Absence of a trapping effect on the kinetic critical radius in nucleation and growth processes

In a recent paper by Erdélyi et al. [Acta Mater. 58 (2010) 5639–5645] it is claimed based on kinetic Monte Carlo simulations that the actual critical nucleus size can deviate from the thermodynamic prediction by orders of magnitude due to effects caused by the difference between the diffusivities in the matrix and the nucleus. Here these issues are critically re-examined both by simulations and theoretical considerations. The growth probability curves for nuclei of different sizes are obtained for different diffusion asymmetries; they show no effect of diffusion asymmetries on the critical size. The time evolution of the simulated nucleus sizes does show an effect, however: while the mean time until a nucleus has either evaporated or grown is comparable, fluctuations on small timescales are much larger for faster diffusion in the nucleus. Finally, it is argued that a dynamic trapping effect is actually in conflict with the principle of detailed balance.