A Study of the Internal Friction Associated with Discontinuous Precipitation in Lead-tin Alloys

Lead alloys containing up to 20 atomic percent (at%) tin were solution treated and then cooled through the solvus temperature while measuring the internal friction. At the start of the precipitation reaction the damping decreased as a result of β-tin (Sn) particles pinning grain boundaries. An internal friction peak was also observed and was found to be associated with the growth stage of the discontinuous precipitation reaction. Cooling rate and frequency dependence of the internal friction peak were observed to obey the previously proposed models for first-order phase transformations. Internal friction peak height increased as the cooling rate was increased and the frequency dependence indicated an inverse relationship with an exponent of 0.59, that is, f-0.59. The frequency dependence, with a slope greater than -1, supports the intergranular mode of discontinuous precipitation of β-Sn from α-lead (Pb) under our experimental conditions. The exact nature of the anelastic strain associated with the internal friction peak is yet unclear, but may be associated with a ledge-terrace mechanism for the cooperative growth of the β-Sn and α-Pb lamellae