Acoustic quality factor of aluminium and selected aluminium alloys from 50 mK to 300 K Original Research Article

The amplitude-independent quality factors (Qs) of polycrystalline aluminium of 99.999 wt% purity, and the commercial aluminium alloys 2024, 6061 and 7075 have been measured in the temperature range 50 mK–300 K. Data were obtained at about 1 kHz from a torsional mode of machined monolithic mechanical resonators. The as-machined alloys exhibited maximum Qs of 1.5 to 9.5 million. The annealed alloys exhibited maximum Qs from 4.3 to 7.4 million. After several weeks at room temperature, substantially higher Qs were observed in all of the annealed materials, as compared to values obtained within a few days subsequent to the thermal treatment. Nevertheless, the maximum Qs were appreciably lower than those which have been measured in aluminium–magnesium alloys. Three loss peaks were observed in the aluminium, and a single peak in two of the alloys, even after annealing. The temperature dependence of the shear moduli was also obtained. This study is part of the continuing effort in this laboratory to evaluate the acoustic loss of high-Q materials at low temperatures, of particular interest for the construction of sensitive detectors of gravitational radiation.