A dislocation-disclination model was proposed which described the heterogeneous nucleation of an embryo of hexagonal close-packed martensite at a tilt grain-boundary segment which contained some extrinsic dislocations. The total energy gain, due to hexagonal close-packed embryo nucleation, was analyzed in detail and the existence of equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) was shown. Depending upon the external conditions, the characteristic embryo size could vary widely. Thus, the equilibrium size increased and the critical size decreased as the external shear stress increased and the temperature decreased. It was also demonstrated that a critical external stress existed which induced athermal embryo nucleation when the nucleation-energy barrier disappeared and the definitions of equilibrium and critical embryo size lost their significance. The critical external stress depended upon the temperature and characteristic parameters of the grain boundary where the face-centered cubic to hexagonal close-packed martensite transformation took place. It was shown, in particular, that the critical external stress increased in direct proportion to the grain-boundary misorientation angle and the temperature.

Dislocation-Disclination Model of Heterogeneous Martensite Nucleation in Transformation-Induced-Plasticity Steels. M.Y.Gutkin, K.N.Mikaelyan, V.E.Verijenko, L.D.Thompson: Metallurgical and Materials Transactions A, 2002, 32[5], 1351-62