Determination of precipitate nucleation and growth rates from ultrasonic harmonic generation

An analytical model based on precipitate-dislocation interactions is presented that predicts a decrease in ultrasonic harmonic generation during the nucleation of precipitates and an increase during precipitate growth up to the point of precipitate-matrix coherency loss. The opposing effects allow independent, indirect assessments of nucleation and growth rates from a curve fit of the model equation to experimental data. Application to the assessment of S′ precipitation during the artificial aging of AA2024 yields a constant nucleation rate of 5.4×1015 m−3 s−1 and a growth rate increasing as the 1.2 power of time in the range 72–432 min during heat treatment at 463 K.