INTRODUCTIONMany low temperature «20 K) conductor applications require materials with particular combinations of properties such as resistivity, magnetoresistivity, and strength. Depending on the situation, superconducting composites, metal-metal composites, or monolithic conductors are used. In all cases, however, oxygen-free high-conductivity (OFHC) copper and high-purity aluminum are considered and are often in direct competition. Furthermore, for any application, optimization of the resistivity, magnetoresistivity, and strength of these high-purity metals is necessary in order to achieve the best possible performance of the conductor.The ideal characteristics for low-temperature conductors are high strength combined with low resistivity. Unfortunately, methods of improving strength typically degrade the resistivity.! With conventional alloying, for example, the increase in residual resistivity (p 42K) due to the solute elements can be extremely large because the solute concentrations commonly associated with alloying are high. By using relatively small amounts (-500 weight ppm [wppm]) of solute to form dilute aluminum alloys, limited strengthening can be achieved and the residual resistivity can be kept reasonably low. 2 This dilution can be taken a step further by limiting the solute concentrations to microalloy levels (<100 wppm). In addition, by selecting elements that affect resistivity to a lesser extent, relatively higher solute levels can be specified and higher levels of strengthening can be achieved. The work described in this article represents such an approach to improving the strength and resistivity properties of high-purity aluminum.
EXPERIMENTAL PROCEDURESSpecimens were cast, worked, and machined by Sumitomo Chemical Company, Ltd. (Tsukuba, Japan). The fabrication steps for the 150 mm x 10 mm diameter samples are described in Table 1. Steps 11-13 in Table I were carried out at Texas A&M University.Residual resistivity measurements of the specimens were made at 4.2 K using the Eddy Current Decay (ECD) technique. 3 ,4 The ECD technique is a simple, contactless method that is more easily applied to bulk aluminum samples than is the conventional four point potential method. The accuracy in resistivity measurement is within ±2.0% (for resistivities greater than about 0.5 nQcm, the accuracy is within ±1.0%).Mechanical strength measurements were also made on the test specimens. Although it will not be discussed in this communication, resistivity degradation due to low-temperature plastic deformation in these materials was of interest. During cyclic deformation experiments, which are described in Reference 5, the flow stress values were obtained at a strain level of 0.1 %. The entire testing apparatus is also described in detail in Reference 6.Chemical composition analyses using corrected Glow Discharge Mass Spectrometry (GDMS) were performed by Sumitomo for accurate determination of solute concentrations; the maximum concentration of residual impurities was found to be 3.9 wppm. Optical analyses of g...
