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

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 σ _A-M required for this phase transformation increases with decreasing grain size d. The stress σ _A-M goes up from 150 MPa to 450 MPa as d decreases from ∼600 nm to ∼10 nm, and a rapid increase of stress σ _A-M is observed as d goes below 50 nm. This relationship between grain size d and stress σ _A-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.