The effects of low temperature fatigue on the RRR and strength of pure aluminum

Low temperature fatigue effects on residual resistivity ratio (
RRR = \rho_{273 K}/\rho_{4.2K}
) and strength of 300 and 1000 RRR aluminum are reported. The objective of this investigation is to select the best initial purity for the stabilizer aluminum used in energy storage magnets. Monolythic centimeter diameter specimens were fatigued at 4.2 K to strains (ε) reaching 0.3 percent. The resistivity ratio rapidly decreases during the first 100 cycles and approaches saturation (RRR
f
) after about 1000 cycles for all strains tested. The RRR
f
values are different for different initial resistivity ratio (RRR
i
) values, but all tend to come together at 0.3% strain independent of RRR
i
. The maximum specimen stress (
\sigma_{\max}
) is reached after about 1000 cycles also, and approaches a common value (
\sigma_{\max} = εE/2
, where ε is the strain range and E the elastic modulus) independent of RRR
i
. Thus high purity aluminum becomes "fully hard" at equilibrium and behaves elastically. The impact of fatigue damage on conductor design and choice of stabilizer purity is considered.