Nucleation pathway in coherent precipitation

Abstract

The non-classical nucleation pathway of coherent precipitates has been computed through minimisation of the nucleation barrier in the composition (c)–size (R) space to predict the evolution of nucleus composition. The generalized Gibbs model, developed by Schmelzer et al. [J. Chem. Phys. 112 (2000) p.3820; J. Colloid Interface Sci. 272 (2004) p.109], has been extended to include misfit elastic energy. The composition of critical embryos c* was found to be independent of the interfacial constant. The composition of critical nuclei (c*) decreased with supersaturation. The elastic energy increased both c* and the nucleation barrier, as well as R*. The evolution of nucleus composition (c) as a function of size (R) along the minimum energy pathway was computed. Nucleation only starts when a size threshold is exceeded. Then, rapid enrichment to the expected composition (c _β) precedes a constant composition regime. However, for small supersaturations, the change in cluster composition can occur sharply for a very small radius and then the composition slowly increased with a significant change in size. Coherency misfit energy was found to slow down the evolution of nuclei composition with R. The model was compared to experimental results.

Keywords:

• solid state phase transformations
• precipitation
• non-classical nucleation
• minimum energy pathway

Acknowledgements

The authors acknowledge C. Pareige, B. Radiguet (GPM, Rouen) and G. Martin (CEA, Saclay) for fruitful discussions. The first author also acknowledges N. Masquelier for his help in the use of the Thermo-Calc® software.