Characterization of the nucleation and growth behavior of copper precipitates in low-carbon steels

The nucleation and growth behavior of copper precipitates in ferrite was investigated both theoretically and experimentally for two low-carbon steels with and without niobium additions in samples cooled directly to the desired aging temperature from the austenitizing temperature. Theoretical nucleation and growth rate models were constructed using calculated thermodynamic data in conjunction with classical theories. The maximum nucleation and growth rates for Cu were experimentally determined to be 8.0 × 10²¹ nuclei/m³ s at 612 °C and 0.12 nm/s at 682 °C, respectively. Using an experimentally determined “effective” activation energy for the diffusion of copper, the theoretical nucleation rate curve compared very well with the hardness data for the first 5 minutes of aging. The growth behavior of the Cu precipitates was investigated through use of a conventional transmission electron microscope (TEM) for samples directly aged at 550 °C. For aging times up to 21 hours, the average precipitate size scaled with a time dependence of t ^1/2.