Internal friction in alloys due to plasticity of the diffusion phase transformation

An analytic expression is obtained for the time dependence Q(-1)(t) of internal friction associated with plasticity of a phase transformation. Time dependences Q(-1)(t) of internal friction of the Pb-62Sn and Pb-1.9Sn alloys (wt.%) alloys were studied in the regime of continuous excitation of resonant flexural vibrations. The measurements of the Q(-1)(t) dependences for 1 h at room temperature and a fixed strain amplitude epsilon(0) = 10(-3) reveal an exponential decrease followed by two internal friction peaks (at t(m) approximate to 7 and 19 min) for the Pb-62Sn alloy. For the Pb-1.9Sn alloy under the same conditions, an exponential decrease followed by an internal friction peak (at t(m) approximate to 7 min) is observed. It is shown numerically that the above singularities of internal friction are formed by processes of intermittent phase decomposition of Pb-Sn alloys in the cyclic stress field produced by an external load. Experimental data on Q(-1)(t) are used for reconstructing the kinetic curves describing the decomposition (conversion) ratio as a function of time and for calculating the corresponding values of parameters K and n of the Avrami kinetic equation for the Pb-62Sn alloy.