Silver-Copper Composite Wires Prepared by Spark Plasma Sintering and Wire Drawing for Non-Destructive Pulsed High Magnetic Fields—Influence of Alloying on Electrical Resistivity and the Tensile Strength

LNCMI-Toulouse produces some of the most intense non-destructive pulsed magnetic fields in the world with a European record of 98.8 T and aims at reaching more than 100 T. The generation of high pulsed magnetic requires the use of coils wound of wires with exceptional properties. To limit the heating, wires need to show a high conductivity and they also must have a high strength to be able to resist the Lorentz forces. Classical methods for strengthening copper wires involve the preparation of composite wires, such as copper-stainless steel (Cu-SS) and copper-niobium (Cu-Nb) or alloyed wires such as copper/silver (Cu/Ag). The Cu/Ag microstructure presents Cu-rich and Ag-rich solid solutions. These Cu/Ag wires exhibit a UTS value higher than 1 GPa but also a high electrical resistivity.<br/><br/>For several years, LNCMI and CIRIMAT have been working on the preparation of Ag-Cu composite wires by powder metallurgy, spark plasma sintering (SPS) and wire drawing with the aim of obtaining Ag-Cu wires with a low electrical resistivity. We have already shown that Ag-Cu (0.5 - 10 vol. % Ag) composite wires present an UTS similar to that of Cu/Ag alloy wires containing about 20 times more Ag, and interestingly also show an electrical resistivity about 1.5 times lower.<br/><br/>In the present work, we prepare and study 1 vol. % Ag-Cu composite wires and Cu/Ag alloy wires. The samples were prepared using the same silver and copper powders, mixing and wire drawing processes but the cylinders (wire precursors 8 mm in diameter and 33 mm long) were sintered by SPS at different temperature.<br/><br/>Ag nanowires (diameter 0.2 µm, length 30 µm) were synthesized in-house and mixed with a micrometric Cu powder. One cylinder is sintered at 400 °C, where the solubility of Ag in Cu is below 0.1 vol. %, in order to obtain a composite microstructure, i.e. pure Ag nanowires dispersed in a pure Cu matrix. The second sample is sintered at 600 °C, where the solubility of Ag in Cu is equal to about 2.4 vol. %, which allows the Ag to dissolve into the Cu to form Cu/Ag alloy nanowires.<br/><br/>The diameter of the cylinders is reduced by wire drawing at room temperature, in several passes, thus producing progressively finer wires (diameter 1 - 0.2 mm). Wires with ultrafine elongated (200-700 nm) Cu grains oriented along the drawing axis are thus prepared. The pure Ag nanowires or the Cu/Ag alloy nanowires are dispersed along the grain boundaries of the Cu matrix.<br/><br/>The 1 vol. %Ag composite wires and alloy wires show similar UTS (1100 MPa at 77 K), reflecting an equivalent strengthening effect for pure Ag and Cu/Ag alloy nanowires. However, it is shown that the formation of a Cu/Ag alloy, despite it is highly localized, results in a significantly higher electrical resistivity compared to un-alloyed wires (0.56 <i>vs</i> 0.49 µΩ.cm, at 77 K).<br/><br/>This study confirms the importance of preparing composite wires in order to obtain the most suitable properties for high magnetic fields.