Effects of grain size on the rate-dependent thermomechanical responses of nanostructured superelastic NiTi

We investigated the effects of grain size on the rate-dependent thermomechanical responses of polycrystalline superelastic NiTi (with an average grain size from 10 to 90 nm) under both monotonic and cyclic tensile loading-unloading. Measurements of stress-strain curves, hysteresis loop area, and temperature fields are synchronized using in situ infrared thermography in the strain rate range from ε̇ = 4 × 10−5 s−1 to ε̇ = 1 × 10−1 s−1. It is found that with the grain size reduction to the nanoscale, the rate dependence of the transformation stress and the hysteresis loop area gradually weakens and finally tends to vanish for a grain size of 10 nm. Under cyclic loading, the non-isothermal cyclic stability of the polycrystal is significantly improved as manifested by 64% reduction in heat accumulation and 91% reduction in stress variations when the grain size is reduced from 90 to 27 nm. It is shown that such significant improvements in the cyclic stability and decrease of the rate sensitivity (while preserving large (≈5%) recoverable strain) originate from the rapid decrease of internal heat sources (the latent heat and the hysteresis heat) and the rapid decrease of the temperature dependence of the transformation stress with the grain size. This work strongly implies that the heat accumulation in cyclic loading of superelastic NiTi SMAs, as one of the sources of poor fatigue response, can be reduced by extreme grain refinement.