Kao, Yi-Tang (2014-12). Effect of Severe Plastic Deformation and Subsequent Heat Treatment on Hardness and Electrical Conductivity of Oxygen-Free High Conductivity (OFHC) Copper, Commercial Pure Copper, and Copper Chromium Alloy. Master's Thesis.
Thesis
Samples of oxygen-free high conductivity (OFHC) copper (C101), commercially pure copper (C110), and copper chromium alloy (C182) were subjected to severe plastic deformation (SPD) using equal-channel angular extrusion (ECAE) to determine the effect of large amounts of plastic strain on the hardness and electrical conductivity for electrical conductor applications. Different levels of plastic strain and strain orientation combinations were applied by ECAE at room temperature. Heat treatments in the range 100?C to 500?C for times from 10 minutes to 4 days were applied to the materials after ECAE. The electrical conductivity and hardness were determined by four-point probe measurement and Vickers microhardness measurement. The hardness of all test materials increased significantly and the electrical conductivity decreased after ECAE, presumably because of the higher density of dislocations caused by the plastic strain. The properties changed most dramatically after a strain of ~2.3 and reached a near plateau after a strain of ~4. A post-strain heat treatment for temperatures at and above 250?C and for times of at least 1 hr. caused the conductivity and hardness to return to pre-strain levels (near 100 % IACS and VH 50) in C101 and C110, with the change occurring more rapidly for higher temperature annealing. For the case of C182, the post-strain heat treatment induced the highest hardness (VH 162) at 450?C for which the material had a conductivity of 76 %IACS. Copper 101 and 110 showed a plateau in hardness and conductivity after 3 hours heat treated at 150?C and higher; the hardness and conductivity of C182 did not reach stable values at 350?C and 450?C after 48 hours. SPD and post SPD heat treatment successfully improved the combination of hardness and electrical conductivity of the three Cu-based alloys studied for room temperature electrical conductor applications. The best combination of hardness and conductivity (99 %IACS and VH 137) occurred in C110 after two passes of ECAE (plastic strain of 2.3) and heat treated at 100 oC for 1 hour, for which the hardness increased by 58% over the fully annealed condition. The results of this work can be applied to other metals such as aluminum and silver contemplated for electrical conductor applications.
