Microstructural evolution and mechanical properties of oxygen-free copper processed by severe plastic deformation

Alawadhi, Meshal, Y (2017) Microstructural evolution and mechanical properties of oxygen-free copper processed by severe plastic deformation. University of Southampton, Doctoral Thesis, 378pp.

Record type: Thesis (Doctoral)

Abstract

This thesis presents a study on the microstructural evolution and mechanical properties of oxygen-free copper processed by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature. Experiments were systematically conducted to examine microstructural stability and deformation mechanisms, and their influence on the mechanical properties.

A significant grain refinement was attained after deforming the material by 24 passes of ECAP and 10 turns of HPT. The microstructure in the steady-state condition consisted of equiaxed ultrafine grains (UFGs) with high-angles of misorientation. Hardness values increase with increasing number of ECAP passes and HPT turns, and the microhardness distribution was relatively homogenous.

ECAP and HPT samples were pulled to failure at room temperature using strain rates of 1.0×10^-4 s^-1, 1.0×10^-3 s^-1 and 1.0×10^-2 s^-1. The direct influence of recovery behaviour on the tensile properties was investigated. A simultaneous increase in strength and ductility were observed with increasing number of ECAP passes as well as HPT turns. This is due to occurrence of dynamic recovery at an equivalent strain of ~12 that decreases the total dislocation density and restores the work hardening ability of oxygen-free copper. Both grain boundary and dislocation strengthening mechanisms contribute to the strength of oxygen-free copper. Higher ductility but lower strength was observed when using lower strain rates.

UFG copper samples produced by HPT were stored at room temperature for 12 months to investigate microstructural stability and self-annealing phenomena. The results show that the samples processed by a low number of turns exhibit lower thermal stability after storage of 12 months in comparison to the samples processed by a high number of turns. A significant decrease in the hardness was recorded near the edges of the discs processed by 1/4, 1/2 and 1 turn due to recrystallization and grain growth whereas a minor drop in hardness values were observed in the samples processed by 3, 5 and 10 turns, and this drop was related to the recovery mechanism. Tensile tests were repeated after 12 months and the results showed that the ductility was enhanced in compensation for strength.

To investigate the deformation mechanism and thermal stability under high strain rates, copper samples were subjected to 1, 4 and 8 passes of ECAP and further deformed by dynamic testing. A significant grain refinement was produced in the ECAP specimens after dynamic testing which is comparable to the grain refinement produced by severe plastic deformation (SPD) techniques such as ECAP and HPT. The grain refinement mechanism was mainly by dislocation slip in the specimen processed by 1 pass whereas it was through dynamic recrystallization for the specimen processed by 8 passes. This is due to the difference in the dislocation densities and stored energy between the ECAP specimens. A 1 pass specimen has better stability than 4 and 8-pass specimens during dynamic testing. It was also shown that increasing the testing temperature and/or the strain rate can highly influence the deformation mechanism.