processing on the mechanical properties and microstructure is evaluated by hardness and electron back-scatter diffraction measurements, respectively. The properties and microstructural features are compared with those obtained in conventional, statically deformed HPT samples