Investigating Anvil Alignment and Anvil Roughness on Flow Pattern Development in High-Pressure Torsion
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Investigating Anvil Alignment and Anvil Roughness on Flow Pattern Development in High-Pressure Torsion Yi Huang1, Megumi Kawasaki2,3, Terence G. Langdon1,2 1
Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, U.K. 2
Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, U.S.A. 3
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea
ABSTRACT High-pressure torsion (HPT) is a processing technique in which samples are subjected to a high pressure and torsional straining. Anvil alignment and anvil roughness are two important factors related to the successful application of the HPT processing technique. Using a two-phase duplex stainless steel as a model material, experiments were conducted by placing the anvils in different amounts of initial misalignment. Experiments show that the flow patterns (the development of double-swirl patterns) in HPT are dependent upon the alignment of the anvils within the HPT facility. Through carefully designed experiments, it is shown that the presence of a double-swirl is a feature of HPT processing when the initial positions of the anvils have a small lateral misalignment. The effect of the double-swirl patterns on the hardness evolution was also evaluated quantitatively. By comparing the flow patterns developed on the disc upper surface using both rough and smooth anvils with a fixed anvil misalignment, it was demonstrated that there are some differences in the flow patterns which are dependent upon the anvil surface roughness.
INTRODUCTION High-pressure torsion (HPT) is a well-known and widespread processing technique for severe plastic deformation which is capable of producing ultrafine-grained and nanocrystalline metals by imposing high strains on various coarse-grained materials [1-3]. The principles of HPT originated from torsion tests carried out by Bridgman [4] who showed that in a torsion test the fracture strain can be further increased by applying hydrostatic pressure. Thereafter, the method of combining torsion and hydrostatic pressure together to deform materials was developed in the 1980s [5] and is now commonly designated as HPT. There are three different types of HPT facilities termed unconstrained HPT, constrained HPT and quasi-constrained HPT, respectively. In unconstrained HPT, the disc is placed between two flat anvils and the lateral flow of the material is not restricted under the applied pressure. In constrained HPT, the disc is placed within a cavity in the lower anvil so that the lateral flow of the material is totally restricted under the applied pressure. In practice, however, most HPT processing is now conducted under quasi-
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