Size effects of NiTi nanoparticle on thermally induced martensitic phase transformation

Molecular dynamics simulations are performed to study the thermally induced martensitic phase transformation (B2-B19') of NiTi free-standing nanoparticles. We successfully reproduce the experimental observations that the martensitic transformation temperature decreases with reducing particle size and the martensite may form self accommodated herringbone morphology. It is found that a core-shell structure with shell thickness of 0.3 nm (approximately equal to one unit cell) exists in NiTi nanoparticles and the B2-019' transformation is dominated by the core region. In addition, three detwinning manners are observed in the reverse martensitic phase transformation (B19'-B2) process. It is concluded that the suppression of B2-B19' transformation in NiTi nanoparticles is due to the lack of nucleation sites and the size dependent martensitic morphology originates from the constraints of surface shell.