Moreover, the application of HPT is no longer limited to metallic materials-the method has been applied to nonmetallic materials such as carbon polymorphs, silicon polymorphs, oxides, oxynitrides and glasses as well.
Besides the popular application of the method to produce UFG materials, it has also been used to control phase transformations, consolidate powders and conduct mechanical alloying. Within the past three decades, the technology of the HPT method has not changed significantly however, the method has been used for a wider range of materials and applications. This study resulted in the development of the Nano-SPD field and accelerated the research activities on the HPT method by many other research groups. introduced the HPT method as an SPD technique to achieve nanograined microstructures in metallic materials. The method was originally introduced by Bridgman in 1935 to examine polymorphic phase transformations under high pressure (see the historical development of the HPT method in ). In this method, as schematically shown in Figure 1, a small disc sample is compressed between two anvils under a pressure of several giga-Pascals and, concurrently, shear strain is introduced in the sample by rotating one of the anvils with respect to the other one ( γ = 2π rN/ h, γ : shear strain, r: distance from disc center, N: anvil turns, h: sample thickness). High-pressure torsion (HPT) is a severe plastic deformation (SPD) method, which is widely used to generate nanograins or ultrafine grains (UFG) in metallic materials.