Effects of Grain Size and Purity on the Low Temperature Cyclic Strain Degradation of Stabilizer Aluminum

Abstract

Aluminum specimens of 99.99% to 99.9998% purity and with grain sizes ranging from 0.1 mm up to 2.2 mm have been mechanically and electrically tested at 4.2 K to determine strain hardening and resistivity degradation characteristics. RRR values of the fully annealed (large grained) materials range from 1,150 up to 10,400. Results show that for a cyclic strain range of ±0.001 at 4.2 K, the effects of grain size are significant when purities are greater than about 99.9995%. At purities below 99.999%, the grain size has a minimal effect on the rate of strain hardening and on resistivity degradation. Mechanisms for the observed behavior are associated with increased strain hardening rates and a reduction in the total number of point defects (vacancies) generated during the low temperature deformation.