Metals (Jul 2021)
Decrease in Electrical Resistivity below 28 nΩm by Aging in Hyperperitectic Al-Zr Alloys Treated at High Temperatures
Abstract
High-voltage and alternating current conductors are mainly made up of a steel core and an envelope made of aluminum wires. The alloys commonly used in the shell are of pure aluminum, which provides low electrical resistivity, but also low mechanical resistance. The correlation between resistance and electrical resistivity has become a design limitation, which limits the development of new alloys. Traditional airline conductors limit their service temperature to a maximum of 80 °C. Al-Zr alloys are susceptible to hardening by aging treatment at high temperatures due to the precipitation of the metastable L12-Al3Zr phase, which allows their working temperature to be limited to 300 °C. This work aims to correlate the resistance and electrical resistivity of two alloys with hyperperitectic compositions, subjected to a solution treatment at 640 °C. At the same time, the aging potential at different temperatures is analyzed using aging duration of up to 900 h. Whereas, the ultimate goal is to correlate the results with electrical resistivity, trying to achieve resistivity values lower than 28 nΩm. It is worth mentioning that there is a slow aging process and the inverse correlation between hardness and electrical resistivity. The optimum result is achieved in an alloy with more than 0.4% Zr subjected to aging at 380 °C in a time range between 425 and 900 h. In these cases, electrical resistivity values lower than 26 nΩm were reached. Transmission electron microscopy (TEM) verified the nanometric size of the L12-Al3Zr precipitates and their coherence with the α(Al) matrix. In turn, the size distribution of the precipitates was analyzed, revealing two different families of precipitates with respective averages of 4 and 19 nm. The precipitates associated with the average of 4 nm are the majority when the aging time is 425 h. However, when the aging time increases to 900 h, sizes close to 20 nm are the majority. This confirms the slow growth of the L12-Al3Zr precipitates during aging and the beginning of the over-aging phenomenon after almost 900 h of aging.
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