Nucleation in a nickel-based superalloy utilizing computational thermodynamics and diffusion kinetics

A model for predicting nucleation kinetics of coherent, homogeneous precipitates using thermodynamic and diffusion kinetic data calculated by computer modeling software was developed. The nucleation model incorporated classical nucleation theory along with derivations of the incubation time using the theory of time reversal symmetry.; An atomic mobility database was developed for a seven-element nickel-based superalloy to allow for the calculation of multicomponent diffusivities to be used in the incubation time calculations. The seven elements modeled in the database were: nickel (Ni), iron (Fe), chromium (Cr), niobium (Nb), titanium (Ti), aluminum (Al), and carbon (C). The process of assessing an atomic mobility database is discussed, and the difficulties of assembling such a database are reviewed.; The nucleation model and atomic mobility database were then applied to modeling the nucleation kinetics of gamma' in two nickel-aluminum binary alloys. The results of the nucleation model were in good agreement with one binary alloy but did not agree well with the results for the second alloy. Enhanced diffusion due to excess vacancy concentrations was identified as the most probable reason for the discrepancy with the second nickel-aluminum alloy.; The nucleation model and atomic mobility database were then tested on an industrial, multicomponent nickel-based superalloy, Inconel alloy 706. The precipitation kinetics of two coherent precipitates, gamma' and gamma'', were modeled and compared to Time-Temperature-Transformation diagrams and Time-Temperature-Hardness diagrams for the alloy. The calculated incubation times for gamma ' and gamma'' were faster than the experimentally observed transformation start times. The results of the gamma' incubation time calculations were in close approximation to the experimental data at higher temperatures. The results of the gamma'' calculations did not agree well with the experimental data.