Springs, Jason Cole (2017-08). Mechanical Properties, Microstructure, and Electrical Resistivity of ECAE Processed OFHC Copper for High Strength and High Conductivity Applications. Master's Thesis.
Thesis
In recent years, superconductors have become a topic of great interest in the scientific and industrial communities due to their ability to carry large currents with zero resistivity. Most superconducting wires have a surrounding matrix material, commonly made of copper, which provides mechanical stability as well as an electrical shut and thermal sink/link. This matrix is vital to the correct and continuous operation of superconductors, and thus must have the correct mechanical and physical properties. Specifically, the matrix material strength and conductivity must be as high as possible. Perhaps the best way to enhance a pure metal's strength without significantly reducing conductivity is through work hardening. By severe plastic deformation (SPD) an ultrafined grained (UFG) material that improved mechanical properties with a minimal increase to resistivity. In this study, oxygen free high conductivity (OFHC) copper was processed by equal channel angular extrusion (ECAE) and then tested for strength, hardness, microstructure, and residual resistivity. Some of the effects of post processing heat treatment and rolling were studied. The objective of this study is to determine the best processing procedure to develop OFHC copper for its use in a superconductor, or any high strength high conductivity application. The ECAE routes studied include 1A, 2A, 4A, 8A, 4B, 8B, 4Bc, 8Bc, 16Bc, 4C, 8C, 4E, 8E, and 16E. In recent years, superconductors have become a topic of great interest in the scientific and industrial communities due to their ability to carry large currents with zero resistivity. Most superconducting wires have a surrounding matrix material, commonly made of copper, which provides mechanical stability as well as an electrical shut and thermal sink/link. This matrix is vital to the correct and continuous operation of superconductors, and thus must have the correct mechanical and physical properties. Specifically, the matrix material strength and conductivity must be as high as possible. Perhaps the best way to enhance a pure metal's strength without significantly reducing conductivity is through work hardening. By severe plastic deformation (SPD) an ultrafined grained (UFG) material that improved mechanical properties with a minimal increase to resistivity. In this study, oxygen free high conductivity (OFHC) copper was processed by equal channel angular extrusion (ECAE) and then tested for strength, hardness, microstructure, and residual resistivity. Some of the effects of post processing heat treatment and rolling were studied. The objective of this study is to determine the best processing procedure to develop OFHC copper for its use in a superconductor, or any high strength high conductivity application. The ECAE routes studied include 1A, 2A, 4A, 8A, 4B, 8B, 4Bc, 8Bc, 16Bc, 4C, 8C, 4E, 8E, and 16E. Tests on these samples included tensile tests, hardness tests, differential scanning calorimetry (DSC) analysis, microscopy, and residual resistivity. Significant results show a maximum as-worked strength of ~440 MPa for ECAE and ~495 MPa for ECAE plus rolled samples. Hardness and strength saturate after four ECAE passes, with only incremental changes in strength for eight and 16 passes. Heat treatments show that recrystallization temperatures have an inverse relationship to applied strain. Route Bc was shown to give the smallest average as-worked and recrystallized grain size at ~415nm and ~1.4um respectively. Residual resistivity testing resulted in decreasing values with respect to strength. Grain size and strength are shown to have a linear relationship, as well as those of residual resistivity ratio with both strength and grain size. Lastly, it was determined that a lower number of ECAE passes results in the best ratio of strength to resistivity.
