Acoustic quality factor of aluminum alloys from 50 mK to 300 K

Measurements of the acoustic quality factor (reciprocal internal friction) of commercial aluminum alloys 2090, 5056, and 6061 in a torsional vibrational mode at a frequency of 1 kHz in the temperature range from 50 mK to 300 K are reported. The temperature dependence of the mode frequency, which is a measure of the shear modulus of the material, is also given. The quality factor (Q) of all alloys exhibited the highest values at the lowest measurement temperature. A heat-treated sample of the 5056 alloy gave the highest Q of 6.5×107 at 53 mK. Maximum Q values of 8.9×106 in 2090 and 6.5×106 in 6061 were observed. Internal friction mechanisms which may explain the measured Q values are discussed. The results provide support for the planned construction and operation of very sensitive 50-mK aluminum-alloy bar gravity-wave detectors.