Grain-size-dependent martensitic transformation in bulk nanocrystalline TiNi under tensile deformation

► Grain-size-dependent stress-induced martensitic transition in TiNi is revealed. ► A relationship between the required stress for the martensitic transition and grain size is obtained. ► Results have implications for controlling functional properties of TiNi. Effect of grain size d and internal stress on the stress-induced phase transformation from austenite to martensite in bulk nanocrystalline (NC) TiNi during room-temperature tensile tests has been investigated. The stress σA–M required for this phase transformation increases with decreasing grain size d. The stress σA–M goes up from 150MPa to 450MPa as d decreases from ∼600nm to ∼10nm, and a rapid increase of stress σA–M is observed as d goes below 50nm. This relationship between grain size d and stress σA–M may be attributed largely to the increased grain boundary density and phase interfaces from grain refinement and various phase compositions in NC TiNi, which inhibit the martensitic transformation. Moreover, the internal stress in NC TiNi has a significant role in hindering this phase transition.