Residual Stress and Sample Size Effect in Equal Channel Angular Pressing (ECAP) Processed OFHC Copper by the Neutron Diffraction and Finite Elements Method

Abstract

A number of techniques have been developed for the Sever Plastic Deformation (SPD) to achieve the ultra-fined grains (UFG) in the materials with excellent strength and ductility. High-pressure torsion (HPT), equal-channel angular pressing (ECAP), accumulative roll-bonding (ARB) twist extrusion (TE), multi-directional forging, repetitive corrugation and straightening (RCS), cylinder covered compression (CCC), and multi directional deformation (MDD) are processes to name a few. ECAP is the well-known process for fabrication of UFG materials since the last few decades to enhance the mechanical properties of metals and other alloying elements without altering the original size of the specimen.SPD by ECAP has been intensively investigated for the last decade due to its capability of producing large fully dense work pieces having UFG microstructures. Since stress and strain, governing the microstructure and property evolutions in ECAP, are inhomogeneous and their histories are complicated, good understanding of the deformation behavior including Residual stresses (RS) needs both experimental and theoretical analyses. Generally, the RS significantly influences the performance and the lifetime of engineering components

hence, investigating the RS is critically important, while the RS in ECAP processed materials have not been investigated as far as I know. In this thesis, the measurement of the RS using Neutron Diffraction (ND) method has been carried out in the ECAP processed OFHC. In the current study Oxygen-Free High Thermal Conductivity (OFHC) was pressed upto two passes for routes A, BC, and C. The ND experiments for the measurement of the RS have been conducted in KAERI HANARO. In case of route A and one pass samples, the bottom part of the specimen, which is less sheared, showed tensile RS and the top of the specimen, which is highly sheared, showed compressive behavior. In the cases of route BC and route C the bottom region of the specimen showed compressive RS because of the orientation of planes involved in respective routes. The evolution of the microstructure and the hardness measured on the plane perpendicular to the extrusion axis verified the experimental RS results. The finite elements method (FEM) simulation was carried out to understand the results obtained from the ND experiments. It has been shown that the stress behavior observed in the FEM simulation is qualitatively in good agreement with the ND results. The experimental results will be not only a valuable data as it is but also good evidence for accessing the validity of the theoretical analysis. Sample size effect has been studied by conducting experiments for specimens with two different diameters. It has been shown that smaller specimen shows enhanced mechanical properties as compared with the specimen with bigger diameter. X-ray diffraction (XRD) analysis has shown that smaller specimen suffers more deformation as compared with the bigger specimen, which resulted in more peak broadening and higher dislocation density in smaller specimen.