Internal friction studies were performed on two aluminum-based alloys: aluminum-indium and aluminum-lead. Each system was processed to obtain microstructures consisting of an aluminum matrix dispersed with inclusions of nominally pure indium or lead. The inclusion volume fraction was varied from 0.02 to 0.07 (6 to 16% by weight) for aluminum-indium alloys and from 0.01 to 0.02 (4 to 8% by weight) for aluminum-lead alloys. Internal friction was measured as a function of time, temperature, temperature scan rate, frequency, and strain amplitude using a dynamic mechanical analyzer.
Sharp internal friction peaks were observed near the melting temperatures of 156 and 327°C for the aluminum-indium and aluminum-lead alloys, respectively. These peaks were associated with a matrix relaxation that accommodated the change in volume associated with the melting transition. The internal friction at the melting transition exhibited many of the internal friction characteristics associated with the polymorphic diffusionless transformations. Internal friction peak height associated with these melting transitions was found to be proportional, but not inear, to the ratio of the heating rate to the frequency of oscillation. Also, the peak height was observed to increase proportionally with the volume fraction of the inclusion. Increasing the applied strain amplitude reduced the internal friction peak height and the peak was observed to disappear completely at strain amplitudes of greater than 0.02 for aluminum-indium and 0.01 for aluminum-lead alloys. An internal friction peak was not observed when a laminate of aluminum and indium was tested. It was postulated that the internal friction peaks observed near the melting transition temperature of the embedded inclusions were related to a matrix relaxation around the inclusion. Only when the volume change of the melting inclusions is fully constrained by the matrix is an internal friction peak observed. Similar effects were observed for cooling experiments where the inclusions solidified.