A dislocation-disclination model is proposed, describing the heterogeneous nucleation of an embryo of hcp martensite at a tilt grain-boundary segment containing some extrinsic dislocations. The total energy gain due to hcp embryo nucleation is analyzed in detail, and the existence of both the equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) is shown. Depending on the external conditions, these characteristic embryo sizes may vary in wide ranges. So, the equilibrium size increases while the critical size decreases as the external shear stress increases and the temperature decreases. It is also demonstrated that a critical external stress exists which induces athermal embryo nucleation when the nucleation-energy barrier disappears and the terms of equilibrium and critical embryo sizes lose their significance. The critical external stress has been studied, depending on the temperature and characteristic parameters of the grain boundary where the fcc-to-hcp martensite transformation takes place. We have shown, in particular, that the critical external stress increases in direct proportion to both the grain-boundary misorientation angle and temperature.
