Stress hysteresis and temperature dependence of phase transition stress in nanostructured NiTi—Effects of grain size

Stress hysteresis (H) and temperature dependence of phase transition stress (dσ/dT) are the two signatures of first-order phase transition in shape memory alloys. We studied the effects of grain size on these two properties in polycrystalline superelastic NiTi with the average grain size from 10 nm to 1500 nm. We identified a critical grain size (∼60 nm) below which both H and dσ/dT rapidly decrease, leading to vanishing hysteresis and breakdown of Clausius-Clapeyron equation. The physics behind such grain size effects are the dominance of interfacial energy in the energetics of the polycrystal and the lack of two-phase coexistence at nano-scales.