Study of Internal Friction in Aluminum - Magnesium Alloys

The internal friction of several commercial aluminum -magnesium alloys has been measured from 4.2K to 300K for the frequency range from 12kHz to 70kHz. The following results were found experimentally. (1) The internal friction in the temperature range 300K to 50K is primarily due to intercrystalline thermal currents. The data was fit to the theoretical model by fitting the data for several different frequencies with a distribution of grain sizes. (2) A small peak in the internal friction was observed in some of the samples at (TURN)100K. An activation energy, (DELTA)H, of 0.144 (+OR-) .001 eV and a relaxation time constant, (tau)(,o), of 2.4 (+OR-) 3 x 10('-12)sec were determined. The thermally activated motion of geometrical kinks on dislocation lines is consistent with the data. (3) For samples annealed at 540(DEGREES)C and cooled slowly or samples with no heat treatment, the Q('-1) for T < 30K was nearly independent of temperature. The Q('-1) can be described by Q('-1) = a + bf. The coefficient a appears to be due to frictional losses between the sample and the support. The coefficient b has a value of (TURN)3 x 10('-13): this is consistent with conduction electron damping of acoustic waves. The highest Q at low temperature was 1.46 x 10('8) in a sample of 5056-F at 12kHz. (4) For samples annealed at around 350(DEGREES)C and for samples quenched from 540(DEGREES)C, the low temperature (T < 50K) Q('-1) is temperature dependent. In one sample, the temperature dependence goes as T('3) and is consistent with dislocation resonance with damping through the fluttering mechanism. This is believed to be the first observation of this effect in an aluminum sample.