Ring-Constraint High-Pressure Torsion Process
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MATERIALS with nano/ultrafine structure became competitive in engineering applications because of demand for advanced materials due to their advantageous mechanical properties. These strong points include high strength, reasonable ductility, and better fatigue strength.[1,2] Recently, several methods to achieve severe plastic deformation (SPD) under high hydrostatic pressure have been developed. They include a number of processes, e.g., equal-channel angular pressing (ECAP), high-pressure torsion (HPT), accumulative roll bonding (ARB), twist extrusion (TE), by which to produce bulk materials with nano/ultrafine structure 20 to 200 nm average grain sizes.[3–5] Among various SPD methods, the HPT process, a novel method for producing nanostructured materials, has been widely investigated because of its ability to impose both extremely high strain and hydrostatic pressure to develop homogeneous nanostructures with high-angle grain boundaries (HAGBs) and high intrinsic stresses.[6] Moreover, the HPT process can be applied not only to solid materials but also for consolidation of powders to produce densely packed bulk materials.[7–10] One interesting merit of the HPT process is its ability for processing hard and brittle materials, without fracturing, due to the high hydrostatic pressure generated. However, this is not always true in cases of extremely brittle materials.
SOO-HYUN JOO, Assistant Professor, is with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. HYOUNG SEOP KIM, Professor, is with the Pohang University of Science and Technology and also with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail: [email protected] Manuscript submitted September 3, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
There are three types of HPT process: unconstrained, quasi-constrained, and constrained HPT.[6] In constrained HPT, a workpiece can be processed without the outward flow of materials due to the presence of an effective back pressure. However, it is technically difficult to achieve a perfectly constrained condition. Therefore, quasi-constrained (QC) anvils are usually used for the HPT process, where the flow of materials from the center to the periphery is unavoidable. In the QC condition, extremely brittle materials cannot be processed due to a sudden fracture, even if high hydrostatic pressure is applied. In general, hydrostatic pressure increases ductility of metallic materials.[11] The fracture of brittle materials occurs due to insufficient compressive hydrostatic stress during the HPT process. Furthermore, hydrostatic pressure is very important in powder metallurgy. A top–down approach that applies intense strains was studied for the production of bulk fully dense ultrafine-grained materials from various metal powders using the HPT process.[7–10] It is known that cracks and failures due to shear deformation occurring during the ECAP process can be effectively prevented by applying back press
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