Temperature Dependence of Sound Attenuation in Aluminum

The object of this experiment was to measure the attenuation of sound waves in commercial aluminum as a function of temperature in the range between the temperature of boiling nitrogen (78*K) and room temperature (300'K). Several attenuation mechanisms have been known for some time; for instance, thermal loss mechanisms, which occur when a solid is compressed. The compressed parts become warmer than the extended parts, and the heat flow that occurs between these parts abstracts energy from the wave. Another example is the scattering of sound in polycrystalline materials in which the random grain orientation causes an impedance mismatch between adjacent grains and produces a scattering of sound and a loss of energy. The tendency of atoms to interchange position under thermal agitation can also cause an ultrasonic attenuation. In recent years, attenuation effects for which there is no theoretical explanation have been observed. The presence of a type of imperfection in metals, known as a dislocation, has provided a partial answer. Dislocation phenomena have been classified into two types. The first, an edge dislocation, is a region in which a plane of atoms has slipped into or out of an otherwise perfect crystal. The other type of dislocation, known as a screw dislocation, is the boundary between a slipped and an unslipped region of the lattice. Measurements at low temperatures indicate the presence of attenuation maxima whose position on the temperature scale is a function of the frequency of measurement. It has been suggested that these peaks may be caused by a relaxation mechanism whereby dislocations are displaced by thermal agitation between their pinning points into adjacent minimum energy wells to form pairs of kinks. These kinks can be formed by stresses less than the Peierls stress. If the angular frequency of the applied stress is greater, or less, than the frequency of thermal formation of kinks, very little energy is lost during each stress cycle (attenuation is small). A maximum attenuation occurs when the angular frequency of the applied stress is close to the frequency of kink formation. The apparatus used in measuring the attenuation of acoustical waves in metal bars