Low-Temperature Heat Capacity of a Superconducting Alloy of Aluminum

Abstract

The specific heat of a precipitation-hardening alloy of aluminum (Alcoa 6063) has been measured down to 1.8°K in both the annealed and hardened conditions. From 1.8° to 4.2°K, the data for both conditions are well represented by $C=\gamma T+1.944\mathrm{}\times {10}^6{\left(\frac{T}{{\theta }_D}\right)}^3$ mjoule/mole-deg with $\gamma =1.33\mathrm{}\pm 0.02$ mjoule/mole-${\mathrm{deg}}^2$ and ${\theta }_D=430\mathrm{}\pm 4°$K for the annealed condition. In this temperature range, both $\gamma$ and ${\theta }_D$ increase for the alloy in the hardened condition. The changes are $\frac{d\gamma }{\gamma }=1.1\mathrm{}\pm 0.5$ percent and $\frac{d{\theta }_D}{{\theta }_D}=1.3\mathrm{}\pm 0.6\%$. No change in ${\theta }_D$ was observed from 10° to 20°K. The change in ${\theta }_D$ is quantitatively consistent with previous work on the shift of the superconducting critical field curve of this alloy with hardening. Both the present change in ${\theta }_D$ and the previously reported shift in $H_c$ indicate an internal compressive stress in the fully hardened alloy which is believed to arise from coherency stress in the early stages of precipitation. The observed change in $\gamma$ seems inconclusive with respect to the coherency stress hypothesis for reasons which are discussed.