A model is presented of the dependence of acoustic harmonic generation in polycrystalline solids on the coherency strains resulting from the lattice mismatch at the interface between the matrix material and a precipitated second phase in the matrix material. The acoustic nonlinearity parameter (a quantitative measure of acoustic nonlinearity) is shown to depend on the second, third, and fourth order elastic constants of the material, the precipitate-matrix lattice misfit parameter, and the volume fraction of precipitates. The model is applied to the artificial aging of aluminum alloy 2024 from the T4 to the T6 temper. Experimental measurements on samples of Al 2024 taken at various heat treatment times not only confirm the predictions of the model but together with the model provide a basis for assessing the influence of precipitate phase transformations on the aging process.