Twenty-five years of ultrafine-grained materials: Achieving exceptional properties through grain refinement

Twenty-five years ago, in 1988, there appeared a classic description of the application of severe plastic deformation (SPD) to bulk solids in order to achieve exceptional grain refinement to the submicrometer level. This report and later publications initiated considerable interest in materials science laboratories around the world and many experiments were subsequently performed to evaluate the principles and practice of SPD processing. The present report provides an overview of the more recent developments in this field, with special emphasis on the opportunities for achieving homogeneity in the as-processed materials and on the general characteristics of the mechanical properties achieved after SPD processing. For simplicity, special emphasis is placed on the two techniques of equal-channel angular pressing and high-pressure torsion as these are currently the most popular procedures for applying SPD processing.