Extended defect structures nucleation

The nucleation rate, growth rate, and transformation rate equations that we developed in the preceding sections are sufficient to provide a general, semiquantitative understanding of nucleation- and growth-based phase transformations. However, it is important to understand that the kinetic models developed in this introductory text are generally not sufficient to provide a microstructurally predictive description of phase transformation for a specific materials system. It is also important to understand that real phase transformation processes often do not reach completion or do not attain complete equilibrium. In fact, extended defects such as grain boundaries or pores should not exist in a true equilibrium solid, so nearly all materials exist in some sort of metastable condition. Many phase transformation processes produce microstructures that depart wildly from our equilibrium expectation. The limited atomic mobilities associated with solid-state diffusion can frequently cause (and preserve) such nonequilibrium structures. In this section, we will focus more deeply on solidification (a liquid-solid phase transformation) as a way to discuss some of these issues. In particular, we will examine a few kinetic concepts/models... 

Fronts under bistable conditions are best studied using a Pt(ll 1) surface, since here no complications arise from structural changes. The behavior found [106] is essentially what is expected for spatially extended bistable systems [62] for detailed modeling see [107]. The critical radii turned out to be in the /xm-range. It is very improbable that such large nuclei are formed via concentration fluctuations. The experimental observations are therefore attributed to heterogeneous nucleation triggered by surface defects. 

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