Martensitic transformation of NiTi nanocrystals embedded in an amorphous matrix Original Research Article

The influence of the size on the martensitic phase transformation of spherical NiTi nanocrystals embedded in an amorphous matrix was studied by transmission electron microscopy. The transformation from B2 austenite to B19′ martensite is suppressed by a transformation barrier that increases with increasing transformation strains and decreasing diameter of the nanocrystals. Minimizing the strain and interface energy yields a two step transformation from B2 to the R-phase and finally to B19′. The transformation starts at the crystalline/amorphous interface by heterogeneous nucleation. In the case of the R-phase to B19′ transformation large strains occur causing after a certain point fine compound twinning during growth that finally leads to a complete transformation right up to the interface. In the isolated nanocrystals a lack of collective transformation effects yields larger transformation barriers and causes larger critical diameters than those observed in the grains of nanocrystalline NiTi.