*4.2. The E*ff*ect of High-Pressure-Torsion*

HPT-processing of a material results in significant increases of densities of dislocations, vacancies and agglomerates during or after HPT-processing. As the dislocations partially arrange into grain boundaries, a decrease in grain size causing an increase in strength and/or hardness takes place.

With HPT, a material workpiece can be exposed to very large torsional strains under hydrostatic pressures up to 10 GPa [55,56]. The large hydrostatic pressure suppresses the annihilation of lattice defects and thus provides grain refinements down to several nanometers and even until an amorphous state. Grain sizes in Mg and other alloys HPT-processed at room temperature can reach around 100 nm [57–62].

As already mentioned, during HPT-deformation of the investigated alloys to 0.5 rotation (γ*T*~20) (Figure 11) at room temperature, microhardness increased by up to 130% compared to the furnace-cooled IS, while the increase reached only 80% compared to the quenched IS. When deforming the samples for two rotations and more (γ*<sup>T</sup>* =20-100), microhardness increased further, up to 190% for the furnace-cooled, and up to 100 % for the quenched samples. Again, the samples in the IS (furnace-cooled) showed slightly larger hardness at γ*<sup>T</sup>* > 20 than the samples in the IS (quenched). This effect may be explained by the fact that in the furnace-cooled samples—in contrast to the quenched ones—some precipitates may exist before HPT, which stimulates the formation of deformation-induced defects, contributing to hardening. Nevertheless, after HPT-processing, both the conditions—furnace-cooled and quenched—reached almost the same hardness level.
