Developing ultrafine-grained microstructures through the use of severe plastic deformation

Thermo-mechanical processing is often used to achieve grain refinement in bulk solids but the grains produced in this way generally have sizes no smaller than within the micrometer range. Additional refining, to the submicrometer or even the nanometer range, may be achieved in bulk solids through the application of severe plastic deformation (SPD). Procedures available for applying SPD include Equal-Channel Angular Pressing (ECAP) where a sample is pressed through a special die constrained within a channel which is bent through an abrupt angle and High-Pressure Torsion (HPT) where a sample is subjected to a very high pressure together with torsional straining. Both of these procedures are effective in introducing a high density of dislocations and these dislocations subsequently re-arrange to give an array of grain boundaries having reasonably high angles of misorientation. This paper examines these two SPD processes and compares the results when both processes are applied to the same materials.