Surface Abrasive Torsion for Improved Mechanical Properties and Microstructure
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lastic deformation (SPD) process is an efficient process to manufacture high-strength ultrafine-grained (UFG) and nanocrystalline (NC) materials.[1,2] However, most of the SPD process results in poor ductility of the material. Recently, the adjustment of gradient structure has attracted great attention as a solution to low ductility, which is a critical drawback of UFG and NC materials. Lu and co-workers proposed surface mechanical attrition treatments for surface gradient nanostructuring,[3,4] and several studies on gradient structures have reported superior mechanical properties.[5–7] Among the various SPD processes, high-pressure torsion (HPT) is extremely effective in producing NC materials by imposing very high strains, but it has limitations in upscaling. This can be overcome by use of the simple torsion (ST) process. It should be noted that ST is a suitable process for manufacturing large bulk UFG and NC materials with gradient
JI HYUN MOON, SEUNG MI BAEK, SEOK GYU LEE, JAE IK YOON, SUNGHAK LEE, and HYOUNG SEOP KIM are with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea. Contact e-mail: [email protected] Manuscript submitted January 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
structure by application of high strain.[8,9] Recently, it was reported that the performance of ST can be improved by cyclic forward/reverse torsion[10–12] and by controlling the annealing temperature.[10,13] Theoretically, ST can proceed without any limitations on the imposed strain because the deformed geometry is identical to the initial one. However, from a practical point of view, infinite deformation cannot be applied due to the occurrence of surface fracture in the workpiece after a certain number of turns. Under the ideal condition, i.e., a specimen with a smooth surface, the maximum number of turns in ST is determined by the maximum stress reaching the critical value of the material. However, the maximum number of turns that can be achieved, in reality, with a rough surface is much lower than the critical value due to the effect of stress localization as a result of surface roughening during deformation.[14] To overcome this limitation, a new surface abrasive torsion (SAT) process is proposed in the present study. In this proposed process, the roughened surface is removed, thereby preventing the stress/strain localized regions generated during torsion. As a result, a larger amount of shear deformation can be applied to the specimen by SAT than by ST. The effect of SAT on microstructure, surface roughness, and hardness was compared with ST. The result of grain refinement after SAT shows the potential for greater torsion as SPD. In addition to the experimental results, a finite element method (FEM) simulation was also carried out to compare the effect of SAT and ST on the stress localization. For experimental study, low carbon steel produced by POSCO (Gwangyang, South Korea) was annealed at 900 C for 90 minutes and then furnace cooled for ful
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