  A model for phase transitions initiated on grain boundaries is proposed and
tested against numerical simulations: this approach based on a grain explicit
model (GEM) allows to consider the granular structure, yielding accurate
predictions for a wide span of nucleation processes. Comparisons are made with
classical models of homogeneous (JMAK) as well as heterogeneous (Cahn)
nucleation. A transition scale based on material properties is proposed,
allowing to discriminate between random and site saturated regimes. Finally, we
discuss the relationship between an Avrami type exponent and the transition
regime, drawing conditions for its extraction from experiments.
