Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B19' martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.