Nominal and effective strains in severe plastic deformation processes

Severe plastic deformation (SPD) processes may be used for obtaining materials with a nanocrystalline structure and enhanced properties. Recent work on such methods is briefly reviewed, with special emphasis on processes resulting in only minor shape changes, i.e. multi-step forging (MSF), high-pressure torsion (HPT) and equal-channel angular extrusion (ECAE). The nominal strain levels achieved by these processes are discussed, taking into account various possible conditions of deformation (at constant or variable thickness for HPT, according to the exact shape of the die for ECAE). In actual operation, strain values may deviate from the calculated ones, particularly due to strain inhomogeneities (along the sample radius or through the thickness for HPT, longitudinal or transverse inhomogeneities in ECAE). The domains of application for these methods are considered.