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

Abstract

Large thermal hysteresis (Thys) 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 Thys 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 Thys 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 Thys in our single-phase NiTiNb SMA and that in an equiatomic NiTi SMA, we attribute the large Thys to the enhanced kinetic resistance of transformation by the small grains. The connection between the Thys and the kinetic resistance is demonstrated by a modified dislocation-based kinetic model.