Producing Bulk Ultrafi ne-Grained Materials by Severe Plastic

In practice, the presence of a large fraction of high-angle grain boundaries is important in order to achieve advanced and unique properties. 4 Processing by SPD refers to various experimental procedures of metal forming that may be used to impose very high strains on materials leading to exceptional grain refi nement. A unique feature of SPD processing is that the high strain is imposed without any signifi cant change in the overall dimensions of the workpiece. Another feature is that the shape is retained by using special tool geometries that prevent free fl ow of the material and thereby produce a signifi cant hydrostatic pressure. The presence of this hydrostatic pressure is essential for achieving high strains and introducing the high densities of lattice defects necessary for exceptional grain refi nement. The principles of SPD processing