Early Stages of Cu Precipitation in 15-5 PH Maraging Steel Revisited -Part II: Thermokinetic Simulation

In this part II of the authors' investigation of Cu-precipitation in 15-5 PH steel, the authors complement the experimental analysis of part I with thermokinetic modeling and simulation to aid in the interpretation of processes and mechanisms involved in the two precipitation reactions between around 300 and 500 degrees C. The kinetic simulations are founded on extended classical nucleation theory, a mean-field model for precipitate growth, a predictive model for interface energy calculation, and CALPHAD-based Gibbs-energy and mobility databases. The authors modeling supports the interpretation that the first peak in the continuous differential scanning calorimetry (DSC) signal is the result of nucleation of Fe-rich Cu clusters, which leads to a partial Cu-depletion of the matrix and produces the first observed DSC signal. The second DSC peak has its origin in the Cu-enrichment of precipitate clusters above approximately 500 degrees C and the associated enthalpy change. A quantitative reproduction of both exothermic reactions is performed within the framework of the thermokinetic simulation.