Martensitic phase transformations in nanocrystalline NiTi studied by TEM Original Research Article

By high pressure torsion (HPT) deformation almost complete amorphization is obtained in bulk Ni–50.3at.%Ti containing B19′ martensite. During low temperature annealing tiny crystallites retained after the HPT deformation are acting as nuclei and trigger the nanocrystallization of B2 austenite. It is shown that the density of the nuclei is a function of the HPT strain and determines together with the annealing temperature the grain size of the nanocrystals ranging from 5 to 350 nm. Upon cooling the nanostructures transform to B19′ partially since the grain boundaries hinder the autocatalytic formation of martensite. The large transformation strains of B19′ are reduced by very fine (0 0 1) compound twins. With decreasing grain size an increasing energy barrier arises and the martensitic transformation is completely suppressed in grains smaller than 60 nm. The R-phase transformation causing only small transformation strains is observed in grains between 15 and 60 nm. Whereas in grains below 15 nm B2 remains indicating that no transformation occurs at all.