Large thermal hysteresis in a single-phase NiTiNb shape memory alloy

Large thermal hysteresis (T_hys) is favorable in shape memory alloys (SMAs) because it renders wide service and storage temperature range of the SMA-made parts. In this letter, we show that large intrinsic T_hys up to 100 K is achieved in a low Nb content (2 at.%), β-Nb-free and single-phase NiTiNb SMA fabricated by casting, forging, wire-drawing and annealing. Such a large T_hys is comparable to that of the widely used dual-phase NiTiNb SMAs in which high Nb content and β-Nb are required. By exploiting the grain-size-dependence of T_hys in our single-phase NiTiNb SMA and that in an equiatomic NiTi SMA, we attribute the large T_hys to the enhanced kinetic resistance of transformation by the small grains. The connection between the T_hys and the kinetic resistance is demonstrated by a modified dislocation-based kinetic model.