Y Wang, A G Khachaturyan/Materials Science and Engineering A 438-440(2006)55-6 Reloading O/G Martensitic variant 1, with the trigonal axis along Martensitic variant 3, with the trigonal axis along Grain boundary Fig. 4. The hysteresis loop and the 3D microstructures obtained at different stresses for a cubic- trigonal martensitic transformation in a polycrystalline system 15 of martensitic variants, autocatalytic effect, and transformation faults and grain boundaries [36, 53-59]. The strain fields assoc hysteresis, it offers little knowledge about the heterogeneous ated with the martensitic particles may be partially or completely processes occurring at the dislocation level. Even though homo- cancelled by the strain fields generated by the lattice defects. geneous nucleation of martensite through the formation of mul- This would lower significantly or eliminate completely the acti- tivariant polytwinned embryos under large undercooling has vation barriers of the kinetic pathways of nucleation and growth been demonstrated in the phase field simulations and homo- Therefore, it is essential to include dislocation-level activities geneous nucleation of martensite has been observed in small in models of MTs. The recent development of the phase field particle experiments [52], most MTs occur through heteroge- approach to dislocation dynamics has offered a unique oppor- neous nucleation at lattice defects such as dislocations, stacking tunity to treat rigorously and self-consistently heterogeneous60 Y. Wang, A.G. Khachaturyan / Materials Science and Engineering A 438–440 (2006) 55–63 Fig. 4. The hysteresis loop and the 3D microstructures obtained at different stresses for a cubic→trigonal martensitic transformation in a polycrystalline system [15]. of martensitic variants, autocatalytic effect, and transformation hysteresis, it offers little knowledge about the heterogeneous processes occurring at the dislocation level. Even though homo￾geneous nucleation of martensite through the formation of mul￾tivariant polytwinned embryos under large undercooling has been demonstrated in the phase field simulations and homo￾geneous nucleation of martensite has been observed in small particle experiments [52], most MTs occur through heteroge￾neous nucleation at lattice defects such as dislocations, stacking faults and grain boundaries [36,53–59]. The strain fields associ￾ated with the martensitic particles may be partially or completely cancelled by the strain fields generated by the lattice defects. This would lower significantly or eliminate completely the acti￾vation barriers of the kinetic pathways of nucleation and growth. Therefore, it is essential to include dislocation-level activities in models of MTs. The recent development of the phase field approach to dislocation dynamics has offered a unique oppor￾tunity to treat rigorously and self-consistently heterogeneous